<figure><img src="image_84.png" /><figcaption>Figure 1: French archeointensity data over the past eight centuries after Genevey et al. [2009].</figcaption></figure>

link existing between geomagnetic secular variation and regional and/or global climatic fluctuations during the Holocene [Gallet et al., 2009a, Hartmann et al., 2010]. This challenging topic is closely connected to the necessity of better knowing the geomagnetic field behavior over centennial time scales, including new constraints on the origin of the so-called archeomagnetic jerks [Gallet et al., 2009b], and, from this point of view, joins the first theme discussed above.

The interest for the different applications of archeomagnetism in geomagnetism, archeology and climatology rapidly and significantly grew over the past few years within the French and international communities of Paleomagnetism and Geomagnetism; there is no doubt that this interest will be even more important in the near future.

## References

J. Carlut, G. Chazot, H. Dessales, and E. Letellier. Trace element variations in an archeological carbonate deposit from the antique city of ostiaă: Environmental and archeological implications. *C. R. Geosciences*, 341:10–20, 2009.

Y. Gallet, M. Le Goff, A. Genevey, J. Margueron, and P. Matthiae. Geomagnetic field intensity behavior in the middle east between 3000 bc and 1500 bc. *Geophys. Res. Lett.*, 2008.

Y. Gallet, A. Genevey, M. Le Goff, N. Warme, J. Gran-Aymerich, and A. Lefevre. On the use of archeology in geomagnetism, and viceversa: Recent developments in archeomagnetism. *C. R. Phys.*, 10, 2009a. doi: 10.1016/j.crhy.2009.08.005.

Y. Gallet, G. Hulot, A. Chulliat, and A. Genevey. Geomagnetic field hemispheric asymmetry and archeomagnetic jerks. *Earth Planet. Sci. Lett.*, 284:179–186, 2009b. doi: 10.1016/j.epsl.2009.04.028.

A. Genevey, Y. Gallet, C. Constable, M. Kornte, and G. Hulot. Archeointä: An upgraded compilation of geomagnetic field intensity data for the past ten millennia and its application to the recovery of the past dipole moment. *Geochem., Geophys., Geosyst.*, 2008.

A. Genevey, Y. Gallet, R. J., and M. Le Goff. Evidence for rapid geomagnetic field intensity variations in western europe over the past 800 years from new archeointensity french data. *Earth Planet. Sci. Lett.*, 2009.

M. Gomez-Paccard, A. Chauvin, P. Lanos, and J. Thiriot. New archeointensity data from spain and the geomagnetic dipole moment in western europe over the past 2000 years. *J. Geophys. Res.*, 2008.

G. Hartmann, A. Genevey, Y. Gallet, R. Trindade, C. Etchevarne, M. Le Goff, and M. Afonso. Archeointensity in northeast brazil over the past five centuries. *Earth Planet. Sci. Lett.*, 296:340–352, 2010.

G. Hervé, E. Schneppe, A. Chauvin, P. Lanos, and N. Nowaczik. Archaeomagnetic results on three early iron age salt-kilns from moyenvic (france). *Geophys. J. Int.*, 2011.

E. Schneppe, P. Lanos, and A. Chauvin. Geomagnetic paleointensity between 1300 and 1750 AD derived from a bread oven floor sequence in Lubeck, Germany. *ggg*, 10, 2009. doi: 10.1029/2009GC002470.

J.-P. Valet, E. Herrero-Bervera, J.-L. Le Mouël, and G. Plénier. Secular variation of the geomagnetic dipole during the past 2000 years. *Geochem., Geophys., Geosyst.*, 9, 2008.

N. Warmé. L'archéomagnétisme appliqué aux fours culinaires du haut moyen âge: dix ans d'activité. L'habitat rural du haut Moyen Âge en Île de France, 2ème supplément au Bulletin archéologique du Vexin Français et du Val d'Oise, Guiry-en-Vexin, 2009.
# Paleomagnetism and Climate

Guillaume Le Hir

Université Paris D. Diderot, Institut de Physique du Globe de Paris, Université Paris-Cité, INSU/CNRS, F-75005 Paris

**Abstract** The Earth's surface has undergone successive episodes of formation of continental landmasses through Wilson cycles, which formed supercontinents, such as Pangea, followed by their dispersal. As a consequence, the land-sea distribution, as well as its distribution with respect to latitude has evolved. Using oceanic magnetic anomalies and palaeomagnetic data, we are able to reconstruct the locations of continents in the past until the Late Jurassic. If paleomagnetic methods are used by many researchers, very few work on the potential influence of the continental drift on the climate change. A French team of scientists has intensively investigated this interplay, and has shown that the paleogeography is among the first mechanisms proposed to explain the climate change at the long term scale.

Indeed it has long been recognized that paleogeography may potentially influence the climate via the continental silicate weathering which is the main carbon sink at the long term scale [Francois and Walker, 1992]. The two climatic factors affecting the rate of continental silicate weathering are the air temperature and the continental runoff, both of these factors being strongly linked to atmospheric $pCO_2$ via the greenhouse effect. The essence of the mechanism is that accumulation of CO2 in the atmosphere would be compensated by an increase in its removal through silicate weathering under enhanced greenhouse conditions, the $pCO_2$ increase would halt at the point where the silicate weathering rate balances the rate of input by volcanic outgassing. On the basis of the existence of this relationship, the continental silicate weathering feedback governs the $pCO_2$ and therefore the Earth's climate at the geological timescale [Walker et al., 1981].

Five years ago, a French team has developed a new tool to investigate the interplays between tectonic and climate [Donnadieu et al., 2006]. This model, named GEOCLIM, includes a General Circulation Model (GCM) [Jacob, 1997] coupled to a box model of the geological carbonalkalinity cycles (COMBINE) [Godderis et al., 2005]. The spatial resolution of GEOCLIM allows to capture the evolution of the main features of geographies and their impacts on the carbon cycle. To illustrate the potential magnitude of continental drift control on the long term atmospheric $CO_2$ and climate, an international team, mainly composed by French researchers [Donnadieu et al., 2006] has quantified the $pCO_2$ evolution

in response of the waltz of continents over the course of the Mesozoic. It should be noted that, regarding the abundance of the oceanic crust, paleogeographies prior the Jurassic are less well constrained.

Looking for the Fig. 1, we see that the general pattern of the simulated $CO_2$ curve illustrates the fundamental importance of the paleogeography in controlling atmospheric $CO_2$ and then the Earth climate. From a global point of view, the breakup of Pangea causes a transition from a generally dry climatic regime during the Triassic to a wetter one during the Cretaceous. In the same time, global and continental air temperature largely decreases in response of the $pCO_2$ reduction. Hence the paleogeography, by influencing the continental weathering pattern, governs the global climate and could explain the Earth cooling or warming at the long-term scale.

As shown with this brief example, the paleomagnetism is an essential tool to reconstruct the past land-sea distributions and therefore the Earth climate. However, three years ago, a second paleomagnetism application has emerged, allowing us to capture the Deccan trap impact on the global climate. Indeed, by a flow-by-flow analysis of paleomagnetic directions, Chenet et al. [2009] have reconstructed the eruptive history of the Deccan trap. They have discovered that the volcanism duration was shorter than previously expected and constituted by volcanic pulses. Using this fact, climate modelers are now able to reconstruct the climatic consequences of the Deccan eruption, and determine its potential impact on the Life. In conclusion, the paleomagnetism and its recent developments contribute to
<figure><img src="image_85.png" /></figure>

Figure 1: (left) Links between paleogeography and runoff using a climate model. Mean annual runoff (cm/yr) for two paleogeographies spanning the Mesozoic obtained in fixing the pCO₂ at the same level (here 1680ppmv). (right) Evolution of atmospheric CO₂ through the Mesozoic. Bars represent atmospheric CO₂ estimates (min/max values) coming from the data sets built by [Royer et al., 2004] and available online at http://www.geosociety.org/pubs/ft2004.htm. The squares represent the modeled atmospheric CO₂ in the reference run under boundary conditions (full description available in [Donnadieu et al., 2006]).

increase our understanding of the past climate
changes and the Earth evolution.

References

A.-L. Chenet, V. Courtillot, F. Fluteau, M. Ger-
ard, X. Quidelleur, S. F. R. Khadri, K. V. Sub-
barao, and T. Thordarson. Determination of
rapid deccan eruptions across the cretaceous-
tertiary boundary using paleomagnetic secu-
lar variation: 2. constraints from analysis of
eight new sections and synthesis for a 3500-
m-thick composite section. *J. Geophys. Res.*,
114, 2009. doi: 10.1029/2008JB005644.

Y. Donnadieu, Y. Godderis, R. Pierrehumbert, G. Dromart, F. Fluteau, and R. Jacob. A GEOCLIM simulation of climatic and biogeochemical consequences of Pangea breakup. *Geochem., Geophys., Geosyst.*, 7, 2006. doi: 10.1029/2006GC001278.

L. Francois and J. Walker. Modeling the phanero-
zoic carbon-cycle and climate constraints
from the sr-87-sr-86 isotopic ratio of seawater.
*Am. J. Sci.*, 292:81–135, 1992.

Y. Godderis, Y. Donnadieu, M. Tombozafi,
R. Pierrehumbert, J. Gaillardet, L. R. Kump,
and B. Dupre. Links between climate, paleo-
geography and silicate rock weathering: A cre-

taceous vs present day comparative study with
the geoclim model. gca, 69:688–688, 2005.

R. L. Jacob. *Low frequency variability in a simulated atmosphere ocean system*. PhD thesis, The University of Wisconsin - Madison, 1997.

D. L. Royer, R. A. Berner, I. P. Montanez, N. J. Tabor, and D. J. Beerling. Co2 as a primary driver of phanerozoic climate. *GSA Today*, 14 (3):4–10, 2004.

J. C. G. Walker, P. B. Hays, and J. F. Kasting. A negative feedback mechanism for the long-term stabilization of earths surface- temperature. *J. Geophys. Res.*, 86:9776–9782, 1981.
# Environmental Magnetism and Magnetotactic Bacteria

Aude Isambert

Institut de Physique du Globe de Paris, Sorbonne Paris Cité, Université Paris Diderot, INSU/CNRS, F-75005 Paris

**Abstract** Studying the crystallographic and magnetic properties of magnetite crystals produced by magnetotactic bacteria present a great interest in many fields such as physics of fine particles, biomineralization, medicine and environmental and planetary sciences. Indeed, (1) biomagnetites represent an interesting object of study in the field of fine particles magnetism and a source of potential medical application, (2) studying the biological mechanisms implied in the formation of magnetite crystals is a means to better understand the different processes of biomineralization in living organisms, (3) and last, fossil magnetosomes could significantly contribute to magnetic properties of sediments and soils, their spatial organisation in chains being considered as a potential biomarker in terrestrial or extraterrestrial environments.

Magnetotactic bacteria mineralize nanometer-sized magnetite (Fe₃O₄) or greigite (Fe₃S₄) crystals in intracellular vesicles, also called magnetosomes. The sizes, morphologies, crystallographic characteristics, and chemical compositions of the biogenic crystals of magnetite are controlled by the bacteria and can thus be used as potential biosignatures (Fig. 1). Magnetosomes are generally arranged in chains that act as compass needles so that the bacteria tend to align passively along the Earth's magnetic field lines. This phenomenon called magnetotaxis, coupled with flagellar motility and aerotaxis, allows the cells to locate and maintain an optimal position for their growth in vertical chemical gradients within aquatic environments.

The French community studying magnetotactic bacteria, due to the transversal aspect of this subject of research, is composed of specialists of different domains including biophysicists, biochemists, microbiologists and geophysicists. One first aim of the working group is to better understand the contribution of fossil magnetosomes to magnetic properties of sediments. Various morphotypes of magnetotactic bacteria have been observed in aqueous environments. The freshwater magnetotactic population found in the Seine River [Isambert et al., 2007] is an example of the variety of both magnetic cells and crystals shapes. Different morphotypes were recognized using morphological criteria, which rely on the number of magnetite crystals and their organization within cells, the size and shape of the cells and their statistical distribution. Among the collected rod-shaped bacteria, a type similar to Magnetobacterium

bavaricum has been found (Fig. 2). This species is indeed easily recognizable by the fact that these large rod-shaped magnetic cells, up to 12 µm in length, may contain more than 600 highly anisotropic bullet-shaped magnetosomes in a unique cell, arranged in 3 up to 5 bundles of magnetosomes chains. Those bullet-shaped magnetites may contribute significantly to sedimentary magnetism. To begin to answer, magnetic measurements on enrichments of freshly collected natural living cells and on magnetic extracts of river sediments are in progress.

In parallel, to improve the knowledge of MTB distribution and ecology, we have just started to investigate the temporal variation of MTB communities in the Seine River by combining the study of phenotypic criteria with phylogenetic analysis and cultivation-independent molecular techniques [Gerard et al., 2005]. This technology, applied for the first time to natural samples could give us specific information on MTB species. Indeed, the biomineralization process of bullet shaped magnetites is still poorly understood and culturing these bacteria has been unsuccessful until now.

A second aim is to better understand the anthropogenic effect on the development of magnetotactic populations and to precise the natural (biomagnetite, sedimentary magnetite) versus anthropogenic input in magnetic particles in the Seine River system. A challenge is to trace both geological, biological and human induced processes and particularly to identify the potential effect of pollution on fine magnetic particles present in these fluvial sediments. A first analysis of the regional distribution of suspended mag-
<figure><img src="image_86.png" /><figcaption>Figure 1: TEM images showing the diversity of magnetosomes morphologies observed in the Seine River (from left to right: cuboctahedral, truncated prismatic, tooth-shaped and elongated prismatic magnetite).</figcaption></figure>

netic material has been conducted on sediment trap samples [Franke et al., 2009]. A combination of rock magnetic and advanced scanning electron microscopic techniques has shown an increase in magnetic concentration coupled with a coarsening in magnetic grain size downstream of the Seine river system. A significant correlation between anthropogenic antimony-rich iron oxide particles and the magnetic concentration has been observed. In the continuity of this study, it seems important to determine the contribution of magnetic fine grains of biological origin at nanometre scale, which are the principal carriers of remanence in sediment, and to observe the potential adsorption of pollutant on those magnetic nanograins.

<figure><img src="image_87.png" /><figcaption>Figure 2: Rod-shaped bacteria extracted from the Seine River, similar to Magnetobacterium bavaricum, containing more than 600 highly anisotropic bullet-shaped magnetites.</figcaption></figure>

In addition to microscopic approach [Isambert

et al., 2007], magnetic measurements, because of the nature of the studied object, represent a useful analysis tool able to serve to identify the contributions of individual tributaries in the Seine river system [Alphandery et al., 2010, Vasiliev et al., 2008].

## References

E. Alphandery, L. Liejour, Y. Lalatonne, and L. Motte. Different signatures between chemically and biologically synthesized nanoparticles in a magnetic sensor: a new technology for multiparametric detection. *Sensors & Actuators B*, 147:786–790, 2010.

C. Franke, C. Kissel, E. Robin, P. Bonte, and F. Lagroix. Magnetic particle characterization in the seine river system: Implications for the determination of natural versus anthropogenic input. *Geochem., Geophys., Geosyst.*, 10, 2009. doi: 10.1029/2009GC002544.

E. Gerard, F. Guyot, P. Philippot, and P. Lopez-Gardia. Fluorescence in situ hybridisation coupled to ultra-small immunogold detection to identify prokaryotic cells using transmission and scanning electron microscopy. *J. Microbiol. Methods*, 63:20–28, 2005.

A. Isambert, N. Menguy, E. Larquet, F. Guyot, and J. P. Valet. Transmission electron microscopy study of magnetites in a freshwater population of magnetotactic bacteria. *Amer. Min.*, 92:621–630, 2007.

I. Vasiliev, C. Franke, J. D. Meeldijk, M. J. Dekkers, C. G. Langereis, and W. Krijgsma. Putative greigite magnetofossils from the pliocene epoch. *Nature Geoscience*, 1: 782–786, 2008.
# Planetary Magnetic Fields and Magnetized Environments

Benoit Langlais

Univrsité de Nantes and CNRS, Laboratoire de Planétologie et Géodynamique de Nantes, 44322 Nantes cedex 3

**Abstract** There exists magnetic fields around other planets than the Earth. The closest body, the Moon, possesses a weak remanent magnetic field, of the order of a few nT at 10 km altitude or so. Mars has a similars magnetic field in origin, but its amplitude reaches up to 1500 nT at 90 km, as it was measured by Mars Global Surveyor. These intense crustal fields strongly interact with the light Martian atmosphere and the solar wind, creating minimagnetospheres. Mercury has the most 'earth-like' magnetic signature, with a magnetic field of core origin. Observations and models of these planetary magnetic fields, as well as measurements on extraterrestrial samples, help to better understand the geodynamo processes.

The existence of a global magnetic field around a planet is important, as it may help to protect and shield the atmosphere against the solar wind [Dehant et al., 2007]. Thus, the magnetic exploration of our solar system is important, as it brings constraints on the conditions under which dynamo action is possible [Breuer et al., 2010]. Here we briefly review some recent results dealing both with planetary magnetic fields and their effects on their environment.

## Mars

The Martian magnetic field was measured by the Mars Global Surveyor (MGS, 1996-2006) probe, with a very good accuracy and a very good and complete coverage. Soon after the first measurements, the nature of the martian field revealed its complexity: intense and large magnetic anomalies are heterogeneously located, mainly above the southern and highly cratered highlands. The northern hemisphere, as well as the largest volcanic provinces and the largest impact craters, seems to be devoid of significant anomalies, meaning that the dynamo which was active when units acquired a remanent magnetization shut down early in Mars' history [Langlais and Quesnel, 2008], although this was recently challenged [Michel and Forni, 2011].

Recently, the absence of magnetization above the northern lowlands were tentatively reinterpreted. Rather than invoking a giant impact, possibly affecting a whole hemisphere, Quesnel et al. [2009] suggested large scale serpentinization processes, affecting the southern hemisphere only. This metamorphic process produces magnetite, and also decrease the density,

which may explain the topographic dichotomy. Another explanation may be the existing oxygenation conditions that existed at Mars' surface while the dynamo was active [Hood et al., 2010]. This would explain why some volcanic units, post-dating the large demagnetized impacts, are still magnetized. An alternate is a different dynamo regime: recently, computations proved that a heat flux dichotomy at the CMB creates a hemispherical dynamo [Amit et al., 2011].

The existence of these strong localized magnetic field modify the way the planet and its environment interacts with the solar wind. For instance,

<figure><img src="image_88.png" /></figure>

Figure 1: Map of the probability to be in a closed field line region at 400 km in altitude on the Martian nightside as calculated from MGS-ER. Also plotted are the trajectories of Mars Express (black line) below 1000 km altitude for four orbits with aurora events (crosses). After Leblanc et al. [2008].
aurorae were recorded by SPICAM on board Mars Express above areas where crustal magnetic field lines are not closed, i.e. organized into cusp-like structures [Leblanc et al., 2008]. 3-D hybrid simulations of the solar wind/Mars interaction were also performed, to quantify the importance of the solar wind source to the helium balance of the Martian atmosphere [Chanteur et al., 2009]. The importance of the dayside induced magnetic field in the ionosphere were also studied [Akalin et al., 2010].

## Mercury

Mercury's magnetic field will be partially monitored by the US probe MESSENGER very soon, giving the first hemispheric (but not global) view of the Hermean magnetic field. While it is of core origin [Wicht et al., 2007], the exact conditions under which a dynamo action is possible in such a small planet remain unknown. Recently, Malavergne et al. [2010] suggested a layered core, due to the presence of sulfur and silicon. Such a structure would indeed limit the area in which the dynamo action is possible, explaining the weak observed magnetic field.

## Lunar and other rocks

Measurements and experiments on extraterrestrial rocks may constrain the nature and the extent of the lunar magnetic fields. The origin of the weak lunar magnetic field is still unknown [Langlais et al., 2010]. Has it a core, external or in-situ origin? [Gattacceca et al., 2010] performed experimental acquisition of shock remanence by lunar rocks in the 0.1-2 GPa range, and showed that shock explanation is plausible as the origin of lunar magnetization. This is confirmed by analyses of lunar Apollo and Luna returned samples [Rochette et al., 2010].

Paleomagnetic studies of chondritic and small-body achondritic meteorites have revealed a diversity of magnetic field records [Weiss et al., 2010]. When coupled with new theoretical insights into the possibility of dynamo generation on small bodies, these records indicate that some planetesimals formed metallic cores and early dynamos within just a few million years of solar system formation.

## Titan

The Cassini mission has provided much information about the Titan environment, with numerous low altitude encounters with the moon. The structure of its ionosphere was studied by Ågren et al. [2009], who highlighted the dependence of electron number density and electron temperature w.r.t. the Solar Zenith Angle. The atmospheric escape is strongly related to these quantities too [Edberg et al., 2010], as it was predicted by a hybrid model [Modolo and Chanteur, 2008].

## References

K. Ågren, J. Wahlund, P. Garnier, R. Modolo, J. Cui, M. Galand, and I. Müller-Wodarg. On the ionospheric structure of Titan. *Planet. Space Sci.*, 57, 2009. doi: 10.1016/j.pss.2009.04.012.

F. Akalin, D. D. Morgan, D. A. Gurnett, D. L. Kirchner, D. A. Brain, R. Modolo, M. H. Acuna, and J. R. Espley. Dayside induced magnetic field in the ionosphere of Mars. *Icarus*, 206, 2010. doi: 10.1016/j.icarus.2009.03.021.

H. Amit, U. R. Christensen, and B. Langlais. The influence of degree-1 mantle heterogeneity on the past dynamo of Mars. *Phys. Earth Planet. Int.*, Submitted, 2011.

D. Breuer, S. Labrosse, and T. Spohn. Thermal Evolution and Magnetic Field Generation in Terrestrial Planets and Satellites. *Space Sci. Rev.*, 152(1-4):449-500, 2010. doi: 10.1007/s11214-009-9587-5.

G. M. Chanteur, E. Dubinin, R. Modolo, and M. Fraenz. Capture of solar wind alpha-particles by the Martian atmosphere. *Geophys. Res. Lett.*, 36, 2009. doi: 10.1029/2009GL040235.

V. Dehant, H. Lammer, Y. N. Kulikov, J.-M. Grießmeier, D. Breuer, O. Verhoeven, Ö. Karatekin, T. van Hoolst, O. Korablev, and P. Lognonné. Planetary magnetic dynamo effect on atmospheric protection of early Earth and Mars. *Space Sci. Rev.*, 129, 2007. doi: 10.1007/s11214-007-9163-9.

N. J. T. Edberg, J. Wahlund, K. Ågren, M. W. Morooka, R. Modolo, C. Bertucci, and M. K. Dougherty. Electron density and temperature measurements in the cold plasma environment of Titan: Implications for atmospheric
escape. *Geophys. Res. Lett.*, 37, 2010. doi: 10.1029/2010GL044544.

J. Gattacceca, M. Boustie, L. Hood, J.-P. Cuq-Lelandais, M. Fuller, N. S. Bezaeva, T. de Resseguier, and L. Berthe. Can the lunar crust be magnetized by shock: Experimental groundtruth. *Earth Planet. Sci. Lett.*, 299, 2010. doi: 10.1016/j.epsl.2010.08.011.

L. L. Hood, K. P. Harrison, B. Langlais, R. J. Lillis, F. Poulet, and D. A. Williams. Magnetic anomalies near Apollinaris Patera and the Medusae Fossae formation in Lucus Planum, Mars. *Icarus*, 208, 2010. doi: 10.1016/j.icarus.2010.01.009.

B. Langlais and Y. Quesnel. New Perspectives on Mars' Crustal Magnetic Field. *C. R. Geosciences*, 340, 2008. doi: 10.1016/j.crte.2008.08.006e.

B. Langlais, V. Lesur, M. E. Purucker, J. E. P. Connerney, and M. Mandea. Crustal Magnetic Fields of Terrestrial Planets. *Space Sci. Rev.*, 152, 2010. doi: 10.1007/s11214-009-9557-y.

F. Leblanc, O. Witasse, J. Lilensten, R. A. Frahm, A. Safaenili, D. A. Brain, J. Mouginot, H. Nilsson, Y. Futaana, J. Halekas, M. Holmström, J. L. Bertaux, J. D. Winningham, W. Kofman, and R. Lundin. Observations of aurorae by SPICAM ultraviolet spectrograph on board Mars Express: Simultaneous ASPERA-3 and MARSIS measurements. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2008JA013033.

V. Malavergne, M. J. Toplis, S. Berthet, and J. Jones. Highly reducing conditions during core formation on Mercury: Implications for internal structure and the origin of a magnetic field. *Icarus*, 206, 2010. doi: 10.1016/j.icarus.2009.09.001.

N. Michel and O. Forni. Mars mantle convection: Influence of phase transitions with core cooling. *Planet. Space Sci.*, In Press, 2011. doi: 10.1016/j.pss.2011.02.013.

R. Modolo and G. M. Chanteur. A global hybrid model for Titan's interaction with the Kronian plasma: Application to the Cassini Taflyby. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2007JA012453.

Y. Quesnel, C. Sotin, B. Langlais, S. Costin, M. Mandea, M. Gottschalk, and J. Dyment. Serpentinization of the martian crust during Noachian. *Earth Planet. Sci. Lett.*, 277, 2009. doi: 10.1016/j.epsl.2008.10.012.

P. Rochette, J. Gattacceca, A. V. Ivanov, M. A. Nazarov, and N. S. Bezaeva. Magnetic properties of lunar materials: Metorites, Luna and Apollo returned samples. *Earth Planet. Sci. Lett.*, 292, 2010. doi: 10.1016/j.epsl.2010.02.007.

B. P. Weiss, J. Gattacceca, S. Stanley, P. Rochette, and U. R. Christensen. Paleomagnetic Records of Meteorites and Early Planetesimal Differentiation. *Space Sci. Rev.*, 152, 2010. doi: 10.1007/s11214-009-9580-z.

J. Wicht, M. Mandea, F. Takahashi, U. R. Christensen, M. Matsushima, and B. Langlais. The Origin of Mercury's Internal Magnetic Field. *Space Sci. Rev.*, 132, 2007. doi: 10.1007/s11214-007-9280-5.
# Balloon Campaigns Dedicated to the Study of the Dynamics and the Chemistry of the Atmosphere

Jean-Baptiste Renard

Université Orléans and CNRS, LPC2E, F-45071 Orléans 2, France

**Abstract** Several campaigns involving different kinds of balloons were dedicated to the study of the atmosphere between 2006 and 2010. These campaigns were conducted by CNES. The main campaigns were the ENVISAT validation missions between 2006 and 2008, VASCO (climate variability above the Indian Ocean) in 2007, SCOUT (troposhere - stratosphere interactions in tropical areas) in 2008, StrapolÉté (summertime polar stratosphere), Kiruna 2010 (water vapour, ozone, nitrogen dioxide and carbon dioxide in the stratosphere), and Concordiasi (validation campaign of the IASI instrument).

## Envisat validation (2006)

Since its launch in 2002, several validation campaigns of the European satellite Envisat have been conducted at various latitudes using stratospheric balloons [Renard et al., 2008]. The 2006 campaign, conducted from Kiruna, Sweden, was mainly dedicated to the validation of NOx measurements, in particular in the lower stratosphere by combining in situ and remote sensing measurements.

It has been shown that remote-sensing measurements can be biased in the lower stratosphere when the stratosphere is dynamically perturbed, as in the outer part of the winter polar vortex [Berthet et al., 2007].

## VASCO (2007)

This campaign was dedicated to better understand the climate variability in the Indian Ocean [Vialard et al., 2009]. In tropical regions, the atmosphere undergoes marked fluctuations of temperature, pressure and humidity. These fluctuations, which are particularly intense in the Indian Ocean, cause disturbances over differing time-scales.

The method involves using low-flying aeroclipsers and stratospheric balloons that drift with disturbances over the oceans and in the atmosphere's boundary layer, where the circulation of airstreams is influenced by surface relief and by thermal exchanges with the surface, both land and sea. The programme will run for several years in the form of annual campaigns during

which balloons will be launched from January until mid-February.

## SCOUT / Envisat validation (2008)

7 flights were performed. The campaign was in the (sub)tropical regions with the following objectives [Cairo et al., 2010]:

* to investigate transport from the tropical tropopause layer into the stratosphere;

* to study the role of deep convection on the transport of species into the stratosphere;

* to investigate the microphysics of aerosols and clouds and their impact on the composition of this atmospheric region.

As Teresina (Brasil) is truly tropical (lat 5S), the campaign adds value and gives more insight into the tropical trace gas distribution in the upper troposphere, lower and middle stratosphere. The Quasi-Biennial Oscillation (QBO) is an important modulator for the transport insight the tropical pipe. Because of the QBO modulation of the wind direction and speed in the equatorial stratosphere, it is often possible to manage rather long balloon flights by taking advantage of different wind directions at different layers and using the valve and ballast to stabilize the balloon at the preferred altitude levels.

Flight were also dedicated to the validation of the MIPAS instrument onboard the Envisat satellite. The SCOUT project is funded by the European commission, and the Envisat valuation is funded by the European Space Agency.
<figure><img src="image_89.png" /><figcaption>Figure 1: Launch of a stratospheric balloon from Kiruna (Sweden).</figcaption></figure>

## Strapolet é (2009)

The summer polar stratosphere was poorly documented in the past, since most of the previous polar campaigns were dedicated to the ozone-hole studies [Huret, 2010].

The core of the project is based on measurements obtained during an Arctic campaign involving seven flights of balloon-borne instruments in August-September 2009 from Kiruna, Sweden (Fig. 1). The data set obtained from balloon-borne instruments and satellites will allow us to provide a detailed picture of the polar stratosphere in August for the content in NOx and halogenes species, and in aerosols

Concerning the modelling approach the goal is twofold with firstly an assessment of their operational results from comparisons with measurements, and secondly detailed investigations to highlight key processes controlling the stratosphere composition and to improve the model calculations. The project is funded by ANR (French National Research Agency) and IPEV (Institut polaire français Paul Emile Victor).

## Kiruna (2010)

This small campaign was mainly dedicated to the study of the water vapour and aerosol content trend in the stratosphere.

In particular, a flight was performed up to an altitude of 42 km, has unambiguously confirmed the presence of (solid) aerosols in significant amount at such altitude [Renard et al., 2010].

## Concordiasi (2010)

Concordiasi [Rabier et al., 2010] is an international project, currently supported by the following agencies: MÃıtÃ’o-France, CNES, CNRS/INSU, NSF, NCAR, University of Wyoming, Purdue University, University of Colorado, the Alfred Wegener Institute, the Met Office and ECMWF. Three field experiments are part of Concordiasi, two which have occurred during the autumn 2008 and 2009 (Austral spring) in Antarctica and a third one planned in Austral spring 2010.

Additional in-situ measurements include radiosoundings at the Concordia station at Dome C and at Dumont d'Urville in 2008, at Concordia in 2009 and high altitude balloons able to drop dropsondes, launched on demand under a parachute and measuring atmospheric parameters on their way down over Antarctica in 2010. Some of the balloons will carry experimental instruments measuring ozone and particles at flight level. Some will also be enhanced to carry GPS receivers in order to perform radiooccultation measurements.

Numerical simulations will be use to complete the experiment data. The main goal is to provide validation data to improve the usage of polar-orbiting satellite data over Antarctica, in particular IASI radiances (IASI is a sounder instrument onboard the Metop satellite measuring the infrared spectrum with an improved accuracy for temperature and humidity sounding in the troposphere and stratosphere).

## References

G. Berthet, J.-B. Renard, V. Catoire, M. Chartier, C. Robert, N. Huret, F. Coquelet, Q. Bourgeois, E. D. Riviere, B. Barret, F. Lefevre, and A. Hauchecorne. Remote-sensing measurements in the polar vortex: Comparison to in situ observations and implications for the simultaneous retrievals and analysis of the NO2 and OCIO species. *J. Geophys. Res.*, 112, 2007. doi: 10.1029/2007JD008699.

F. Cairo, J. P. Pommereau, K. S. Law, H. Schlager, A. Garnier, F. Fierli, M. Ern, M. Streibel, S. Arabas, S. Borrmann, J. J. Berthelier, C. Blom, T. Christensen, F. D'Amato, G. Di Donfrancesco, T. Deshler, A. Diedhiou, G. Durry, O. Engelsen, F. Goutail, N. R. P. Harris, E. R. T. Kerstel, S. Khaykin, P. Konopka, A. Kylling,
N. Larsen, T. Lebel, X. Liu, A. R. MacKenzie, J. Nielsen, A. Oulanowski, D. J. Parker, J. Pelon, J. Polcher, J. A. Pyle, F. Ravegnani, E. D. Riviere, A. D. Robinson, T. Rockmann, C. Schiller, F. Simoes, L. Stefanutti, F. Stroh, L. Some, P. Siegmund, N. Sitnikov, J. P. Vernier, C. M. Volk, C. Voigt, M. von Hobe, S. Viciani, and V. Yushkov. An introduction to the SCOUT-AMMA stratospheric aircraft, balloons and sondes campaign in West Africa, August 2006: rationale and roadmap. *Atm. Chem. Phys.*, pages 2237–2256, 2010.

N. Huret. Le projet strapolété pour l'étude de la stratosphère polaire en été. *Météorologie*, 68, 2010.

F. Rabier, A. Bouchard, E. Brun, A. Doerenbecher, S. Guedj, V. Guidard, F. Karbou, V. H. Peuch, L. El Amraoui, D. Puech, C. Genthon, G. Picard, M. Town, A. Hertzog, F. Vial, P. Cocquerez, S. A. Cohn, T. Hock, J. Fox, H. Cole, D. Parsons, J. Powers, K. Romberg, J. Vanandel, T. Deshler, J. Mercer, J. S. Haase, L. Avalione, L. Kalnajs, C. R. Mechoso, A. Tangborn, A. Pellegrini, Y. Frenot, J. N. Thepaut, A. McNally, G. Balsamo, and P. Steinle. The Concordia project in Antarctica. *Bull. Am. Met. Soc.*, 91, 2010. doi: 10.1175/2009BAMS2764.1.

J.-B. Renard, G. Berthet, C. Brogniez, V. Catoire, D. Fussen, F. Goutail, H. Oelhaf, J.-P. Pommereau, H. K. Roscoe, G. Wetzel, M. Chartier, C. Robert, J.-Y. Balois, C. Verwaerde, F. Auriol, P. Francois, B. Gaubicher, and P. Wursteisen. Validation of GOMOS-Envisat vertical profiles of O-3, NO2, NO3, and aerosol extinction using balloon-borne instruments and analysis of the retrievals. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2007JA012345.

J.-B. Renard, G. Berthet, V. Salazar, V. Catoire, M. Tagger, B. Gaubicher, and C. Robert. In situ detection of aerosol layers in the middle stratosphere. *Geophys. Res. Lett.*, 37, 2010. doi: 10.1029/2010GL044307.

J. Vialard, J. P. Duvel, M. J. McPhaden, P. Bouruet-Aubertot, B. Ward, E. Key, D. Bourras, R. Weller, P. Minnett, A. Weill, C. Cas sou, L. Eymard, T. Fristedt, C. Basdevant, Y. Dandonneau, O. Duteil, T. Izumo, C. Montegut, S. Masson, F. Marsac, C. Menkes, and S. Kennan. CIRENE Air-Sea Interactions in the Seychelles-Chagos Thermocline Ridge Region. *Bull. Am. Met. Soc.*, 90, 2009. doi: 10.1175/2008BAMS2499.1.
# French Satellite Programme for the Study of the Troposphere and Climate

Jean-Baptiste Renard

Université Orléans and CNRS, LPC2E, F-45071 Orléans 2

**Abstract** Four main programmes have been conducted during the last years by the French Space Agency for the study of the troposphere and climate from space. The objectives are a better understanding of the water vapour cycle, the better detection of the aerosols, pollution studies, meteorological provisions, and role of solar irradiance on climate. These programmes are CALLISPSO (tropospheric aerosols), PARASOL (clouds and aerosols properties), IASI (study weather forecast and atmospheric chemistry), and PICARD (effects of sun irradiance on climate).

## CALIPSO

The Franco-American CALIPSO mission (Cloud Aerosol Lidar and Infrared Pathfinder Satellite Observations) was selected by the NASA for the Earth System Science Pathfinder (ESSP) program to supply a unique data set of atmosphere vertical profiles measured by a Lidar on-board a satellite [Vernier et al., 2009, Winker et al., 2010]. The CALIPSO satellite has a payload composed of one backscattering Lidar at 532 and 1,060 nm, with polarization measurements, a 1 meter telescope, a visible camera, and an infrared imager provided by CNES. This minisatellite provides vertical distribution of the properties of aerosols and thin clouds to better understand the aerosol-cloud-radiation interactions. In particular, CALIPSO has detected the ashes of the Eyjafjöll volcano above Paris (Figure 1). CALIPSO has been launched on 28 April 2006.

CALIPSO flies in tandem with CLOUDSAT radar satellite, and both integrated the A-TRAIN, constellation of satellites dedicated to the observation of clouds, aerosols and the water cycle (Fig. 2).

## PARASOL

PARASOL, for *Polarization and Anisotropy of Reflectances for Atmospheric Science coupled with Observations from a Lidar* is the second microsatellite in the Myriade series, developed by CNES. It carries the POLDER widefield radiometer (Polarization and Directionality of the Earth's Reflectances), designed with the support of the LOA atmospheric optics laboratory

(CNRS-USTL). POLDER measures the directional characteristics and polarization of light reflected by the Earth and atmosphere to further our understanding of the radiative and micophysical properties of clouds and aerosols. For example, this innovative observing technique will allow scientists to distinguish natural and manmade aerosols more accurately [Fan et al., 2008]. PARASOL is also in the A-Train. PARASOL was launched on 18 December 2004.

## IASI

IASI, Infrared Atmospheric Sounding Interferometer, is a key payload element of the METOP series of European meteorological polar-orbit satellites. It is developed by CNES in the framework of a co-operation agreement with EUMET-SAT. The first flight model was launched in 2006 onboard the first European meteorological polar-orbiting satellites, METOP-A. The second and third instruments will be mounted on the METOP-B and C satellites with launches scheduled in April 2012 and October-November 2016.

IASI has been designed for operational meteorological soundings with a very high level of accuracy (specifications on Temperature accuracy of 1K for 1 km and 10% for humidity) being devoted to improved medium range weather forecast. It is also designed for atmospheric chemistry aiming at estimating and monitoring trace gases like ozone, methane or carbon monoxide on a global scale [Clerbaux et al., 2009]. The IASI system includes the 3 instruments. The measurement technique is based on passive IR remote sensing using an accurately calibrated
<figure><img src="image_90.png" /><figcaption>Figure 1: Ashes of the Eyjafjöll volcano above Paris (from IPSL, NASA and CNES).</figcaption></figure>

<figure><img src="image_91.png" /><figcaption>Figure 2: The "A-Train".</figcaption></figure>

Fourier Transform Spectrometer operating in the 3.7 - 15.5 µm spectral range and an associated infrared imager operating in the 10.3 - 12.5 µm spectral range. The optical configuration of the sounder is based on a Michelson interferometer. The integrated infrared imager allows the co registration of the IASI sounder with AVHRR imager on-board METOP.

METOP (Fig. 3) was launched on 19 October 2006.

<figure><img src="image_92.png" /><figcaption>Figure 3: The METOP satellite.</figcaption></figure>

## PICARD

PICARD is a micro-satellite dedicated to the simultaneous measurement of the absolute total and spectral solar irradiance, the diameter and solar shape, and to the Sun's interior probing by the helioseismology method [Thuillier et al., 2009, Schmutz et al., 2009]. These measurements obtained all along the mission will allow to study their variations as a function of the solar activity. The PICARD payload is composed of the following instruments: SOVAP SOlar VARIability PICARD: composed of a differential absolute radiometer and a bolometric sensor to measure the total solar irradiance (previously called solar constant) ; PREMOS PREcision MOnitor Sensor: a set of 3 photometers to study the ozone formation and destruction, and to perform helioseismologic observations, and an absolute differential radiometer to measure the total solar irradiance; SODISM SOlar Diameter Imager and Surface Mapper: an imaging telescope accurately pointed and a CCD which allows to measure the solar diameter and shape with an accuracy of a few milliarc second, and to perform helioseismologic observations to probe the solar interior.

The PICARD measurements will help to better understand the effect of the Sun irradiance on Earth climate. PICARD was launched on 15 June 2010.

## References

C. Clerbaux, A. Boynard, L. Clarisse, M. George, J. Hadji-Lazaro, H. Herbin, D. Hurtmans, M. Pommier, A. Razavi, S. Turquety, C. Wespes, and P. F. Coheur. Monitoring of atmospheric composition using the thermal infrared
IASI/MetOp sounder. *Atm. Chem. Phys.*, 9: 6041–6054, 2009.

X. Fan, P. Goloub, J. L. Deuze, H. Chen, W. Zhang, D. Tanre, and Z. Li. Evaluation of PARASOL aerosol retrieval over North East Asia. *Rem. Sens. Env.*, 112, 2008. doi: 10.1016/j.rse.2007.06.010.

W. Schmutz, A. Fehlmann, G. Huelsen, P. Meindl, R. Winkler, G. Thuillier, P. Blattner, F. Buisson, T. Egorova, W. Finsterle, N. Fox, J. Groebner, J. F. Hochedez, S. Koller, M. Meftah, M. Meisonnier, S. Nyeki, D. Pfiffner, H. Roth, E. Rozanov, M. Spescha, C. Wehrli, L. Werner, and J. U. Wyss. The PREMOS/PICARD instrument calibration. *Metrologia*, 46, 2009. doi: 10.1088/0026-1394/46/4/S13.

G. Thuillier, T. Foujols, D. Bolsee, D. Gillotay, M. Herse, W. Peetermans, W. Decuyper, H. Mandel, P. Sperfeld, S. Pape, D. R. Taubert, and J. Hartmann. SOLAR/SOLSPEC: scientific objectives, instrument performance and its absolute calibration using a blackbody as primary standard source. *Sol. Phys.*, 257, 2009. doi: 10.1007/s11207-009-9361-6.

J. P. Vernier, J. P. Pommereau, A. Garnier, J. Pelon, N. Larsen, J. Nielsen, T. Christensen, F. Cairo, L. W. Thomason, T. Leblanc, and I. S. McDermid. Tropical stratospheric aerosol layer from CALIPSO lidar observations. *J. Geophys. Res.*, 114, 2009. doi: 10.1029/2009JD011946.

D. M. Winker, J. Pelon, J. A. Coakley Jr., S. A. Ackerman, R. J. Charlson, P. R. Colarco, P. Flamant, Q. Fu, R. M. Hoff, C. Kittaka, T. L. Kubar, H. Le Treut, M. P. McCormick, G. Megie, L. Poole, K. Powell, C. Trepte, M. A. Vaughan, and B. A. Wielicki. The CALIPSO mission, a global 3D view of aerosols and clouds. *Bull. Am. Meteo. Soc.*, 91, 2010. doi: 10.1175/2010BAMS3009.1.
# The terrestrial ionosphere and its connections

## Pierre-Louis Bleilly (1) and the French community in ionospheric physics

(1) Institut de Recherche en Astrophysique et Planétologie, Toulouse, France

**Abstract** The ionosphere is a good tracer of the interactions between the Sun and the terrestrial environment, as it is connected to the different regions of the magnetospheric system and it is in strong interaction with the thermosphere. Our studies aim at taking advantage of this specificity to access the different levels of couplings within the Earth system in the frame of the scientific part of an international Space Weather program. For doing that we jointly use experimental facilities and complex numerical models of the ionospheric subsystem.

The community is strongly involved in studies aiming at understanding the key role of the thermosphere-ionosphere subsystem within the magnetospheric system, in the frame of Solar-Terrestrial connection programs. The activity of the community is based on a combined use of experiment facilities and numerical models, with clear scientific objectives related to the couplings at high latitudes and applications to the Space Weather. Moreover, our scientific interests evolve towards the couplings at low latitude (interhemispheric dynamics) and couplings with low and mid-altitude regions which may be significant contributors to the overall dynamics.

## Experimentation

The instruments we base our studies on are basically EISCAT incoherent scatter radars, SuperDARN coherent scatter radars, satellites (DEME TER, Cluster) and optical instruments. However, the four-year period has been marked by the death of Jean-Paul Villain, the Principal Investigator of the French contribution to SuperDARN chain. This imposed us to reconsider our involvement in SuperDARN community and we decided to refocus our activity around Kerguelen radar. Moreover, France decided to withdraw from EISCAT association and our involvement is now restricted to the purchase of radar hours around dedicated campaigns. Beside this, a new optical instrument devoted to the measurement the polarity of the oxygen red line emission has been developed and deployed in the polar region (Svalbard) for joint campaigns with EISCAT-ESR radar.

These facilities are located at high latitudes, and our studies mainly deal with solar-wind / magnetosphere couplings. In particular, the penetra-

tion within the magnetosphere of particles and energy coming from the solar wind, observed at high altitude with Cluster, has been studied in term of ionospheric signatures in radar measurement, either on EISCAT or SuperDARN. Following this, a mapping at ionospheric level of the different regions of the magnetosphere has been handled. In connections with these couplings, we have made several studies on the dynamics of the ionospheric plasma:

* ULF waves, which are observed on ionospheric profiles and magnetometer radars at either local scales or global scales;

* relationships between convection and plasma field aligned transport in the vicinity of auroral arcs.

The red line emission is polarized because of the existence of a static magnetic field, and we were able to detect, for the first time this polarity, with the new optical instrument.

More recently, the eclipse au August 1st 2008 was a good opportunity to study the couplings of the ionospheric plasma with the atmosphere in the auroral and polar regions by using our numerical tools (Fig. 1).

## Numerical models

Numerical models are important part of our contribution, as they help us in analyzing and interpreting the experimental data (see figure 1). We have developed a series of ionospheric model, the TRANSCAR family, which are intended to provide a very good insight of the ionospheric plasma dynamics, controlled by the couplings with the atmosphere and the magnetosphere.
<figure><img src="image_93.png" /></figure>

Figure 1: temporal profiles of the electron temperature at ESR during the eclipse period. The black lines are the contours between 500 and 3000 K (by step of 550 K) of our simulation-with TRANSCAR and should be compared to the same contours for the experiment (in white).

Two new models were built, one dedicated to the inclusion of the impact of energetic protons on the atmosphere and one dedicated the interhemispheric transport of plasma within the plasmasphere. Beside this, we also built a new model for the runaway electrons which are one of the energetic contributions of the ionosphere to the global system.

## Space Weather activity

Space weather is the physical and phenomenological state of natural space environments which focus on the possible impacts on biological and technological systems. Our contribution to the Solar-Terrestrial physics is completely embedded in the scientific aspects of this large program. We made some efforts to provide some parameters, like proxies, characteristics of the magnetospheric system activity which may help to forecast the solar and non-solar driven perturbations that affect Sun-Earth system.

## Low and mid-atmosphere couplings

There are more and more evidences that the couplings between the ionosphere and the middle and low atmosphere play a significant role in the dynamics of the Earth system. Thus, we decided to extend our studies to the low ionosphere (D region) where chemical processes can produce molecules (e.g. NOx) that can be transported to lower regions (in the stratosphere). In this perspective we are involved in TARANIS project which will study the couplings between thunderstorms and the magnetosphere.

## References

P.-L. Bleilly, D. Alcayde, and A. P. van Eyken. A new analysis method for determining polar ionosphere and upper atmosphere characteristics from ESR data: Illustration with IPY period. *J. Geophys. Res.*, 115, 2010. doi: 10.1029/2009JA014876.

G. Garcia and F. Forme. A kinetic model of the ionospheric return currents. *Geophys. Res. Lett.*, 36, 2009. doi: 10.1029/2009GL039972.

C. Hanuise, J. C. Cerisier, F. Auchere, K. Bocchialini, S. Bruinsma, N. Cornilleau-Wehrlin, N. Jakowski, C. Lathuillere, M. Menvielle, J.-J. Valette, N. Vilmer, J. Watermann, and P. Yaya. From the Sun to the Earth: impact of the 27-28 May 2003 solar events on the magnetosphere, ionosphere and thermosphere. *Ann. Geophys.*, 24:129-151, 2006.

T. Johansson, G. Marklund, T. Karlsson, S. Lileo, P.-A. Lindqvist, A. Marchaudon, H. Nilsson, and A. Fazakerley. On the profile of intense high-altitude auroral electric fields at magnetospheric boundaries. *Ann. Geophys.*, 24:1713-1723, 2006.

A. Kullen, S. Buchert, T. Karlsson, T. Johansson, S. Lileo, A. Eriksson, H. Nilsson, A. Marchaudon, and A. N. Fazakerley. Plasma transport along discrete auroral arcs and its contribution to the ionospheric plasma convection. *Ann. Geophys.*, 26:3279-3293, 2008.

C. Lathuillere and W. Kofman. A short review on the F1-region ion composition in the auroral and polar ionosphere. 37, 2006. doi: 10.1016/j.asr.2005.12.014.

C. Lathuillere and M. Menvielle. Comparison of the observed and modeled low- to midlatitude thermosphere response to magnetic activity: Effects of solar cycle and disturbance time delay. *Adv. Spa. Res.*, 45, 2010. doi: 10.1016/j.asr.2009.08.016.

C. Lathuillere, M. Menvielle, A. Marchaudon, and S. Bruinsma. A statistical study of the observed and modeled global thermosphere response to magnetic activity at middle and low latitudes. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2007JA012991.

J. Lilensten, C. Simon, M. Barthelemy, J. Moen, R. Thissen, and D. A. Lorentzen. Considering the polarization of the oxygen ther-
mospheric red line for space weather stud-
ies. *Space Weather*, 4, 2006. doi:
10.1029/2006SW000228.

J. Lilensten, J. Moen, M. Barthelemy, R. Thissen, C. Simon, D. A. Lorentzen, O. Dutuit, P. O. Amblard, and F. Sigernes. Polarization in aurorae: A new dimension for space environments studies. *Geophys. Res. Lett.*, 35, 2008. doi: 10.1029/2007GL033006.

S. Lileo, G. T. Marklund, T. Karlsson, T. Johansson, P.-A. Lindqvist, A. Marchaudon, A. Fazakerley, C. Mouikis, and L. M. Kistler. Magnetosphere-ionosphere coupling during periods of extended high auroral activity: a case study. *Ann. Geophys.*, 26:583–591, 2008.

G. Lointier, T. D. de Wit, C. Hanuise, X. Vallieres, and J.-P. Villain. A statistical approach for identifying the ionospheric footprint of magnetospheric boundaries from SuperDARN observations. *Ann. Geophys.*, 26:305–314, 2008.

R. Lukianova, C. Hanuise, and F. Christiansen. Asymmetric distribution of the ionospheric electric potential in the opposite hemispheres as inferred from the SuperDARN observations and FAC-based convection model. *J. Atmo. Terr. Phys.*, 70, 2008. doi: 10.1016/j.jastp.2008.05.015.

A. Marchaudon, J.-C. Cerisier, J.-M. Bosqued, C. J. Owen, A. N. Fazakerley, and A. D. Lahiff. On the structure of field-aligned currents in the mid-altitude cusp. *Ann. Geophys.*, 24:3391–3401, 2006.

M. Menvielle, C. Lathuillere, S. Bruinsma, and R. Viereck. A new method for studying the thermospheric density variability derived from CHAMP/STAR accelerometer data for magnetically active conditions. *Ann. Geophys.*, 25: 1949–1958, 2007.

M. Messerotti, F. Zuccarello, S. L. Guglielmino, V. Bothmer, J. Lilensten, G. Noci, M. Storini, and H. Lundstedt. Solar Weather Event Modelling and Prediction. *Space Sci. Rev.*, 147, 2009. doi: 10.1007/s11214-009-9574-x.
# Cluster's insights in solar wind-magnetosphere coupling

## Aurélie Marchaudon

Laboratoire de Physique et Chimie de l'Environnement et de l'Espace, CNRS, Orléans

**Abstract** Despite more than 10 years spent in space, the four spacecraft Cluster mission is still making important progress on the understanding of the Earth's solar-wind magnetosphere coupling. The multi-point nature of this mission allows discriminating between temporal and spatial structures. Recent results include new insights on large-scale and small-scale properties of magnetopause reconnection, dynamics of polar cusp, polar cap and of its boundaries.

## Reconnection process at the magnetopause

Magnetopause reconnection has long been recognized as the main process by which energy momentum and plasma can be transferred from the solar wind to the magnetosphere. The Cluster mission flying in tetrahedron configuration through the high-latitude magnetopause has improved our understanding of this region and of the reconnection process, especially when used in conjunction with other missions, such as the Chinese Double Star mission.

Berchem et al. [2008] have compared Cluster and Double Star magnetopause data to results of MHD model. They have been able to discriminate between anti-parallel and 'component' reconnection models; in the latter magnetic fields do not need to be strictly anti-parallel for reconnection to occur. Pitout et al. [2008] investigated properties of the magnetosheath at different altitudes with Cluster and Double Star showing that unlike predictions from gasdynamic modeling, the density is found lower near the nose of the magnetopause than further downstream.

Pitout et al. [2008] and Owen et al. [2008] also used the four-point measurement capability of Cluster to discriminate between sporadic reconnection signatures known as Flux Transfer Events (FTE) and magnetopause pressure pulses caused by solar wind/magnetosheath compressions. Owen et al. [2008] confirmed for the first time without ambiguity that specific magnetopause signatures, known as 'Crater' FTE, are caused by reconnection.

Magnetopause studies for very quiet and very disturbed solar wind conditions have also been possible with Cluster. Maggiolo et al. [2008] showed that very efficient magnetopause reconnection process can occur even during extremely low solar wind density conditions, ex-

plaining why the plasmasheet and magnetotail regions are maintained during such conditions. Dandouras et al. [2009] studied unusual day-side magnetosphere compression, penetration of high-energy particles in the magnetotail, and ring current development following Interplanetary Coronal Mass Ejections (ICME) shock arrival on the magnetopause.

## Dynamics of the polar cusp

Cluster is also a powerful tool for studying polar cusp dynamics, region providing direct access to solar wind plasma into the magnetosphere via magnetopause reconnection process. When crossing the cusp at mid-altitude, the Cluster spacecraft adopt a string-of-pearl configuration following each other on the same orbit with a certain time delay (usually several minutes). With this configuration, temporal variations of the cusp precipitation can be studied and location of the magnetopause reconnection site with respect to Interplanetary Magnetic Field (IMF) variations can be inferred.

By means of statistical analysis, Pitout et al. [2006] studied the size and location of the polar cusp. Pitout et al. [2009] recently completed this study by investigating large-scale cusp morphology. They showed that the occurrence of clearly dispersed ion structures in the cusp, caused by magnetopause reconnection, is 48% and corresponds generally to stable IMF conditions. In all other cases, the cusp exhibits a more disturbed behaviour in terms of ion structures and generally correlates with variable or rotating IMF.

By studying evolution of cusp precipitation with the four Cluster spacecraft, Escoubet et al. [2008a,b] showed that the cusp responds quickly to IMF rotation causing generally a relocation
<figure><img src="image_94.png" /><figcaption>Figure 1: Summary sketch of the suggested current geometry for a polar cap crossing by northward IMF. The red (green) arrows point toward the Earth (magnetosphere) and represent the downgoing (upgoing) currents mostly carried by the electron beams (ion structures) [Teste et al., 2007].</figcaption></figure>

of the magnetopause reconnection site. During the transition period between two different stable IMF orientations, particles coming from two distinct magnetopause reconnection sites are simultaneously observed.

Marchaudon et al. [2009] used a conjunction between Cluster, Double Star and ionospheric SuperDARN radars, to study properties of cusp re-connected flux tubes issued from magnetopause reconnection. Using a method developed in a previous paper [Marchaudon et al., 2006], they determined the transverse sizes (0.5-2 RE), the bean-shape like, the velocity (20-40 km.s-1) and the field-aligned current distribution (upward/downward current pair) of these flux tubes. They thus gained a complete picture of the flux tubes electrodynamics.

## Electrodynamics of the polar cap and of its boundaries

Field-aligned electron and ion beams have long been observed in the auroral zones. On the other hand, polar cap mainly connected to the magnetospheric lobe region, has long been regarded as an almost void region, apart from weak ion outflows originating from the dayside region and transported anti-sunward with the solar wind flow.

Cluster, crossing the polar cap between 5 and 9 RE altitude, discovered complex particles signatures associated with broadband electrostatic emissions during northward IMF periods. The polar ionosphere suddenly appeared as a wide

region of successive extended particle populations forming a complex electrodynamical circuit in the polar cap. Maggiolo et al. [2006] first identified ion outflows originating from the ionosphere and accelerated locally by potential drop as confirmed by their inverted-V shaped energy-time signatures. Teste et al. [2007] identified exactly between these ion outflow structures, narrow upward electrons beams with large fluxes, also escaping the ionosphere and responsible for broadband electrostatic emissions (Fig. 1). Teste et al. [2010] showed recently that these beams are likely to destabilize Langmuir waves and could be responsible for the appearance of electrostatic solitary waves above the polar cap.

A large set of phenomena occur at polar cap boundaries and can be observed by Cluster combined with ionosphere measurements (EIS-CAT radars, ground-based magnetometers, low-latitude spacecraft...). Aikio et al. [2008] observed zig-zag motion of the polar cap boundary (PCB) with a 30 minutes period during sub-storm late expansion phase. They proposed that the poleward motions of the PCB are produced by bursts of enhanced reconnection at the near-Earth neutral line (NENL) in the magnetotail. The subsequent equatorward motions of the PCB represent the recovery of the merging line towards the equilibrium state. Echim et al. [2009] studied a stable discrete auroral arc associated with fluxes of field-aligned accelerated electrons and ions at the interface of the polar cap. They compared Cluster and DMSP observations with a quasi-stationary magnetosphere-ionosphere coupling model. The matching data
and model results suggest a quasi-stationary field-aligned acceleration of auroral electrons and ions, probably associated with a convergent perpendicular electric field at the polar cap boundary.

## References

A. T. Aikio, T. Pitkanen, D. Fontaine, I. Dandouras, O. Amm, A. Kozlovsky, A. Vaivdas, and A. Fazakerley. EISCAT and Cluster observations in the vicinity of the dynamical polar cap boundary. *Ann. Geophys.*, 26, 2008.

J. Berchem, A. Marchaudon, M. Dunlop, C. P. Escoubet, J. M. Bosqued, H. Reme, I. Dandouras, A. Balogh, E. Lucek, C. Carr, and Z. Pu. Reconnection at the dayside magnetopause: Comparisons of global MHD simulation results with Cluster and Double Star observations. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2007JA012743.

I. S. Dandouras, H. Reme, J. Cao, and C. P. Escoubet. Magnetosphere response to the 2005 and 2006 extreme solar events as observed by the Cluster and Double Star spacecraft. *Adv. Spa. Res.*, 43, 2009. doi: 10.1016/j.asr.2008.10.015.

M. M. Echim, R. Maggiolo, M. Roth, and J. De Keyser. A magnetospheric generator driving ion and electron acceleration and electric currents in a discrete auroral arc observed by Cluster and DMSP. *Geophys. Res. Lett.*, 36, 2009. doi: 10.1029/2009GL038343.

C. P. Escoubet, J. Berchem, J.-M. Bosqued, K. J. Trattner, M. G. G. T. Taylor, F. Pitout, H. Laakso, A. Masson, M. Dunlop, I. Dandouras, H. Reme, A. N. Fazakerley, and P. Daly. Effect of a northward turning of the interplanetary magnetic field on cusp precipitation as observed by Cluster. *J. Geophys. Res.*, 113, 2008a. doi: 10.1029/2007JA012771.

C. P. Escoubet, J. Berchem, J.-M. Bosqued, K. J. Trattner, M. G. G. T. Taylor, F. Pitout, C. Vallat, H. Laakso, A. Masson, M. Dunlop, H. Reme, I. Dandouras, and A. Fazakerley. Two sources of magnetosheath ions observed by Cluster in the mid-altitude polar cusp. *Adv. Spa. Res.*, 41, 2008b. doi: 10.1016/j.asr.2007.04.031.

R. Maggiolo, J.-A. Sauvaud, D. Fontaine, A. Teste, E. Grigorenko, A. Balogh, A. Fazakerley, G. Paschmann, D. Delcourt, and

H. Reme. A multi-satellite study of accelerated ionospheric ion beams above the polar cap. *Ann. Geophys.*, 24, 2006.

R. Maggiolo, J. A. Sauvaud, I. Dandouras, E. Lucek, and H. Reme. A case study of dayside reconnection under extremely low solar wind density conditions. *Ann. Geophys.*, 26, 2008.

A. Marchaudon, J.-C. Cerisier, J.-M. Bosqued, C. J. Owen, A. N. Fazakerley, and A. D. Lahiff. On the structure of field-aligned currents in the mid-altitude cusp. *Ann. Geophys.*, 24, 2006.

A. Marchaudon, J.-C. Cerisier, M. W. Dunlop, F. Pitout, J.-M. Bosqued, and A. N. Fazakerley. Shape, size, velocity and field-aligned currents of dayside plasma injections: a multi-altitude study. *Ann. Geophys.*, 27, 2009.

C. J. Owen, A. Marchaudon, M. W. Dunlop, A. N. Fazakerley, J.-M. Bosqued, J. P. Dewhurst, R. C. Fear, S. A. Fuselier, A. Balogh, and H. Reme. Cluster observations of "crater" flux transfer events at the dayside high-latitude magnetopause. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2007JA012701.

F. Pitout, C. P. Escoubet, B. Klecker, and H. Reme. Cluster survey of the mid-altitude cusp: 1. size, location, and dynamics. *Ann. Geophys.*, 24, 2006.

F. Pitout, M. W. Dunlop, A. Blagau, Y. Bogdanova, C. P. Escoubet, C. Carr, I. Dandouras, and A. Fazakerley. Coordinated Cluster and Double Star observations of the dayside magnetosheath and magnetopause at different latitudes near noon. *J. Geophys. Res.*, 113, 2008. doi: 10.1029/2007JA012767.

F. Pitout, C. P. Escoubet, B. Klecker, and I. Dandouras. Cluster survey of the mid-altitude cusp - Part 2: Large-scale morphology. *Ann. Geophys.*, 27, 2009.

A. Teste, D. Fontaine, J.-A. Sauvaud, R. Maggiolo, P. Canu, and A. Fazakerley. CLUSTER observations of electron outflowing beams carrying downward currents above the polar cap by northward IMF. *Ann. Geophys.*, 25, 2007.

A. Teste, D. Fontaine, P. Canu, and G. Belmont. Cluster observations of outflowing electron distributions and broadband electrostatic emissions above the polar cap. *Geophys. Res. Lett.*, 37, 2010. doi: 10.1029/2009GL041593.
RAPPORT QUADRIENNAL DU CNFGG A L'UGGI

QUADRENNIAL REPORT OF CNFGG TO IUGG

SECTION 5 – METÉOROLOGIE ET
SCIENCES DE L'ATMOSPHÈRE

SECTION 5 – METEOROLOGY AND
ATMOSPHERIC SCIENCES
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This section has not provided report on 01/07/2011
RAPPORT QUADRIENNAL DU CNFGG A L'UGGI

QUADRENNIAL REPORT OF CNFGG TO IUGG

SECTION 6 – SCIENCES

HYDROLOGIQUES

SECTION 6 – HYDROLOGICAL
SCIENCES
Cette section n'a pas fourni de rapport au 01/07/2011

This section has not provided report on 01/07/2011
RAPPORT QUADRIENNAL DU CNFGG A L'UGGI

QUADRENNIAL REPORT OF CNFGG TO IUGG

SECTION 7 – SCIENCES PHYSIQUES
DE L'OCÉAN

SECTION 7 – PHYSICAL SCIENCES
OF THE OCEAN
# Rapport quadriennal de la Section VII Sciences
physiques des Océans
pour le congrès IUGG 2011

## Contenu

1/ *Observations of the ACC across the Kerguelen Plateau: TRACK project* - Young-Hyang Park

2/ *The DRAKE project - Experimental aims and design* - Christine Provost

3/ *The OVIDE Project Documenting and understanding the inter annual to decadal variability of the North Atlantic Subpolar gyre* - Herlé Mercier, Pascale Lherminier, Nathalie Daniault, Virginie Thierry, Bruno Ferron, Thierry Huck

4/ *Ocean fine scales – micro-scales horizontal and vertical mixing* - Gilles Reverdin (LOCEAN, Paris), Pascale Bouruet-Aubertot (LOCEAN, Paris), Bruno Ferron (LPO, Brest), Louis Marié (LPO, Brest), Ariane Koch-Larrouy (LEGOS, Toulouse)

5/ *Mesoscale and submesoscale turbulence* - Patrice Klein and Guillaume Lapeyre

6/ *A global numerical model to study the interannual variability of the ocean* - Anne Marie Treguier

7/ *Inter-decadal modulation of ENSO nonlinearity and its evolution in a warming climate* - Boucharel J., Dewitte B., du Penhoat Y., Garel B., Yeh, S.-W., Kug J.-S.

8/ *Observed freshening and warming of the western Pacific Warm Pool* - Sophie Cravatte, Thierry Delcroix, Dongxiao Zhang, Michael McPhaden and Julie Leloup

9/ *Low-frequency variations of the large-scale ocean circulation and heat transport in the North Atlantic from 1955–2008 in situ temperature and salinity data* - T. Huck, A. Colin de Verdière, P. Estrade, F. Gaillard, R. Schopp, P. Bellec, R. Dussin

10/ *Ocean wave research in France, 2007-2010* - Fabrice Ardhuin

11/ *Coastal patterns tracking: a lagrangian approach* - Philippe FRAUNIE
# Observations of the ACC across the Kerguelen Plateau: TRACK project

PI: Young-Hyang Park (yhpark@mnhn.fr)

LOCEAN/DMPA, Muséum National d'Histoire Naturelle, Paris, France

## Rational and objectives

The Kerguelen Plateau is the largest near-meridional submarine topographic obstacle, diverting the Antarctic Circumpolar Current (ACC) over a great distance between the subtropical and subpolar regions. It acts as a natural barrier for closing the eastern boundary of the Weddell Gyre to the west and developing the western boundary of the Australian-Antarctic Gyre to the east of the plateau, thus providing an important meridional pathway for the equatorward evacuation of transformed subpolar water masses. Despite its indisputable role in the ACC dynamics and the Southern Ocean meridional circulation, our knowledge on the physics and dynamics of the ACC across the Kerguelen Plateau has been seriously hampered until recently due to lacking of high-quality oceanographic observations. The TRACK (Transport across the Kerguelen Plateau) cruises carried out as part of a French contribution to the International Polar Year activities, were designed to measure directly the top-to-bottom physical properties, currents and transport over the Kerguelen Plateau, especially across the Fawn Trough and the Deep Western Boundary Current (DWBC) on the eastern flank of the southern plateau.

## TRACK cruises and preliminary results

In February-March 2009 closely-spaced, full-depth hydrographic and direct current measurements in the Fawn Trough area were made during the TRACK cruise (Chief Scientist: Y.-H. Park) on R/V Marion Dufresne II. This cruise occupied in particular a near-meridional section across the Fawn Trough and repeated the western boundary segment of the WOCE I8S section at 58°S (Fig. 1). Three lines of current meter mooring with a total of 12 current meters were maintained for about 1 year across the Southern ACC Front (SACCF), which were recovered in January 2010 during the TRACK recovery cruise (Chief Scientist: F. Vivier). The preliminary results from the 2009 TRACK cruise (Park et al., 2009) are summarised below, while the velocity time series data from recovered current meters are under analysis as part of a thesis work.

Vertical profiles of cross-track lowered acoustic Doppler current profiler (LADCP) velocities and corresponding transport across the western (SACCF) and southern (DWBC) sections are shown in Figure 2. In the western section most of the eastward flow is tightly concentrated in the Fawn Trough, with the strongest flow of 0.6 m s-1 at the surface decreasing to 0.3 m s-1 at 1000 m, below which the velocity is nearly constant reaching >0.2 m s-1 at 2000 m, indicating that both the baroclinic and barotropic components of current are equally important. A secondary eastward flow branch with a depth-averaged velocity on the order of 0.2 m s-1 is observed on the near-shore slope just south of the Heard/McDonald Islands. The net transport across this 750 km-long western section amounts to 50 Sv (1 Sv = 106 m3 s-1) northeastward, 43 Sv of which are concentrated at the SACCF and 6 Sv immediately south of the Heard/McDonald Islands. In the southern section the northwestward flowing DWBC is highly barotropic and mostly confined within a narrow (~75 km) continental slope. It is also characterized by a bottom-intensified flow, with the highest velocity >0.3 m s-1 being found at the bottom at station 48. The total transport of the DWBC amounts to 43 Sv, which is largely compensated by a poleward recirculation transport of 34 Sv in the offshore area.

The TRACK cruise has permitted us to evaluate for the first time reliable transport values of different ACC branches crossing the Kerguelen Plateau (Fig. 3). Including ~2 Sv inferred at the Polar Front (PF) branch south of the Kerguelen Islands, the net ACC transport to the south of the islands amounts to 58 Sv: 43 Sv at the SACCF passing through the Fawn Trough; 6 Sv south of the Heard/McDonald Islands; 7 Sv at the Southern Boundary (SB) of ACC through the northern Princess Elizabeth Trough. The latter 7 Sv was obtained by considering only those stations in the southern section where the bottom water has a temperature >0.1°C, while the major component (36 Sv) of the DWBC with a bottom temperature <0.1°C should be fed by the northward turning of the Antarctic Slope Current along the western limb of the cyclonic subpolar gyre. According to previous work, the ACC main branch associated with the Subantarctic Front (SAF) should carry 89-94 Sv north of the Kerguelen Islands and ~6 Sv at the PF just east of the islands. Combining these with the TRACK transports sums up to 147-152 Sv as the total ACC transport in the Kerguelen area, which are not significantly different from the 147±10 Sv at 140°E south of Australia and the Drake Passage transport of 137±8 Sv augmented by 10-15 Sv of the Indonesian Throughflow.

## Reference

PARK Y.-H., VIVIER F., ROQUET F. & KESTENARE E., 2009. Direct observations of the ACC transport across the Kerguelen Plateau, Geophys. Res. Lett., 36: L18603, doi:10.1029/2009GL039617.
<figure><img src="image_95.png" /><figcaption>Fig. 1_Park. (a) Surface streamlines from the altimetry-derived mean dynamic height, with bold streamlines standing for four circumpolar fronts and a red box for the TRACK cruise area. (b) the 2009 TACK cruise map showing the grid of 60 CTD stations, with two southernmost circumpolar fronts being superimposed. A dotted line is the Jason altimeter ground track #94. Adapted from Park et al. (2009).</figcaption></figure>

<figure><img src="image_96.png" /><figcaption>Fig. 2_Park. Vertical profiles of cross-track LADCP velocities and cumulative top-to-bottom transport in the (a) western (Fawn Trough) and (b) southern (DWBC) sections. Adapted from Park et al. (2009).</figcaption></figure>
<figure><img src="image_97.png" /></figure>

Fig. 3_Park. Schematic of major pathways and transports (in Sv) of the ACC system (bold continuous lines) and DWBC of the Australian-Antarctic Gyre (bold discontinuous lines). Adapted from Park et al. (2009).
**Figure captions**

Fig. 1_Park. (a) Surface streamlines from the altimetry-derived mean dynamic height, with bold streamlines standing for four circumpolar fronts and a red box for the TRACK cruise area. (b) the 2009 TACK cruise map showing the grid of 60 CTD stations, with two southernmost circumpolar fronts being superimposed. A dotted line is the Jason altimeter ground track #94. Adapted from Park et al. (2009).

Fig. 2_Park. Vertical profiles of cross-track LADCP velocities and cumulative top-to-bottom transport in the (a) western (Fawn Trough) and (b) southern (DWBC) sections. Adapted from Park et al. (2009).

Fig. 3_Park. Schematic of major pathways and transports (in Sv) of the ACC system (bold continuous lines) and DWBC of the Australian-Antarctic Gyre (bold discontinuous lines). Adapted from Park et al. (2009).
## The DRAKE project - Experimental aims and design

Christine Provost

The DRAKE project is a recently concluded experiment consisting of in situ measurements made over a period of about 3 years (February 2006 to April 2009), which are tightly coupled to satellite altimetry (TOPEX/POSEIDON and Jason). A major goal of DRAKE is to attempt to combine in situ current meter observations with simultaneous satellite data from Jason to gain a better understanding of the relationship between sea surface height (SSH) and volume transport. The measurement array consisted of 10 subsurface current meter moorings deployed below Jason track 104, with individual moorings located at altimeter crossover points (Fig. 1). High resolution hydrography and LADCP measurements were made on the cruises that serviced the moorings [Provost et al., 2011]. All cruises were performed from R.V. Polarstern. A total of 5 full-depth hydrographic sections were performed.

### DRAKE early results

Early results from analyses of satellite data and the Drake hydrographic data are summarized below, while the time series from the moorings are under analysis as parts of two Ph.D. thesis.

The altimetric time-series documented the long-term trends in sea surface height, the recurrence of major frontal meanders and statistical links between them (Barré et al., 2011). Trends are not homogeneous in Drake Passage, rather they change sign, suggesting a regional effect caused by the complicated bathymetry and geometry (Fig. 2). Topography favors the recurrence of some meanders and eddies in specific spots in Drake Passage. For example a dipole occurring with a close to annual periodicity is observed at the entrance to DP over the Phoenix Antarctic Ridge (PAR) and corresponds to adjacent meanders of the SAF and PF (Barré et al., 2011) (Fig. 3). An anticyclonic meander of the PF was found to be recurrent over the Ona sea floor depression (OSD) to the northwest of the Ona Basin (54°W, 58°S) and constitutes an important element of the cyclonic recirculation in the Ona Basin (Barré et al., 2008).

Barre et al. [2011] used isolines of absolute dynamic topography from satellite altimetry data to map out locations of fronts and eddies, providing a temporal and spatial context for the 2006 DRAKE mooring deployment cruise. Multiple branches of ACC fronts were observed to merge into single jets in the narrowest part of the passage, with two branches of the SAF merging at about 61°W and three branches of the PF merging over the Shackleton Fracture Zone (SFZ) (Fig. 4 a and b). The SACCF branches could also be traced using altimetry. The agreement between the location of the frontal branches and eddies detected by altimetry and the patterns observed in sea surface temperature and ocean color is remarkable in spite of the differences in spatial and temporal resolution (Fig. 4 c, d, e). The crest of the SFZ constitutes a barrier in the south of DP, causing the two SACCF branches to separate by about 400 km, creating sheltered conditions in partial isolation from the ACC, while promoting an active recirculation region in the Ona Basin (Fig. 4 c, d, e). This recirculation, marked by cyclonic eddies carrying cold fresh and oxygenated water from south of the Southern Boundary of the ACC (Fig. 4 c, d, e), causes effective ventilation of the whole Circumpolar Deep Water (CDW) density range (Provost et al., 2011).

The 2006 cruise comprised a hydrographic section under Jason-1 track 104 repeated within 3 weeks, providing a unique opportunity to document full depth in situ variability at about a 10-day interval. Between the two occupations, the contributions of frontal meanders and eddies to the total volume transport changed notably, although the net transport changed by only 10% and agreed within confidence limits with prior WOCE and ISOS estimates [Renault et al., 2011]. Due to high quality of the data and a fine horizontal resolution, transport was estimated (143 Sv) with the best accuracy ever achieved of 7-10 Sv.

Considerable differences in properties between the 10-day-apart sections are observed throughout the whole water column with values as high as 0.2°C in temperature, 0.01 in salinity, 0.03 kg.m-3 in neutral density and 10 µmol.kg-1 in dissolved-oxygen concentration found below a depth of 3000 m [Provost et al., 2011; Sudre et al., 2011]. Only part of the differences is attributable to frontal or eddy displacements along the section. The other part results from the spatial heterogeneity of water properties upstream the section and the funnelling of the flow due to the topographic constraints of the Shackleton Fracture Zone (SFZ). The considerable short-term differences in water properties in rather large-scale structures that cannot be accounted for by frontal motions along the section, call for caution when interpreting differences in hydrographical properties obtained by oceanographic cruises that are years apart in terms of climatic signals.
**References:**

Barré N., C. Provost, N. Sennéchael, J. Hak Lee, 2008: Circulation in the Ona Basin, southern Drake Passage. J. Geophys. Res., 113, doi:10.1029/2007JC004549

Barré N., C. Provost, A. Renault, N. Sennéchael, 2011: Mesoscale activity in Drake Passage during the January-February 2006 cruise: a satellite perspective. Deep Sea Res., Part II, Topical Studies in Oceanography (in press)

Provost C., A. Renault, N. Barré, N. Sennéchael, V. Garçon, J. Sudre and O. Huhn, 2011: Two repeat crossings of Drake Passage in austral summer 2006: short term variations and evidence for considerable ventilation of intermediate and deep waters. Deep Sea Res., Part II, Topical Studies in Oceanography.

Renault A., C. Provost, N. Sennéchael, N. Barré and A. Kartavtseff, 2011: Two LADCP surveys in the Drake Passage in 2006: transport estimates. Deep Sea Res., Part II, Topical Studies in Oceanography, (in press).

Sudre J., V. Garçon, C. Provost, N. Sennéchael, O. Huhn, and M. Lacombe, 2011: Short-term variations of deep water masses in Drake Passage revealed by a multiparametric analysis of the ANT-XXIII/3 bottle data Deep Sea Res., Part II, Topical Studies in Oceanography, (in press).
<figure><img src="image_98.png" /></figure>

Figure 1: Location of the 10 Drake current meter moorings M1 through M10) along Jason ground track #104. Background is bathymetry in meters. Major bathymetric features are labeled: YB Yaghan Basin, OB Ona Basin, PAR: Phoenix Antarctic Ridge, SFZ: Shackleton Fracture Zone, WSR: West Scotia Ridge, OSD: Ona Sea floor Depression,.

<figure><img src="image_99.png" /></figure>

Figure 2: Linear trends in dynamic topography (mm per year) from January 1993 to December 2009. The significance of the trend was computed using a two-sided Student t-test with a confidence limit of 99%. Areas where the trend is not significant are colored in green. White areas correspond to the regions where the time series are incomplete and the data from over the continental slope (depth less than 500 m) are disregarded. Black contours represent the bathymetry between 4000 m and 1000m with contour intervals of 500 m. The black diagonal line indicates the Jason track 104. Black dashed lines represent XBT surveys from the US Antarctic supply vessel (west of track 104) and the repeat hydrographic section SR1b (east of track 104). (updated from Barré et al, 2011).
<figure><img src="image_100.png" /><figcaption>Figure 3:</figcaption></figure>

a Time series of sea level anomaly (SLA) over a 1°x1° box centered at 68°W and 58.5°S (location P1). The linear trend has been removed.

b Regression of SLA, in Drake Passage, on the normalized time-series in Fig.3a. Solid black contours represent the correlation at the 90% confidence level; dashed black contours represent the 95% confidence level. The regression map suggests that the strong anomaly (P1) on the western side of the PAR can be associated with an anomaly of the opposite sign (N1) on the eastern side of the PAR. Thin black lines are bathymetry isobaths (2000, 3000 and 4000m). (updated from Barré et al., 2011)
<figure><img src="image_101.png" /><figcaption>Figure 4:</figcaption></figure>

a Schematic of the frontal branch and eddy location during the ANT-XXIII/3 cruise period derived from altimetry (composite map from 15 to 21 January 2006). SAF branches in orange; PF branches in purple; SACCF branches in green; and SB, in black. Blue patches are the major cyclonic eddies and red patches are the major anticyclonic eddies.

b One-week (17-24 January 2006) composite image of chlorophyll-a concentration (mg.m-3) from MODIS. Black contours are the branches of the ACC fronts and eddies like in a.

c, d, e: Satellite views of the Ona Basin late January 2006:

c: Sea surface temperature (°C) from 22 January 2006 (MODIS). Superimposed are the front and eddy locations derived from the 7-day composite altimetry map centered on January 25: PF-S (purple dotted line); SACCF-N (green solid line), SACCF-S (green dashed line); and SB (black line). The white dotted line is the 2.36 mg.m-3 isoline in Chlorophyll-a concentration and the continuous white line the northern limit to surface waters colder than 2.3°C.

d: Chlorophyll-a concentration (mg.m-3) from 22 January 2006 (MODIS). Contours like in c.

e: Surface geostrophic velocity field derived from the 7-day composite map of altimetric absolute dynamic topography centered on January 25. The color code indicates absolute dynamic topography values (in centimeters). Schematics of the front and eddy locations correspond to the date of the map (same color code as in c and d).

On all maps, thin black contours represent the bathymetry between 4000m and 1000m with contour intervals of 500m. Bottom depths shallower than 3000 m are shaded in grey. The black diagonal line indicates the Jason track 104. (adapted from Provost et al., 2011)
## The OVIDE Project
Documenting and understanding the inter annual
to decadal variability of the North Atlantic Subpolar gyre

Herlé Mercier, Pascale Lherminier, Nathalie Daniault, Virginie Thierry, Bruno Ferron, Thierry Huck
Laboratoire de Physique des Océans, CNRS, Ifremer, IRD, UBO, Plouzané, France

Since 2002, the Ovide project occupies every other year, in June-July, the A25 hydrographic section between the southern tip of Greenland at 60°N and Portugal at 40°N (Figure 1). This is a repeat of a line that was previously occupied during the International Geophysical year and, in the framework of the Fourex project, in September 1997. The Ovide project is a contribution to CLIVAR. During each cruise, CTDO2 and L-ADCP measurements are conducted from the surface down to the sea-floor at about 95 hydrographic stations and VM-ADCP is running during all cruises. Nutrients, CFCs, pH and alkalinity are measured from water samples obtained from the 28 bottle rosette. The Ovide cruises also contributes to maintaining the Argo array by deploying ~15 floats during each cruise. The cruises are a collaborative work with P. Morin (station biologique de Roscoff, France) and A. Rios and F. Perez (IIM Vigo, Spain). We summarize below one of the results obtained from the analysis of the Ovide data set that was complemented by a reanalysis of Fourex.

For the first time in the subpolar North Atlantic, a large variability in the amplitude of the meridional overturning circulation (MOC) and the associated heat flux (HF) has been documented. Estimates are based on an inverse model analysis of the Ovide and Fourex hydrographic sections (Lherminier et al., 2007, Lherminier et al. 2010, Gourcuff et al. 2011). Figure 2 shows that across the Fourex/Ovide hydrographic section the main balance in terms of transport is between the North Atlantic Current (NAC, σ1 < 32.1), east of the subarctic front (SAF) and the net transport of dense water (σ1 > 32.1) west of the SAF. This balance defines the MOC and shows that the NAC transport variability is a good proxy of the MOC variability. Focussing now on the MOC, the maximum amplitudes were observed in September 1997 (19 Sv and 0.69 PW for the MOC and the HF, respectively) and the minimum amplitudes were observed in June 2006 (11 Sv and 0.29 PW for the MOC and the HF). Values for 2002, 2004 and 2008 are in between. Although it is tempting to relate the decrease in the MOC amplitude between Fourex and Ovide 2006 to the spin-down of the subpolar gyre that has been reported by Hakkinen and Rhines (2009), it should be stressed here that those measurements quantify the variability of the MOC and associated heat flux but the associated time scales have still to be determined. For instance, Daniault et al. (2011) showed from an analysis of a current meter array in the East Greenland Irminger Current that at the time of the Ovide 2006 the transport of this current, which is an essential component of the MOC, reached a minimum that lasted for a few weeks, associated with a relatively weak wind stress curl in the Irminger Gyre (as compared to the 20-year average).

## Références

* Daniault, N., P. Lherminier, H. Mercier, 2011: Circulation and transport at the south east tip of Greenland. J. Phys. Oceanogr., in press.

* Gourcuff, C., P. Lherminier, H. Mercier, P. Y. LeTraon, 2011: Altimetry combined with hydrography for ocean transport estimation. J. Atmosph. Ocean. Tech., submitted.

* Hakkinen, S., P. Rhines, 2004: Decline of the subpolar North Atlantic Circulation during the 1990's. Science, 304, 555-559.

* Lherminier, P., H. Mercier, C. Gourcuff, M. Alvarez, S. Bacon, C. Kermabon, 2007: Transports across the 2002 Greenland-Portugal Ovide section and comparison with 1997. J. Geophys. Res. Oceans., 112, C07003, doi:10.1029/2006JC003716.

* Lherminier, P. H. Mercier, T. Huck, C. Gourcuff, F. F. Perez, P. Morin, A. Sarafanov, 2010: The Atlantic Meridional Overturning Circulation and the subpolar gyre observed at the A25-OVIDE section in June 2002 and 2004. Deep Sea Res. I, doi:10.1016/j.dsr.2010.07.009.
<figure><img src="image_102.png" /><figcaption>Figure 1: The Fourex (1997) and the Ovide (2002, 2004, 2006, 2008, 2010) hydrographic lines</figcaption></figure>

<figure><img src="image_103.png" /><figcaption>Figure 2: Transport variability in Sv (1 Sv = 106 m3 s-1) contributing to the MOC obtained from an inverse model analysis of Fourex (1997) and OVIDE (2002, 2004, 2006) hydrographic sections. Northward transports are positive. EGIC: East-Greenland Irminger Current; DWBC: Deep Western Boudary Current; Recirc.: Deep recirculation in the Island basin and the eastern Irminger Sea; NAC: North Atlantic Current. SAF: Sub-arctic front. The σ1 = 32.1 isopycnal is the limit between the upper and lower limbs of the MOC.</figcaption></figure>
Ocean fine scales – micro-scales horizontal and vertical mixing

Gilles Reverdin (LOCEAN, Paris), Pascale Bouruet-Aubertot (LOCEAN, Paris), Bruno Ferron (LPO, Brest), Louis Marié (LPO, Brest), Ariane Koch-Larrouy (LEGOS, Toulouse)

During the last 4 years, the French contribution in this field of research has been more extensive on the vertical dimension with studies of fine and micro-scales in different environments.

This includes investigations during cruises aimed at better understanding large scale or meso-scale circulation and their role on climate or biogeochemical cycles. We can refer to the Ovide cruises in the North Atlantic subpolar gyre (in particular in 2008), with emphasis on finding a structuration of vertical mixing related to bathymetry and understanding how it influences large scale ocean circulation, the Indomix cruise in the Indonesian seas (2010) with emphasis on vertical mixing related to the internal wave field, in particular in the Banda Sea, and its relation to the large internal tides found in these seas. During cruise Boum (summer 2007 in the Mediterranean Sea), the emphasis was on vertical transport of nutrients in the thermocline and the mixed layer during summer in the oligotrophic environment of anti-cyclonic surface eddies of the eastern Mediterranean Sea. During cruise Keops (early 2007, Kerguelen plateau in the Austral Ocean), the emphasis was on understanding the contribution of vertical mixing associated to the internal waves to vertical transport of micro-nutrient over the Kerguelen plateau in the south Indian Ocean. Mixing related to the generation of internal tidal waves near the shelf Break has been investigated during the Mouton summer 2008 cruise in the Bay of Biscay. Some of these data contribute to the parameterization of mixing in numerical models of the regional to large scale ocean circulation.

Other investigations focused on mixing in deep sill environments, in particular near the Lucky Strike segment of the Mid-Atlantic Ridge where deep flows strongly constrained by topography are associated with large vertical mixing, and a very active internal wave field, with a strong contribution of internal tides and background vertical shear to mixing.

A new field of investigation appeared during this period: vertical mixing on the deep shelves surrounding France. This was in particular structured over the Bay of Biscay shelves with the EPIGRAM project, within which the generation of internal waves was investigated in 2010, the dilution by mixing of the freshwater spring plume of the Gironde in 2009-2010, and in 2010, the structuration of mixing near a front (Ushant Front west of Brittany) and how tidal mixing interferes with the structure of this front. This is a fairly new field of research in France, strongly based on the collection of new observations.

Investigation of small horizontal scales and the associated mixing, on the other hand, has been mostly driven by ocean modeling, and surface satellite observations. These satellite observations and model simulations strongly suggest a continuity of scales that structure the ocean dynamics, but also ecosystems and biogeochemical processes. On which scales these structures mix, and the role of small scale vertical processes on this mixing has only been barely touched by the French community on an in situ basis (for example: Gogasmos Bay of Biscay cruises in 2009 and Latex cruise in the Gulf of Lion in 2010).

<figure><img src="image_104.png" /><figcaption>Figure 1</figcaption></figure>

Turbulent kinetic energy Dissipation (log-scale W/kg) measured by VMP micro-structure profiled along Ovide summer 2008 section between Portugal and Greenland. Notice the large values near 4000m (station 40), as well along the Portugal continental slope, and the slightly larger values within the thermocline (east of station 60).
MESOSCALE AND SUBMESOSCALE TURBULENCE

Patrice Klein (LPO, Plouzané, France) and Guillaume Lapeyre (LMD, Paris, France)

The first 500-1000m of the upper ocean have been known for many years to be crucial for the biogeochemical and the atmosphere-ocean coupling systems. At the end of the 20th century, data from conventional satellite altimeters (that capture O(100km) structures) and global-scale oceanic simulations with a 10km resolution have unveiled the strong turbulent character of the mesoscale eddy field in many oceanic regions. These oceanic mesoscale eddies (the ocean weather system), characterized by horizontal scales of O(100km) and depths between 500m and 1000m constitute the dominant part of the total kinetic energy and are known to significantly contribute to the lateral transport of heat, momentum and tracers (Ferrari and Wunsch, ARFM, 2009) and to the biogeochemical system (McGillicuddy, GGC, 2003).

Since 2005 a bunch of studies, both numerical and experimental, have profoundly changed our « turbulent » vision of the upper oceanic layers. They reveal the importance of submesoscales (such as filaments with a 10km width and small-scale vortices with a 5-40km diameter) that are ubiquitous on the Sea Surface Temperature (SST) and Ocean Color images but whose dynamical impacts were totally ignored before. Dynamics of the first 500m of the ocean can now be viewed as captured by a sea of interacting mesoscale eddies and submesoscale structures (Klein et al., JPO, 2008, Capet et al., JPO, 2008, Klein and Lapeyre, ARMS, 2009). These new studies have offered an unprecedented insight on the dramatic impact of these submesoscales. These submesoscales are associated with vertical velocities that can reach 100m/day (typically one order of magnitude larger than those associated with mesoscale eddies) down to 200-500m below the surface (Legal et al., JPO, 2007; Nagai et al., JGR 2006; Klein et al., JPO, 2008; Capet et al., JPO, 2008). As a consequence they impact much more significantly, than previously thought, the global ocean circulation and the biogeochemical system. Such new insight has been made possible because of the development of instruments mounted on towed vehicles, new float devices in the upper layer but, principally, because of the fast development of supercomputers.

Quantification on a global scale indicates that, in oceanic regions overcrowded with mesoscale eddies such as in the Gulf Stream, the Kuroshio and the Antarctic Circumpolar Current, the vertical velocity field associated with submesoscales represent almost 70% of the total vertical velocity field (Lapeyre and Klein, JMR, 2006; Klein et al., JPO, 2008). The remaining 30% being located inside mesoscale eddies. As a result submesoscales explain more than 60% of the total nutrient vertical fluxes. This may provide an answer to the ongoing debate about which physical processes can close the nutrient balance in the ocean (McGillicuddy, GGC, 2003). Furthermore the presence of submesoscales make the primary production in the ocean to be multiplied by a factor 2 (Levy, 2005; Martin et al., GGC, 2006; Klein and Lapeyre, ARMS, 2009). High resolution primitive equation simulations (1km) in large domain performed on the Earth simulator have unveiled the impact of submesoscales on the ocean dynamics at larger scales. Kinetic energy associated with these submesoscales is globally smaller than that associated with mesoscale eddies but their presence explains that the mesoscale kinetic energy is multiplied by a factor 2 (Klein et al., JPO, 2008; and Capet et al., JPO, 2008) and that the sea surface temperature increase by almost 10 K (due to ageostrophic motions) (Klein et al., JPO, 2008)! Roullet and Klein (PRL, 2010) further point out that the submesoscales accelerates the ocean energy circuit – between potential and kinetic energy among the different (large and small) scales - through the vertical velocity field.

The physics that explains the importance of submesoscales in the upper ocean is frontogenesis (the same mechanism that drives atmospheric fronts): density anomalies are stretched by mesoscale eddies into small-scale elongated patterns which increases the density gradient they are associated with and triggers frontogenesis. The resulting vertical velocity field affects the first 500m and provides a pathway between the surface boundary layer (or the surface mixed-layer) and the ocean interior. These submesoscales are part of a surface dynamical mode, first introduced by Blumen (JAS, 1978), extended to the ocean by Held et al. (JFM, 1995) and Lapeyre and Klein (JPO, 2006) and to the atmosphere by Hakim et al. (JAS, 2002) and Tulloch and Smith (JAS, 2009). This mesoscale/submesoscale turbulence in the upper ocean is driven by the surface potential vorticity (that can be estimated from the Sea Surface Height) and by the interior potential vorticity (Lapeyre and Klein, JPO, 2006). The high resolution simulations performed by the French scientists on the Earth Simulator and by the UCLA scientists have provided unvaluable insights on the importance of this mesoscale/submesoscale turbulence. Global observations of the dynamics this mesoscale/submesoscale turbulence are presently lacking but are highly needed to constrain future realistic numerical simulations at these resolutions. Sea Surface Temperature and Ocean Color satellites images have such a high resolution but they are much affected by the clouds and, when available, do not provide any dynamical information. Future satellite altimeter missions (such as the Surface Water and Ocean Topography (SWOT) mission, see Fu and Ferrari, EOS, 2009) should allow to get dynamical information at the ocean surface at a resolution ten times higher than the conventional altimeters and, therefore, reduce the gap between numerical and data resolutions. But one major challenge is how to exploit the full potential information
provided by these new high resolution satellite data - when combined with existing data (such as the Global Argo float database- to retrieve and diagnose these new dynamical impacts related to the submesoscales in particular those related to the vertical velocity field in the first 500m and to the air-sea fluxes. This is one of the major challenge of the present decade.
## A global numerical model to study the interannual variability of the ocean

Anne Marie Treguier, CNRS

Laboratoire de Physique des Oceans, UMR 6523 CNRS-IFREMER-IRD-UBO, IUEM

The DRAKKAR project is a collaboration between French research laboratories (LEGI, Grenoble; LPO, Brest; LOCEAN, Paris), Mercator-Ocean, and other teams in Europe (Southampton, U.K; Kiel, Germany) and Canada. In 2006-2010 a global ocean and sea ice model has been developed, based on the NEMO platform (www.nemo-ocean.eu), with a ¼° resolution: a grid scale ranging from 27 km at the equator to an average of 12 km in the Arctic ocean. A new representation of bottom topography and numerical schemes have brought improvements in the representation of the main current systems and the associated eddies (Barnier et al, 2006; figure 1). Simulations of the ocean variability over the past decades (1958-2007) have been performed, forced by atmospheric data based on ECMWF reanalysis and analyses (Brodeau et al, 2010). The results of these simulations have been distributed to a wide international community. Variability mechanisms have been analyzed globally or regionally (in the Arctic, the Mediterranean sea, the Southern Ocean, the North Atlantic...). Only two examples are given here.

<figure><img src="image_105.png" /><figcaption>Figure 1: Map of a 5-day average of sea level in the DRAKKAR ¼° model (the ice cover is indicated in white). The model reproduces well some eddying regions like the Agulhas retroflection or the Zapiola Anticyclone south of the Brazil-Malvinas confluence zone. The ¼° resolution is still too coarse in other areas, for example in the Gulf stream, the Kuroshio, or west of Australia in the Indian Ocean</figcaption></figure>

### Mechanisms of regional sea level trends (1993-2001).

Lombard et al (2008) have compared the trend in sea level in DRAKKAR simulations and observations. The good agreement (figure 2) allows to use the model to analyse the possible causes of the observed trends. The model (which has no assimilation of satellite nor in situ data) demonstrates that regional changes in sea level over that period are largely forced by the atmosphere. They are mainly due to thermosteric changes in the upper 750m. Wind stress trends over that period explain some of the sea level trends, especially in the tropical Pacific and Indian Ocean.
<figure><img src="image_106.png" /></figure>

Figure 2: Regional trends of sea level between 1993 and 2001 (mm/year).
Top: ORCA025; bottom: observation by satellite altimetry. From Lombard
et al, 2008.

Ice and freshwater transport variability along both sides of Greenland

The ¼° global DRAKKAR model simulates the variability of the Arctic Ocean and its interaction with the Pacific and Atlantic Oceans, with a mesh fine enough to resolve Bering Strait and the main pathways through the Canadian Archipelago. The model allows to propose a quantitative estimate of the liquid freshwater and ice fluxes from the Arctic to the Atlantic (Lique et al, 2009; figure 3). Regarding the mechanisms of interannual variability, the model suggests a striking difference between both sides of Greenland. At Davis Strait to the west, the variability is due to the volume transport, while on the east side at Fram Strait, the variability is due in equal parts to the variations of the volume transport and to variations of the salinity, linked with interannual variability of the ice-ocean exchanges north of Fram Strait.
<figure><img src="image_107.png" /></figure>

Figure 3 (Lique et al, 2009). Schematic view of the Arctic freshwater balance. Mean value of each source and sink is represented (bar, in mSv), as well as the correlation of its variations with the time series of the Arctic freshwater content derivative (circular diagrams). The sign of the freshwater

fluxes indicates if the flux represents a sink or a source of freshwater for the Arctic Ocean, regardless the direction of the volume fluxes (For instance, the inflow branch through Fram Strait brings waters with salinity higher than 34.8, and thus has a negative sign)

Barnier B., G. Madec, T. Penduff, J.-M. Molines, A.-M. Treguier, J. Le Sommer, A. Beckmann, A. Biastoch, C. Böning, J. Dengg, C. Derval, E. Durand, S. Gulev, E. Remy, C. Talandier, S. Theetten, M. Maltrud, J. McClean, and B. De Cuevas, 2006: Impact of partial steps and momentum advection schemes in a global ocean circulation model at eddy permitting resolution. Ocean Dynamics, Vol 4, DOI 10.1007/s10236-006-0082-1.

Brodeau, L., B. Barnier, A.M. Treguier, T. Penduff, S. Gulev, 2009: An ERA40-based atmospheric forcing for global ocean circulation models. Ocean Modelling, 31, 88-104, doi: 10.1016/j.ocemod.2009.10.005

Lombard, A., G. Garric, and T. Penduff, 2009 : «Regional patterns of observed sea level change: Insights from a 1/4° global ocean/sea-ice hindcast ». Ocean Dynamics, 59, 3, 433-449.

Lique, C., Treguier, A.M., Scheinert, M., Penduff, T., 2009: A model-based study of ice and freshwater transport variabilities along both sides of Greenland. Climate Dynamics, 33 (5), 685-705, DOI: 10.1007/s00382-008-0510-7.
Inter-decadal modulation of ENSO nonlinearity and its evolution in a warming climate

Boucharel J. (1), Dewitte B. (1) (2), du Penhoat Y. (1) (2), Garel B. (3), Yeh, S.-W. (4), Kug J.-S. (5)

(1) Université de Toulouse; UPS (OMP-PCA); LEGOS, 14 Av. Edouard Belin, F-31400 Toulouse, France.

(2) IRD; LEGOS, F-31400 Toulouse, France.

(3) Université de Toulouse; INP-ENSEEIHT, Institut de Mathématiques de Toulouse (UPS) France.

(4) Department of Environmental Marine Science, Hanyang University, Ansan, South Korea

(5) Korea Ocean Research and Development Institute, Ansan, South Korea.

**1. Introduction**

El Niño Southern Oscillation (ENSO) is the dominant mode of tropical climate variability, whose variations influence climate and ecosystems as well as many societies around the globe. It is therefore of great interest to understand its past, present and future variations. Over the last century, ENSO properties including amplitude, frequency and propagating features underwent significant changes at decadal to interdecadal timescales (An, 2004), which are partly revealed by the presence of climate shifts (Guilderson and Schrag, 1998) within ENSO typical timeseries. For instance, the two decades after the 1976/77 climate shift were characterised by more frequent and stronger El Niño events compared to previous decades (An and Wang, 2000; An, 2004; Moon et al., 2004). These changes in ENSO characteristics are intimately linked to changes in the mean state of the tropical Pacific because the dominant feedback processes associated with ENSO, namely the zonal advective and thermocline feedbacks, depend explicitly on the zonal mean Sea Surface Temperature (SST) and thermocline gradients, respectively (Zebiak and Cane, 1987; An and Jin, 2001).

Another recently documented characteristic of ENSO is its asymmetry, which reflects the nonlinearity of the tropical Pacific system (An and Jin, 2004). ENSO asymmetry also fluctuates over decadal to interdecadal timescales that are related to changes in mean state (Rodgers et al, 2004; Dewitte et al., 2007). In particular, the latter can result from the residual effect of the ENSO asymmetry variability (Schopf and Burgman, 2006; Dewitte et al., 2007), potentially leading to a tropical Pacific natural coupled mode over decadal timescales (Choi et al., 2009).

Because of this complex interaction between mean state and ENSO at a wide range of frequencies, it is difficult to provide an unambiguous explanation for the impact of global warming on ENSO. As a matter of fact, the current generation of models exhibits a wide range of behaviours with regards to their sensitivity to global warming (Guilyardi et al., 2009). In particular, the evolution of tropical Pacific SST, especially the mean zonal equatorial gradient remains a noteworthy source of uncertainty (DiNezio et al., 2009).

Recently, Yeh et al. (2009) showed, with a group of IPCC models, that most consistent changes in ENSO characteristics due to global warming take place in the central equatorial Pacific (i.e. the Niño4 region (150°E-150°W; 5°S-5°N)). They showed that global warming leads to increased SST variability over the central tropical Pacific in the form of an increased occurrence of the so-called Modoki or Central Pacific (CP) El Niño (Ashok et al., 2007) relatively to the traditional so-called Eastern Pacific (EP) El Niño characterized by positive SST departure from normal in the Niño3 region (150°W-90°W; 5°S-5°N).

In this study, we further examine the relationships between changes in mean state and changes in ENSO properties, especially El Niño flavours and the importance of associated nonlinear mechanisms through an alternative statistical measure. These analyses are carried out on both reconstructed historical datasets and CGCMs outputs.

**2. Statistical diagnosis of ENSO nonlinearity**

Whereas ENSO nonlinearity has usually been considered through its asymmetric character (in particular as El Niño events are stronger and more numerous than La Niña's), recent studies indicate that ENSO also has a signature on higher-order statistical moments (Timmermann, 1999). This tends to emphasize the highly nonlinear nature of the tropical Pacific system and calls for the use of statistical tools that can effectively measure ENSO nonlinearity.

We propose here an alternative to classic Gaussian statistics, namely the □-stable laws, to explicitly consider 2 main features of the ENSO Probability Density Function (PDF) (Figure 1): the asymmetry and the weight of the distribution tail associated with warm strong El Niño episodes.

In brief, non-Gaussian α-stable laws, (heavy tailed laws) are characterized by four main parameters. The main ones, $0 < α ≤ 2$ and $-1 ≤ β ≤ 1$, respectively allow to diagnose the “non-Gaussian degree” (the PDF tails weight) and the asymmetry (~ skewness) of the set to be measured. Simply put, the analyses of these two parameters allow measuring the deviation of a distribution from a Normal law; this deviation being representative of the underlying nonlinear processes. When $α = 2$ and $β = 0$, the result is a Gaussian law. The lower $α$, the more α-stable the distribution and the more nonlinear the associated mechanisms. See Mandelbrot (1963) and Boucharel et al. (2009) for a more complete
description. It is noteworthy that the measurement of nonlinearity through the α parameter does not discriminate between nonlinear processes. Actually, the sources of nonlinearities of the tropical Pacific system remain numerous ranging from turbulent and ocean-mixing processes in the form of tropical instability waves for instance, to high frequency atmospheric forcing (An, 2009).

Koutrouvelis (1980) proposed a trustful regression method to estimate α-stable laws parameters.

<figure><img src="image_108.png" /><figcaption>Figure 1. Histogram of Niño3 SST anomalies from Kaplan et al. (1998) reconstructed dataset. The y-axis of red bars (events greater than standard deviation) has ten times increased scale. Normalized histogram I the upper left quadrant.</figcaption></figure>

**3. Inter-decadal modulation of ENSO nonlinearity (statistics).**

Over the inter-shift periods detected in Boucharel et al. (2009) (statistically confident ruptures detected in 1903, 1976 and 1998 through a bivariate test elaborated by Maronna and Yohai, 1978), the results of the estimation of ☐ and ☑ on distinct warm/cool periods (Figure 2) indicate that the ENSO statistics experienced significant changes. In particular, warm periods were characterised by stronger asymmetry and a greater deviation from Gaussianity (smaller α and ☐ ~ 1) whereas the cool period exhibited a Gaussian symmetrical pattern on average over the tropical Pacific (α ≈ 2 and ☑ ~ 0). A comparable tendency was found in the intermediate complexity Zebiak and Cane (ZC) model. In particular the ZC model had increased (reduced) nonlinearity quantified through nonlinear advection within the mixed layer (Nonlinear Dynamical Heating (NDH)) during warm (cool) periods (An and Jin, 2004; Boucharel et al., 2009). Consistently with recent studies, nonlinear dynamics is found to be responsible for rectification of ENSO variability through changes in ocean background. Although current measures of ENSO nonlinearities (through NDH) have provided meaningful information on the rectification of ENSO variability by changes in mean state, they may not fully account for the complexity of the rectified effect.
<figure><img src="image_109.png" /><figcaption>Figure 2. ☐ and ☑ parameters on distinct warm (1870-1903 and 1976-1998) and cool (1903-1976 and 1998-2009) inter-shifts periods estimated from Kaplan et al. (1998) reconstructed dataset. Confidence intervals for parameters estimation on Niño3 and Niño4 indices are provided below each plot.</figcaption></figure>

In order to test further the relationships between ENSO nonlinearity and the tropical Pacific mean state, we took
advantage of the CMIP3 Coupled General Circulation Models (CGCMs) database and the different scenario
experiments provided by the fourth version of the assessment report of the IPCC. We selected a set of models available
through the CMIP3-IPCC-AR4 data centre at the Program for Climate Model Diagnosis and Intercomparison (PCMDI)
and tested them in different configurations in terms of greenhouse gases concentration (respectively the Pre-Industrial
reference -PICTRL-, the doubling -2xCO2- and quadrupling -4xCO2- greenhouse gases concentration). In particular,
since the focus of the study deals with the diagnostic of high-order statistics and their response to changes in mean state,
we followed the classification established in Table 5 in Boucharel et al. (2009). The latter evaluates the performances of
the whole CMIP3 database in simulating irregular (nonlinear, ☐-stable) ENSO timeseries along with a noteworthy low
frequency modulation of the tropical Pacific mean state (revealed by the presence of climate shifts). Interestingly,
models able to represent a significant decadal to inter-decadal variability are also strongly nonlinear (with regards to the
α parameter analysis). These models (in bold in Table 5 in Boucharel et al. (2009)) will be the models of interest in the
present study. Interestingly, this “nonlinear” classification is consistent with other recent studies dedicated to
intercomparison (Belmadani et al., 2010 among others).
<figure><img src="image_110.png" /><figcaption>Figure 3. Changes in tropical Pacific statistics between different IPCC scenarios for the multi model ensemble mean (2xCO2 – PICTRL on left panels and 4xCO2 – PICTRL on right panels). Changes in: mean SST a) and b), RMS SST c) and d). Dots denote 95% statistical confidence level based on a Student's t-test. Changes in: □ parameter e) and f), □ parameter g) and h). Contours in e), f), g) and h) panels denote □ and □ parameters values for the multi models mean for the PICTRL experiment.</figcaption></figure>

Figure 3 displays the change in mean state for the model ensemble mean considered in this study for the 2xCO₂ and 4xCO₂ scenario. It consists of an El Niño-like pattern (i.e. maximum mean SST changes over the Cold Tongue region, Figure 3a). Interestingly, for the 4xCO₂ scenario, models simulate an enhanced equatorial warming (Figure 3b), rather than an El Niño-like warming. These changes in mean SST are associated with changes in ENSO variability as evidenced by Figures 3cd. Interestingly, the largest changes for these quantities are observed over the Warm Pool region similarly to the study by Yeh et al. (2009) and are less sensitive to the concentration of greenhouse gases, suggesting a nonlinear response of the tropical Pacific.

Figure 3 also displays changes in α and β parameters induced by global climate change under different projection scenarios for the multi-model ensemble mean. The pattern of maps for the β parameter (Figures 3fh) denotes increase (decrease) in negative (positive) asymmetry associated with global warming in the Warm Pool (Cold Tongue) region. Interestingly, for the α parameter, a peculiar pattern of changes associated with increased GHG concentrations emerges (Figure 3eg): it consists of a region of decreased α-stability (decrease in deviation from Gaussianity i.e. Δα>0) in the Cold Tongue region and a region of increased α-stability (Δα<0) in the Warm Pool. In the far western equatorial Pacific the rising concentration of GHG tends to favour a more Gaussian-type behaviour (Δα>0). The decrease in □ over the Warm Pool region indicates a tendency towards more EEs in a warmer climate over this region. See Boucharel et al. (2011) for more explanations and details about the statistical confidence (and consistency among models) of Figure 3efgh.

This is consistent with the recent results by Yeh et al. (2009) who diagnosed an increase of the CP to EP El Niño ratio in a warmer climate. The increase in negative asymmetry over the Warm Pool region also suggests a tendency towards more and/or "stronger" cold events (relative to a warmer climate mean state) consistently with Timmermann (1999).
This raises the question of whether the tendency towards more El Niño events over the Warm Pool region (new "Modoki flavour" of El Niño) is directly related to the tropical Pacific mean state change associated with global warming (seen as an external forcing) or whether it is associated with changes in nonlinearity as an intermediate step in the process. In the next section, we will explore the relationships between the different types of El Niño and tropical Pacific nonlinearity, as measured by the ☐ parameter.

## 4. El Niño flavours and tropical Pacific nonlinearity.

In this section, we investigate the extent to which the change in El Niño type under global warming can be related to the change in nonlinearity. Whereas Yeh et al. (2009) suggest that the increase of CP El Niño frequency in a warmer climate may be associated with changes in mean state (mean thermocline depth and slope), we hypothesize here that it is also related to changes in nonlinearity of the tropical Pacific system. It is noteworthy that timescale interactions (low frequency mean state changes versus ENSO variability) through nonlinear processes are subject to decadal or inter-decadal modulation (see the review by An (2009)) so that diagnostics of global warming on such processes are not straightforward.

In particular, although observational records suggest a tendency towards more frequent CP El Niño in recent years, this may hide fluctuations in statistics on decadal to inter-decadal timescales. For instance, Kug et al. (2010) found a clear phase relationship between the frequency of occurrence of CP El Niño and low frequency changes in mean state in the Warm Pool region in the GFDL-CM2.1 model (see their Figure 13). It is therefore important to verify whether such low frequency variability can also be found in ENSO nonlinearity and the extent to which it can be influenced by global greenhouse warming.

The simple detection test proposed by Yeh et al. (2009) for distinguishing a CP El Niño from an EP El Niño is applied to data and model outputs. However, instead of using a fixed threshold of +0.5°C for the ENSO indices (Niño3 and Niño4 averaged over the boreal winter months of December-January-February) to account for El Niño, we used a threshold that depends on model variability (see Boucharel et al., 2011).

For sake of clarity, we chose to present results for one model among the most sensitive to global warming (seen as a change in tropical SST mean state), i.e. the MRI-CGCM2.3.2.A.

<figure><img src="image_111.png" /><figcaption>Figure 4. Running α parameter (27-year running windows) in blue lines (right panels for Niño3 and left panels for Niño4W) and running El Niño types frequency in red lines (left panels for Central Pacific and right panels for Eastern Pacific)</figcaption></figure>

MRI-CGCM2.3.2.A PICTRL run (a. and b.)

MRI-CGCM2.3.2.A 2xCO2 run (c. and d.)
Figure 4 presents results for this model for the PICTRL and 2xCO₂ scenarios. While the occurrence of so-called Modoki events does not seem to be related to nonlinear processes in a Pre-Industrial climate (correlation of -0.10, Figure 4a.), EP El Niño and over the Niño3 region exhibits a clear out of phase relationship (-0.66, Figure 4b.). Interestingly, the picture is reversed for this particular model under anthropogenic warming hypothesis. The correlation between El Niño Modoki and Niño4 index nonlinearity increases up to -0.80 (Figure 4c.), while the relationship between EP El Niño and Δα over the Niño3 region falls to 0.16 (Figure 4d.). This relationship between Δα and El Niño “flavours” appears to be a robust feature of the Tropical Pacific coupled system as it is clearly simulated by the majority of the selected models (Table 3 of Boucharel et al., 2011).

This raises the question of the evolution of low frequency tropical Pacific coupled modes under global warming hypothesis. Actually, beyond the inter-decadal alternation of favourable EP and CP El Niño periods (e.g. see the out of phase relationships between the red curves in the upper panels of Figure 4), Yeh et al. (2009) and Boucharel et al. (2011) diagnosed a dominance of the “Modoki/CP” mode in a warmer climate. The present study highlights the possible significant role of nonlinear mechanisms in driving this low frequency modulation of El Niño flavours.

## 5. Summary and Discussion

In the present study, we examined the response of ENSO statistics, particularly the high-order statistical moments through an alternative statistical framework, to abrupt changes in the mean climate background. Reconstructed data for the 20th century exhibited a clear modulation of ENSO nonlinearity (deviation of its statistics from a Gaussian distribution) according to the mean state. In particular, warm (cool) periods tend to favour more (less) nonlinear processes. This sensitivity to changes in mean state was further investigated in the CMIP3 database, using different scenario experiments provided by IPCC-AR4 to characterize different mean state. We found drastic changes in ENSO nonlinearity patterns in a warmer climate. The activity of nonlinear mechanisms tends to be translated from the Cold Tongue region to the central equatorial Pacific under greenhouse climate hypothesis.

This is consistent with recent studies which indicate that global warming may have a significant impact on El Niño characteristics, leading in particular to an increased occurrence of the so-called Modoki El Niño located in the central Pacific (Yeh et al., 2009). There are many processes that may be responsible for such changes. They involve complex interactions between mean state, ENSO variability and nonlinearity.

We evidenced here a low frequency modulation of the different El Niño flavours, namely the EP and CP El Niño, both in a Pre-Industrial and in a warmer climate. We found that the EP flavour was directly related to the Eastern Pacific nonlinearity in the PICTRL experiments, whereas the CP flavour does not seem to be driven by the central/western pacific nonlinear mechanisms. Interestingly, the picture is reversed in a warmer climate. These findings provide further arguments to the existence of a tropical Pacific natural coupled mode oscillating on decadal to inter-decadal timescales.

Further study is required to diagnose which of the numerous nonlinear processes is more influential in this low frequency modulation.

## 5. Bibliography

* An, S.-I., and B. Wang, 2000: Interdecadal change of the structure of the ENSO mode and its impact on the ENSO frequency. J. Climate, 13, 2044-2055.

* An, S.-I., and F.-F. Jin, 2001: Collective role of thermocline and zonal advective feedbacks in the ENSO mode. J. Climate, 14, 3421-3432.

* An, S.-I., 2004: Interdecadal changes in the El Niño-La Niña asymmetry. Geophys. Res. Lett., 31:L23210, doi:101029/2004GL021299.

* An, S.-I., and F.-F. Jin, 2004: Nonlinearity and asymmetry of ENSO. J. Climate, 17, 2399-2412.

* An, S.-I., 2009: A review of interdecadal changes in the nonlinearity of the El Nino-Southern Oscillation. Theor. Appl. Climatol., 97, 29-40.

* Ashok, K., S.K. Behera, S.A. Rao, H. Weng, and T. Yamagata, 2007: El Niño Modoki and its possible teleconnection. J. Geophys. Res., 112, C11007.

* Belmadani, A., B. Dewitte, and S.-I. An, 2010: ENSO feedbacks and associated time scales of variability in a multi-model ensemble. J. Climate, accepted.

* Boucharel, J., B. Dewitte, B. Garel, and Y. du Penhoat, 2009: ENSO's non-stationary and non-Gaussian character: The role of climate shifts. Nonlin. Proc. Geophys., 16, 453-473.

* Boucharel, J., B. Dewitte, Y. du Penhoat, B. Garel, S.-W. Yeh, an J.-S Kug 2009: ENSO nonlinearity in a warming climate. In revision in Clim. Dynam.
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Dewitte, B., C. Cibot, C. Périgaud, S.-I. An, and L. Terray, 2007: Interaction between near-annual and ENSO modes in a CGCM simulation: Role of the equatorial background mean state. J. Climate, 20, 1035-1052.

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Observed freshening and warming of the western Pacific Warm Pool

Sophie Cravatte, Thierry Delcroix, Dongxiao Zhang, Michael McPhaden and Julie Leloup

Published in Climate Dynamics, 2009

Cravatte,S., T. Delcroix, D. Zhang, M. McPhaden and J. Leloup (2009), Observed Freshening of the warming Western
Tropical Pacific and extension of the Warm/Fresh Pool in recent decades, Clim Dyn, doi 10.1007/s00382-009-0526-7.

The Warm Pool (WP) of the western tropical Pacific Ocean is characterized by some of the warmest seawaters of the global ocean, with Sea Surface Temperatures (SST) warmer than 28-29°C. Reaching depths of 100 m, the WP covers a large surface area of about 159106 km2 and contains an important warm water volume. Its warm waters supply the atmosphere with water vapor and heat, releasing latent heat and leading to convection and heavy rainfall in excess of 2-3 m per year. This high precipitation rate, associated with light winds in the region, induces Sea Surface Salinity (SSS) lower than 35 pss, and results in the formation of a relatively low density and stable oceanic mixed layer.

At the equator, the Warm and low-salinity waters of the WP are separated from the cold and saltier waters of the central Pacific by a sharp salinity front. The position of the eastern edge of the Warm Pool in the equatorial band, associated with an oceanic convergence zone and a salinity front, is a key parameter for ENSO dynamics and ocean-atmosphere interactions.

<figure><img src="image_112.png" /><figcaption>Figure 1: Mean SST from WOA2000</figcaption></figure>

In a paper published in Climate Dynamics in 2009, S. Cravatte, T. Delcroix from IRD-LEGOS and coauthors from NOAA/PMEL and RSMAS used sea surface temperature and in situ sea surface salinity (SSS) data to address the central questions of whether Warm/Fresh Pool size, eastern extent and properties have changed in recent decades. Their results are striking: a significant surface freshening (up to 0.75 pss per 50 years) concurrent with an important surface warming (up to 1.2°C per 50 years, but typically from 0.2°C to 1°C per 50 years) was detected in the western tropical Pacific since 1955. Both warming and freshening are highly significant, since they reach at some locations five interannual standard deviations of the signal per 50 years. An increase in SSS is, in contrast, observed in regions of high evaporation, in the western Coral Sea and in the subtropical regions. Both geographical and seasonal SSS patterns are enhanced.
<figure><img src="image_113.png" /><figcaption>Figure 2 (@Climate Dynamics): Linear trends in SST (a) and SSS (b). Units are °C/50 years and pss/50 years. Positions of the 28.5°C isotherms (c) and of the 34.8 isohalines (d), averaged during 1956–1965 (black), 1966–1975 (red), 1976–1985 (green), 1986–1995 (blue) and 1996–2003 (light blue). The regions where the linear trends are not significant at the 90% confidence level are hatched in black</figcaption></figure>

In response to these warming and freshening trends, the Warm/Fresh Pool extended both latitudinally and longitudinally at the surface during the past few decades. Interestingly, the warmest and freshest waters expanded the most in relative terms. The eastern edge of the Warm/Fresh Pool migrated eastward along the equator, as indicated by displacements of isohalines and isotherms. Strong decadal variability of the same magnitude as the long-term trend leads to a stronger eastward displacement from 1955 to the mid-1990s, and to a rather negligible displacement from 1978 to 2003. In the Southwestern tropical Pacific, the salinity front located under the South Pacific Convergence Zone also migrated southward and eastward. At decadal timescales, its longitudinal movements are opposite to those of the equatorial front, as they are at interannual timescales. Long-term trends are in the same direction though, which emphasizes that they are not simply the result of decadal variability or increased interannual variability.

Forcing fields that could be used to tentatively explain the observed freshening are not reliable. Alternatively, the “wet get wetter and dry get drier” paradigm associated with quasi-uniform tropical SST warming can provide useful guidance for the interpretation of the long-term SSS trends. Increased heating will enhance evaporation, while increased air moisture due to increase in saturation vapour pressure with temperature will produce more intense precipitation. They use a very simple model to predict the SSS changes using Clausius–Clapeyron scaling. Their results suggest that the increase of the mean hydrological cycle predicted by a simple Clausius–Clapeyron scaling is already happening, consistent with other recent works.

Following many previous studies, the paper suggests that salinity is an important indicator of climate change in the tropics. However, in some regions, decadal variability appears greater or on the same order of magnitude than longer term trends.
<figure><img src="image_114.png" /><figcaption>Figure 3 (@Climate Dynamics): Left: Time series of the surface area covered by waters warmer than SST thresholds. Right: Time series of surface area covered by waters fresher than SSS thresholds.</figcaption></figure>
# Low-frequency variations of the large-scale ocean circulation and heat transport in the North Atlantic from 1955–2008 in situ temperature and salinity data

T. Huck, A. Colin de Verdière, P. Estrade, F. Gaillard, R. Schopp, P. Bellec, R. Dussin

Laboratoire de Physique des Océans (UMR 6523 CNRS IFREMER IRD UBO), Brest, France

## Abstract

On interdecadal timescales, the Atlantic meridional overturning circulation (AMOC) is thought to be in phase with the North Atlantic Sea Surface Temperatures (as measured by the Atlantic Multidecadal Oscillation – AMO – index). However, it appears that we have entered a positive phase of the AMO since 1995-2000 although we fear the Atlantic meridional overturning may be on a declining trend, as suggested by several observational and modelling studies. Here we constrain ocean models with temperature and salinity fields built on observations, and compare the results with various simple methods (namely diagnostic, robust diagnostic and prognostic), models (North Atlantic and global configurations at various resolutions), and forcings. Mean transports of heat and mass are sensitive to the method and model configuration, but their decadal variability is much more coherent and does not depend explicitly on the variations of the surface forcing, its influence being imprinted in the thermohaline structure. Multidecadal variations are of the order of 20% (0.15 PW in heat transport and 4 Sv in overturning), with large transports in the subpolar gyre in the early 1960's and mid 1990's, and low values in the mid 1970's. Declining transports of heat and mass are coherent in several models and methods since 1995, especially in the subpolar gyre, and opposite to the long term tendency from 1958 to 2008.

## 1. Introduction

Variations in the oceanic thermohaline structure have been documented over the last decades: surface intensified warming and changes in salinity, as well as deep water properties and formation rates [Dickson et al. 1996, 2002]. However the associated changes in the large-scale ocean circulation are poorly known, and deserve much interest in the context of the ongoing global warming and possible decay of the thermohaline circulation [Bryden et al. 2005; Gregory et al. 2005], or recent decline observed in the North Atlantic subpolar gyre [Häkkinen and Rhines 2004]. Several ocean models have been forced by atmospheric reanalysis forcings, but these forcings have significant uncertainties and well-known heterogeneities over the last 50 years. The main model deficiencies lie in formulation of subgrid-scale mixing with consequences on deep-water formation, usually impacting the overturning circulation on the long term. In situ data assimilation in such models on long time scales requires complex tools and delicate choices on the method, that largely influence the results. On the other hand, to avoid the need for accurate surface fluxes of heat and freshwater, one can use the observed temperature and salinity (TS) fields. Density providing the baroclinic velocities through the thermal wind relation, the barotropic part is obtained from the vorticity equation forced by the wind and a bottom pressure torque [Sarkisyan and Keonjiyan 1975]. Mellor et al. [1982] integrated this equation along f/H contours, whereas Holland and Hirschman [1972] used the dynamical part of numerical ocean models, although some adjustment of the bottom density field may be necessary [Ezer and Mellor 1994]. These methods have been applied to compare the pentads 1955–59 and 1970–74 [Greatbatch et al. 1991; Ezer et al. 1995], and more recently for 7 pentads from 1950 to 1994 using a finite element formulation [Myers et al. 2005]. NODC has made available global fields of TS pentadal anomalies from 1955–59 to 1994–98 based on hydrographic data. We will diagnose mean ocean currents from these fields to investigate the low-frequency variations of mass and heat transports in the North Atlantic. We first use three simple, well-documented methods: constant tracers, robust diagnostic, and short prognostic. Although the methods provide different results on the mean state, the low-frequency variations are rather coherent. Then, we implement only the robust diagnostic method in a global model continuous simulation with the seasonal cycle and TS anomaly fields updated to 2008.

## 2. First methodological step in a North Atlantic configuration with ROMS

The Regional Ocean Modeling System ROMS [Shchepetkin and McWilliams 2005] is used here, based on topography- following sigma coordinates. A smoothed bottom topography is required for accurate calculations of pressure gradients [Barnier et al. 1998 DSR]. We used a 1/2° resolution and 50 sigma levels to reproduce correctly the ocean bottom topography and capture the signature of the boundary currents in the TS climatologies. The model configuration spans from 10°N to 66°N in the Atlantic. The model is used to produce mean fields of T, S and velocities for each 5-yr period from 1955–59 to 1994–98. The initial TS fields were optimally interpolated on the model grid from the pentadal fields available on a 1°x1° grid and 33 z-levels. These pentadal fields were constructed from objectively analyzed anomalies of T and S down to 3000 m [Levitus et al. 2005; Boyer et al. 2005] and from the associated mean climatology (down to the bottom). Wind stress and surface fluxes are provided by the atmospheric reanalyses from NCEP [Kальнай et al. 1996
BAMS] and ECMWF ERA-40 [Uppala et al. 2005 QJRMS], averaged over the corresponding 5-yr periods. Three semi-diagnostic methods are implemented. Constant Tracers (hereafter CT): T and S are kept constant during the model integration, only the momentum equations are integrated in time and reach a steady state within months [Holland and Hirschman 1972]; the final velocity fields are averaged over months 6 to 12. Robust Diagnostic (RD): the tracer equations are now integrated in time with an additional relaxation to initial values with a timescale of 30 days [Sarmiento and Bryan 1982]; kinetic and potential energy adjusts within 6 months, and the final fields are averaged over the second year of integration. Short Prognostic (PR): the full dynamics and tracer equations are integrated for 45 days such that the barotropic velocities adjusts but the tracers do not drift away from the initial state [Ezer and Mellor 1994]; the final fields are averaged over the days 31 to 45. Rms differences between the initial and final TS fields are similar for both RD and PR methods (around 0.3 K at 100 m, 0.05 K at 1000 m and less than 0.01 K below 2000 m), although the former is in steady-state while the latter drifts rapidly from the initial state and longer prognostic integration would lead to much larger differences. Because the use of annual mean fields instead of seasonal cycle may be arguable, we have tested that the diagnostic transports of mass and heat on the annual mean climatology very closely resemble the mean of these diagnostics for the seasonal climatologies. Results are shown in Fig.1 for 1957-1996.

<figure><img src="image_115.png" /><figcaption>Figure 1: (top) Poleward heat transport maximum in the subpolar gyre (45-60°N) for the 3 diagnostic methods (RD robust diagnostic, CT constant tracers, SP short prognostic) implemented in a North Atlantic ROMS configuration using NODC pentadal TS anomalies and various forcings: ERA-40 (solid) or NCEP (dashed) 5-yr averaged surface fluxes, ERA-40 40-yr-averaged fields (dash-dotted); global prognostic reference simulation of the Drakkar project, ORCA025-G70, annual and pentadal means (green), (middle) Thermohaline circulation and (bottom) barotropic subpolar gyre intensity at 48°N, here for the RD method, show coherent variations with the poleward heat transport.
Source: Huck et al. (2008).</figcaption></figure>

### 3. Implementation of the robust diagnostic method in a global model configuration

Several limitations of the previous methods are addressed in this next step. First, to avoid open boundaries, a global model configuration is used with a ½° resolution, refined towards the equator: OPA ORCA05 [Molines et al. 2006]. The model integration is now continuous in time from 1958 to 2008. The surface forcing set is DFS4 [Brodeau et al. 2010] based on ERA-40 atmospheric reanalysis. The robust diagnostic method is implemented with relaxation time scale of 50 days in the upper 800m and 1 year in the deep ocean [Madec and Imbard 1996], although comparison with a uniform restoring field of 120 day shows no major change. The restoring TS fields, now varying in time and according to the seasonal cycle, are reconstructed from the annual or pentadal anomaly added to the monthly mean climatology. NODC pentadal fields are used for the period 1958 to 1996 as in the previous section. From 1997 to 2008, annual anomaly fields are used down to 2000m [von Schuckmann et al. 2009, Gaillard et al. 2009]. A twin prognostic experiment (ie. with no restoring in the ocean interior) is run for the same period. The restoring term efficiently removes incorrect trends in heat and salt content (not shown), and the evolutions of MOC and MHT diverge within 15 yr but show similar decadal variability (Fig. 2). Comparison is also performed with the Drakkar project reference experiment at ¼° resolution, ORCA025-G70 [Barnier et al. 2006]. In general, although the mean values of heat and mass transport
significantly vary from one model to the other, especially at 24°N, the decadal and longer variations show more coherence.

<figure><img src="image_116.png" /><figcaption>Figure 2: Synthesis of global ORCA05 simulations, prognostic (blue) and robust diagnostic (red), and comparison with Drakkar reference simulation (ORCA025-G70, green) for the evolution of the maximum meridional overturning circulation (MOC) and meridional heat transport (PHT) at 24°N (bottom) and 48°N (top) in the Atlantic. Thin (bold) lines are annual (pentadal) means. Dots correspond to average over the last 2 years for "equilibrium" 12-yr long simulations with restoring and DFS4 forcing of the year repeated (magenta). The decline of mass and heat transport at 48°N since 1995, opposite to the long term 1958-2008 tendency, appears as the most robust signal.</figcaption></figure>

**4. Discussion and conclusion**

This work provides an estimate of the low-frequency variability in the North Atlantic circulation based on in situ TS data using simple methods: diagnostic, robust-diagnostic and short prognostic. Without finely tuning the model configuration or parameterizations, the variability in mass and heat transports associated with the thermohaline changes has been successfully captured in the subpolar gyre, as compared to state-of-the-art prognostic models: energetic barotropic and overturning circulations drive high heat transport in the early 60's and mid 90's, whereas both circulations and heat transport are at the lowest in the mid 70's, and declining since 1995, in agreement with observational estimates attributed to NAO forcing [Curry and McCartney, 2001]. Our methods also point out an apparent phase opposition in heat transport between the subtropical and subpolar gyres, that could result from the delayed adjustment of the meridional overturning at lower latitude to the low-frequency NAO forcing [Eden and Jung 2001]. The original idea of relying on in situ observations rather than changes in the surface forcing to investigate the variations of the ocean circulation provides an alternative to prognostic hindcast models, with or without assimilation, that avoids potential model drift associated with uncertainties in both subgrid-scale processes parameterizations and surface fluxes. Yet let us recall that these are dynamical and not thermodynamical methods: they allow only limited insight in heat or salt budgets for instance. The pentadal/annual TS fields are certainly not perfectly constrained over the last decades, especially at depth, and due to the scarcity of salinity data: the robustness of our results should be investigated with the use of alternative products, analyzed on isopycnal surfaces for instance. The large smoothing in the NODC data set, as discussed by Myers et al. [2005], may also have some influence on our results, as well as the transition from NODC to LPO fields. A radical change occurred in the observing system since 2003 with Argo, that allows to build reliable annual fields of TS for the upper 2000m, and we have shown this is sufficient to reconstruct most of the large-scale circulation changes. See Huck et al. [2008] for a detailed analysis of the results.
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Ocean wave research in France, 2007-2010

Fabrice Ardhuin, Ifremer, Brest, France

Recent developments in the investigations of ocean waves are marked by their connections with other geophysical disciplines, from air-sea interactions and remote sensing to seismology, through coastal oceanography. Indeed the current wave of progress toward ever more realistic numerical models of sea states (e.g. WISE group 2007) has been based on the analysis of a wider range of observations from various types of instruments. Among these, the space-borne synthetic aperture radar data acquired in "wave mode" by the Advanced SAR on board ESA's Envisat mission, a legacy of the ERS 1 and 2 missions thanks to Klaus Hasselmann, has been used to measure the dissipation rates of trans-oceanic swells, with unprecedented accuracy (Ardhuin et al. 2009). The e-folding scales of swells was found to vary systematically with wave steepness and period, from 2000 to over 20000 km for wave periods in the range 13 to 18s. This measurement gave us the biggest missing piece in the energy balance of wave fields at global scale. A tentative theory has been proposed for the observed nonlinear dissipation rate, with stronger dissipation for steeper swells, which may reconcile observations of wave-driven winds and the persistence of swells at global scales. Yet a full understanding of the air-sea boundary layer is still missing.

Based on this observation, the parameterizations of wave dissipation processes have been re-established, with more accurate numerical wave models (Ardhuin et al. 2010). This accuracy was verified against measurements of wave heights, directional wave parameters spectral distributions which include some parameters relevant for many geophysical applications. In particular the mean square slopes of the sea surface can now be estimated with a relative accuracy of about 15% r.m.s., which thus includes the long wave contribution to the slopes. This opens new perspectives for the processing of remote sensing data, in particular for the sea surface salinity retrievals from the SMOS and Aquarius missions, for which the slopes induced by the long waves can produce deviations in the measured quantities (brightness temperatures) equivalent to a salinity change by up to 3 P.S.U. Likewise, these effort make it now possible to improve the sea state bias estimates for more accurate estimates of sea level using altimetry (Tran et al. 2010).

A more challenging area of application, because of the poor knowledge of the directional wave properties, is the estimation of seismic noise sources from ocean wave spectra. Ardhuin et al. (manuscript submitted to J. Geophys. Res.) have demonstrated the capability to reproduce observed seismic spectra with unprecedented accuracy, for most of the global seismic network stations (e.g. figure 1). Using the theory by Hasselmann (1963), this numerical model has revealed that seismic noise sources are strongest in the middle of ocean basins, although many land-based seismometers are mostly sensitive to coastal sources only. This determination of seismic source locations is an important step for better interpretations of seismic noise correlations (Shapiro and Campillo 2004). This seismic noise model was also used to refine estimates of sea states from seismic noise which should provide more accurate analyses of past wave climates, as already proposed by Bernard (1990) or Grevemeyer et al. (2000).

There has also been strong developments in the theoretical formulation of the interaction of waves, currents and turbulence (e.g. Ardhuin et al. 2008), which are now being implemented in three-dimensional primitive equation models for the investigation of nearshore flows with a wide variety of applications: from sediment dynamics to water quality or storm surges. In this area the parameterization of bottom friction and mixing will still require more work, while observations are still scarce and limited. The general improvement in our understanding of wave dynamics should not conceal the formidable questions that are still unanswered. These include the effect of breaking waves on fluxes across the ocean surface. This is probably the most important problem at hand, either for upper ocean mixing (e.g. Rascle and Ardhuin 2009) or for the production of marine aerosols which have a key role in the climate regulation by clouds. On this front we can mention the remarkable measurements by Reul et al. (2008) of modifications of the wind stress by breakers. These observations and others made internationally are feeding the development of the next generation of wave model parameterization, which should aim at predicting wave breaking statistics for many geophysical and other applications.
<figure><img src="image_117.png" /><figcaption>Figure 1: Observed and modelled ground motion caused by secondary microseisms in the North Atlantic, as recorded by two seismic stations. The model is described in Ardhuin et al. (submitted) using the latest improvements in numerical wave modelling. Earthquakes have been filtered out from the measured time series.</figcaption></figure>

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N. Reul, H. Branger, and J.-P. Giovanangeli, "Air flow structure over short-gravity breaking water waves," Boundary-Layer Meteorol., vol. 126, pp. 477-705, 2008.

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WISE Group: «Wave modelling - the state of the art». Progress in Oceanography Vol. 75, pp. 603-674, 2007.
Coastal patterns tracking: a lagrangian approach

Philippe FRAUNIE

Several investigations have been recently conducted in coastal areas in the framework of collaborative field campaigns granted and organized by research agencies and national oceanography administrations (INSU, SHOM, IFREMER) together with university laboratories.

Process oriented measurements and modeling have been performed concerning wind induced and baroclinic small scale features including vortices, filaments, fronts, alongshore current stability, internal tides and waves for better prediction of retention structures and confinement barriers.

In addition to high resolution RV cruises, satellite and HF radar synoptic mapping of the sea surface and vertical profilers and moorings, lagrangian measurements from drifting buoys and gliders have been extensively analysed. Especially, complementary deployment strategy allowed to validate each other measurements and models for high frequency patterns and sudden events both in tidal (bay of Biscay) and microtidal (Gulf of Lions, Adriatic sea) coastal environments. Figures 1 to 3 give examples of investigations in different coastal areas.

<figure><img src="image_118.png" /><figcaption>Fig 1. Drifted buoys trajectories in the bay of Biscay (EPIGRAM project, 2009-2012) (Charria et al) and Navidad event (southern warm waters inflow on the shelf) (Le Cann, Serpette et al)</figcaption></figure>

<figure><img src="image_119.png" /><figcaption>Fig 2. Anticyclonic vortices in the Gulf of Lions (LATEX and ECOLO experiments) ADCP, drifters and HF radar measurements) (Hu et al., 2010, Allou et al 2009)</figcaption></figure>
<figure><img src="image_120.png" /><figcaption>Fig. 3 Model-based directed drifter launches in the Adriatic Sea: Results from the DART experiment, Haza et al., 2007.</figcaption></figure>

New projects are concerning better understanding of coastal and internal waves from theoretical and high resolution long series measurements (Bouruet Aubertot et al, Zeitline et al).

**Selected references :**

Bay of Bisacy project :

Marsaleix P., Ulses C., Pairaud I., Herrmann M.J., Floor J.W., Estournel C., Auclair F., 2009. Open boundary conditions for internal gravity wave modelling using polarization relations. Ocean Modelling, 29, 27-42.

Le Hénaff M., De Mey P., Marsaleix P., 2009. Assessment of observational networks with the Representer Matrix Spectra method-application to a 3D coastal model of the Bay of Biscay. Ocean Dynamics, 59, 3-20

Philippe Grosso, Marc Le Menn, Jean-Louis De Bougrenet De La Tocnaye, Zong Van Wu, Damien Malarde, 2009. Deep-Sea Research 1 57 (2010) 151-156. Practical versus absolute salinity measurements: New advances in high performance seawater salinity sensors.

G. Reverdin, J. Boutin, N. Martin, A. Lourenco, P. Bouruet-Aubertot, A. Lavin, J. Mader, P. Blouch, J. Rolland, F. Gaillard, P. Lazure, 2010. accepted JAOT-O. Temperature measurements from surface drifters.

Michel S., Vandermeirsch F. and Lorance, P., 2009. Aquat. Living Resour., volume 22, 447-461. DOI: 10.1051/alr/2009054. Evolution of upper layer temperature in the Bay of Biscay during the last 40 years.

Bonneton, P., Bruneau, N., Marche, F. and Castelle, B. 2010. DCDS-S, in press. Large-scale vorticity generation due to dissipating waves in the surf zone.

Cienfuegos, R., Barthelemy, E. and Bonneton, P. 2010. J. Waterway, Port Coastal and Ocean Engrg, 136, 10-26. A wave-breaking model for Boussinesq-type equations including mass-induced effects.

Bruneau, N., Castelle, B., Bonneton, P., Pedreros, R., Almar, R., Bonneton, N., Bretel, P., Parisot, J-P. and Sénéchal, N. 2009. Continental Shelf Res, 29,1650–1662, doi:10.1016/j.csr.2009.05.005. Field observations of an evolving rip current on a meso-macrotidal well-developed inner bar and rip morphology.

Lannes D., Bonneton P. 2009. Physics of Fluids 21 (1), 016601 (9 pages), DOI: 10.1063/1.3053183. Derivation of asymptotic two-dimensional time-dependent equations for surface water wave propagation.
Ardhuin, F., B. Chapron, and F. Collard, 2009. Geophysical Research Letters, vol. 36, p. L06607. Strong decay of steep swells observed across oceans.

Ardhuin, F., L. Marié, N. Rascle, P. Forget, and A. Roland, 2009. Journal of Physical Oceanography, 39, 2820—2838. Observation and estimation of Lagrangian, Stokes and Eulerian currents induced by wind and waves at the sea surface.

Collard, F., Ardhuin, F., B. Chapron, 2009. Journal of Geophysical Research, 114, C07023. Routine monitoring and analysis of ocean swell fields using a spaceborne SAR.

V. Rossi, C. Lopez, E. Hernandez-Garcia, J. Sudre, V. Garcon, and Y. Morel, 2009. Nonlin. Processes Geophys., 16, 557–568. Surface mixing and biological activity in the four Eastern Boundary Upwelling.

Cécile Renaudie, Rémy Baraille, Yves Morel, Gwenaëlle Hello, Hervé Giordani, 2009. Ocean Modelling, 30, 178-189. Adaptation of the vertical resolution in the mixed layer for HYCOM.

V. Rossi, Y. Morel and V. Garçon, 2009. Ocean Modelling, doi: 0.1016/j.ocemod.2009.10.002. Effect of the wind on the dynamics of the shelf : formation of a secondary upwelling along the continental margin.

B. Le Cann and A. Serpette, 2009. Continental Shelf Research, doi:10.1016/j.csr.2008.11.015. Intense warm and saline upper ocean inflow in the southern Bay of Biscay in autumn-winter 2006-2007.

Le Boyer A., Cambon G., Daniault N., Herbette S., Le Cann B., Marié L., Morin P, 2009. Cont. Shelf. Res. (29) 1026-1037. Observations of the Ushant tidal front in september 2007

L. Marié, 2009. Nonlin. Processes Geophys. (17), 49-63, 2010. A study of the phase instability of quasi-geostrophic Rossby waves on the infinite beta-plane to zonal flow perturbations.

Rascle, N., F. Ardhuin, 2009. Journal of Geophysical Research, 114, C02016, doi:10.1029/2007JC00446. Drift and mixing under the ocean surface. Part 2: Stratified conditions and model-data comparisons.

Y. Morel and L. Thomas, 2009. Ocean Modelling, doi:10.1016/j.ocemod.2009.01.002. Ekman drift and vortical structures.

**Mediterranean sea projects :**

Hu, Z.H., Petrenko, A.A., Doglioli, A.M., Dekeyser, I. (2011), Numerical Study of eddy generation in the western part of the Gulf of Lion, JGR, submitted

Hu, Z.H., Petrenko, A.A., Doglioli, A.M., Dekeyser, I. (2010), Study of coastal eddies : application in the Gulf of Lion. J. Marine Syst., accepted.

Hu, Z.H., Doglioli, A.M., Petrenko, A.A., Marsaleix, P., Dekeyser, I. (2009), Numerical simulations of eddies in the Gulf of Lion. Ocean Model., Vol. 28/4, pp. 203-208, doi : 10.1016/j.ocemod.2009.02.004

Allou A., Forget P. and J.L. Devenon (2009) Submesoscale vortex structures at the entrance of the Gulf of Lions in the Northwestern Mediterranean Sea. Cont. Shelf Res., 30: 724-732

André G., Garreau P., Fraunié P. (2009) Mesoscale slope current variability in the Gulf of lions. Interpretation of in-situ measurements using a 3D model, Cont. Shelf Res. 29: 407-423

Casella E., Molcard A., Provenzale A. , Simulations of sub-mesoscale eddies in the Ligurian Sea. J. Mar. Syst., to appear.

Dufois F., Garreau P., Le Hir P., Forget P. (2008), Wave and current-induced bottom shear stress distribution in the Gulf of Lions. Continental Shelf Res., 28(15), pp.1920-1934.

Forget P. and G. André (2007), Can satellite-derived chlorophyll imagery be used to trace surface dynamics in coastal zone ? A case study in the Northwestern Mediterranean Sea. Sensors 7 884-904.

Forget, P., M. Saillard, and P. Broche, (2006) Observations of the sea surface by coherent L band radar at low grazing angles in a nearshore environment, J. Geophys. Res., 111 C09015, doi:10.1029/2005JC002900.

Haza A., Ozgokmen T.M., Griffa A., Molcard A., Poulain P.-M., Peggion G. (2010), Transport properties in small scale coastal flows : relative dispersion from VHF radar measurements in the Gulf of La Spezia, Ocean Dynam.,sous presse.

Haza, A. C., A. Griffa, P. Martin, A. Molcard, T. M. Ozgokmen, A. C. Poje, R. Barbanti, J. Book, P. M. Poulain, M. Rixen, P. Zanasca (2007), Model-based directed drifter launches in the Adriatic Sea: Results from the DART Experiment, Geophys. Res. Letters 34 L10605, doi:10.1029/2007GL029634

Jouon A., Douillet P., Ouillon S., Fraunié P. (2006), Calculations of hydrodynamic time parameters in a semi-opened coastal zone using a 3D hydrodynamical model, Cont. Shelf Res., 26, 1395-1415.
Langlais C., Barnier B., Molines J.M., Fraunié P., Jacob D., Kotlarski S. (2009) Evaluation of dynamically downscalled atmospheric reanalysis in the prospect of forcing long term simulations of the ocean circulation in the Gulf of Lions., Ocean Modelling 30: 270-286

Maps F, Plourde S, Zakardjian B (2010) Control of dormancy by lipid metabolism in Calanus finmarchicus: a population model test, Mar. Ecol. Progr. Ser. 403 165-180), doi:10.3354/meps08525

Molcard A., Poulain P.M., Forget P., Griffa A., Barbin Y., Gaggelli J., De Maistre J.C., Rixen M. (2009), Comparison between VHF radar observations and data from drifter clusters in the Gulf of La Spezia (Mediterranean Sea), J. Mar. Sys., 78:79-89./

Taillandier, V., A. Griffa, A. Molcard (2006) A variational approach for the reconstruction of regional scale Eulerian velocity fields from Lagrangian data, Ocean Modelling 13(1): 1-24

F. Bouchut and V. Zeitlin, A robust well-balanced scheme for multi-layer shallow water equations, 2010, Discr. and Cont. Dyn. Systems, 13, 739 – 758.

F. Bouchut, E. Scherer, and V. Zeitlin, Nonlinear adjustment of a front over escarpment, 2008, Phys. Fluids, 20, 016602-1 – 016602-12.

Bouruet-Aubertot, P., van Haren, H., Lelong, M.P. 2010 “Stratified inertial subrange in the boundary layer of Rockall channel”, J. Phys. Oceanogr., 40, 2401-2417.

Bouruet-Aubertot, P., Mercier, H., Gaillard, F., Lherminier, P. 2005. Evidence of strong inertia-gravity waves during POMME experiment, J. Geophys. Res., 110, doi 10-1029/2004JC002747

Ferrarri, R., Wunsch, C. 2010. The distribution of eddy kinetic and potential energies in the global ocean. Tellus, 62A, 92-108.

J. Gula, R. Plougonven, and V. Zeitlin, Ageostrophic instabilities of fronts in a channel in a stratified rotating fluid, 2009, J. Fluid. Mech., 627, 485 - 507.

J. Gula and V. Zeitlin, Instabilities of buoyancy-driven coastal currents and their nonlinear evolution in the two-layer rotating shallow water model. I. Passive lower layer, 2010, J. Fluid Mech. 659, 69 - 93.

J. Gula, V. Zeitlin and F. Bouchut, Instabilities of buoyancy-driven coastal currents and their nonlinear evolution in the two-layer rotating shallow water model. II. Active lower layer, 2010, J. Fluid Mech. – in press.

Komori, N., Ohfuchi, W., Taguchi, B., Sasaki H., Klein, P. 2008 « Deep ocean inertia-gravity waves simulated in a high-resolution global coupled atmosphere-ocean gcm » Geophys. Res. Lett., 35, 1897-1915.

Le Vaillant, X., Cuypers, Y., Bouruet-Aubertot, P., Vialard, J., McPhaden, M. 2010. «Generation and propagation of near-inertial baroclinic waves during Cirene experiment : energy fluxes and impact on vertical mixing», submitted to J. Geophys. Res.

Medvedev, S.B., Zeitlin, V. "Turbulence of near-inertial waves in the continuously stratified fluid" Phys. Letters A, v. 371, p.221-227

G. Reznik and V. Zeitlin, Resonant excitation of trapped waves by free inertia-gravity waves in coastal waveguide, and their nonlinear evolution, 2010 J. Fluid Mech. – accepted.

Scherer E. and V. Zeitlin, Stability and nonlinear evolution of coupled geostrophic density fronts, 2008, J. Fluid. Mech., 613, 309 - 327.

Winters, K., Bouruet-Aubertot, P., Gerkema, T. 2010. “Critical reflexion and abyssal trapping of near-inertial waves on a beta plane”, submitted to J. Fluid Mech., in revision

Zervakis, V., Levine, M.D. 1995. Near-inertial energy propagation from the mixed layer: theoretical considerations. J. Phys. Oceanogr., 25, 2872-2889.
# RAPPORT QUADRIENNAL DU CNFGG A L'UGGI
## QUADRENNIAL REPORT OF CNFGG TO IUGG

### SECTION 8 – SCIENCES DE
LA CRYOSPHERE

### SECTION 8 – CRYOSPHERIC
SCIENCES
<figure><img src="image_121.png" /></figure>

GPS measurements on Chhota Shigri,
India (PhotoP. Wagnon)

In this report are found a selection of papers related to cryosphere with an important
contribution of french scientists. The objective is to give a general view of the research
conducted in France in this domain. The period of interest in 2007-2010.

The main contributors are from the following main laboratories:

- Laboratoire de Glaciologie et Géophysique de l'Environnement, Grenoble
  (http://www-lgge obs ujf-grenoble fr/)

- Centre National de Recherche Météorologique, Météo-France, Toulouse et Grenoble
  (http://www.cnrm-game.fr/)

- Laboratoire des Sciences du Climat et de l'Environnement, Paris
  (http://www.lsce.ipsl.fr/)

- Laboratoire d'Etudes en Géophysique et Océanographie Spatiales, Toulouse
  (http://www.legosobs-mip.fr/)

- Laboratoire des Transferts en Hydrologie et Environnement, Grenoble
  (http://www.lthe.fr/LTHE/)

- Laboratoire d'Océanographie et du Climat, Paris (https://www.locean-ipsl.upmc.fr/)

- Hydrosciences, Montpellier (http://www.hydrosciences.org/)

- GIPSA-LAB, Grenoble (http://www.gipsa-lab.inpg.fr/)
Papers were divided in a few main categories as follows.

<table><thead><tr><th></th><th>Page</th></tr></thead><tbody><tr><td><strong>1 – SNOW</strong></td><td></td></tr><tr><td>1-1 Snow modelling (including snow-climate interactions)</td><td>3</td></tr><tr><td>1-2 Snow observation (including remote sensing).</td><td>12</td></tr><tr><td>1-3 Snow chemistry (including water isotopes)</td><td>16</td></tr><tr><td><strong>2 – GLACIERS</strong></td><td>22</td></tr><tr><td><strong>3 – SEA ICE</strong></td><td>33</td></tr><tr><td><strong>4 – ICE SHEETS</strong></td><td></td></tr><tr><td>4-1 Dynamics and modelling</td><td>37</td></tr><tr><td>4-2 Surface mass-balance</td><td>40</td></tr><tr><td><strong>5 – ICE CORES AND POLAR FIRN</strong></td><td></td></tr><tr><td>5-1 Ice cores</td><td>44</td></tr><tr><td>5-2 Polar firn</td><td>49</td></tr><tr><td><strong>6 – PERMAFROST</strong></td><td>51</td></tr></tbody></table>
**1-1 Snow modelling (including snow-climate interactions)**

**1-1-1 Main references with abstracts**

Lejeune, Y., P. Wagnon, L. Bouilloud, P. Chevallier, P. Etchevers, E. Martin, E. Sicart and F. Habets (2007) : **Melting of snow cover in a tropical mountain environment in Bolivia : Processes and modeling**. Journal of Hydrometeorology, 8(4), 922-937.

***Abstract :*** To determine the physical processes involved in the melting and disappearance of transient snow cover in nonglacierized tropical areas, the CROCUS snow model, interactions between Soil-Biosphere-Atmosphere (ISBA) land surface model, and coupled ISBA/CROCUS model have been applied to a full set of meteorological data recorded at 4795 m MSL on a moraine area in Bolivia (16°17'S, 68°32'W) between 14 May 2002 and 15 July 2003. The models have been adapted to tropical conditions, in particular the high level of incident solar radiation throughout the year. As long as a suitable function is included to represent the mosaic partitioning of the surface between snow cover and bare ground and local fresh snow grain type (as graupel) is adapted, the ISBA and ISBA/CROCUS models can accurately simulate snow behavior over nonglacierized natural surfaces in the Tropics. Incident solar radiation is responsible for efficient melting of the snow surface (favored by fresh snow albedo values usually not exceeding 0.8) and also for the energy stored in snow-free areas (albedo = 0.18) and transferred horizontally to adjacent snow patches. These horizontal energy transfers (by conduction within the upper soil layers and by turbulent advection) explain most of the snowmelt and prevent the snow cover from lasting more than a few days during the wet season in this high-altitude tropical environment.

Brzoska, J.B., F. Flin and J. Barckicke (2008) : **Explicit iterative computation of diffusive vapour field in the 3-D snow matrix : some preliminary results for low flux metamorphism**. Annals of Glaciology, **48**, 13-18.

***Abstract :*** The metamorphism of seasonal snow is classically considered as limited by vapour diffusion in the pore phase. To account for the lack of knowledge of the ice-vapour reaction coefficient near 0°C, the assumption of a reaction-limited metamorphism was first tested in three-dimensional simulations at low and very low temperature gradients; however, the validity of such results is difficult to verify experimentally. By a reasoned use of traditional iterative schemes, vapour diffusion is now simulated in three dimensions on tomographic snow data, mapping the gradient of vapour pressure near the grains. Repeating this process may provide a way to simulate the isothermal metamorphism without grain packing at a reasonable expense of computation time. Preliminary results are compared with existing computations made within the reaction-limited hypothesis.

Flin, F. and J.B. Brzoska (2008) : **The temperature gradient metamorphism of snow: vapour diffusion model and application to tomographic images**. Annals of Glaciology, vol. **49**, pp 17-21.

***Abstract :*** A simple physical model describing the temperature-gradient metamorphism of snow is presented. This model, based on Kelvin's equation and Fick's law, takes into account the local variation of the saturating vapour pressure
with temperature. It can determine locally whether the ice is condensing or subliming, depending on both the pressure and temperature fields in the snow structure. This model can also explain the formation of facets that occurs during the metamorphism. Using X-ray microtomographic images of snow samples obtained under low to moderate temperature-gradient conditions, this model has been tested and compared to the reaction-limited model proposed in a previous work.

Bouilloud, L.; Martin, E.; Habets, F.; Boone, A.; Moigne, P. L.; Livet, J.; Marchetti, M.; Foidart, A.; Franchistéguy, L.; Morel, S.; Noilhan, J. and Pettré, P. (2009), **Road Surface Condition Forecasting in France**, Journal of Applied Meteorology and Climatology, 48, 2513-2527.

***Abstract :*** A numerical model designed to simulate the evolution of a snow layer on a road surface was forced by meteorological forecasts so as to assess its potential for use within an operational suite for road management in winter. The suite is intended for use throughout France, even in areas where no observations of surface conditions are available. It relies on short-term meteorological forecasts and long-term simulations of surface conditions using spatialized meteorological data to provide the initial conditions. The prediction of road surface conditions (road surface temperature and presence of snow on the road) was tested at an experimental site using data from a comprehensive experimental field campaign. The results were satisfactory, with detection of the majority of snow and negative road surface temperature events. The model was then extended to all of France with an 8-km grid resolution, using forcing data from a real-time meteorological analysis system. Many events with snow on the roads were simulated for the 2004/05 winter. Results for road surface temperature were checked against road station data from several highways, and results for the presence of snow on the road were checked against measurements from the Météo-France weather station network.

Durand, Y., G. Giraud, M. Laternser, P. Etchevers, L. Mérindol and B. Lesaffre (2009) : **Reanalysis of 47 years of climate in the French Alps (1958 – 2005): climatology and trends for snow cover.** Journal of Applied Meteorology and Climatology, 48(12), 2487-2512.

***Abstract :*** Since the early 1990s, Météo-France has used an automatic system combining three numerical models to simulate meteorological parameters, snow cover stratification, and avalanche risk at various altitudes, aspects, and slopes for a number of mountainous regions in France. Given the lack of sufficient directly observed long-term snow data, this “SAFRAN”-Crocus-“MEPRA” (SCM) model chain, usually applied to operational avalanche forecasting, has been used to carry out and validate retrospective snow and weather climate analyses for the 1958–2002 period. The SAFRAN 2-m air temperature and precipitation climatology shows that the climate of the French Alps is temperate and is mainly determined by atmospheric westerly flow conditions. Vertical profiles of temperature and precipitation averaged over the whole period for altitudes up to 3000 m MSL show a relatively linear variation with altitude for different mountain areas with no constraint of that kind imposed by the analysis scheme itself. Over the observation period 1958–2002, the overall trend corresponds to an increase in the annual near-surface air temperature of about 1°C. However, variations are large at different altitudes and for different seasons and regions. This significantly positive trend is most obvious in the 1500–2000-m MSL altitude range, especially in the northwest regions, and exhibits a significant relationship with the North Atlantic Oscillation index over long periods.
Precipitation data are diverse, making it hard to identify clear trends within the high year-to-year variability.

Langlois, A., L. Brucker, J. Kohn, A. Royer, C. Derksen, P. Cliche, G. Picard, M. Fily and J.M. Willemet (2009) : **Simulation of Snow Water Equivalent (SWE) using Thermodynamic Snow Models in southern Québec between 2005 and 2006.** Journal of Hydrometeorology, 10(6), 1447-1462.

**Abstract:** Snow cover plays a key role in the climate system by influencing the transfer of energy and mass between the soil and the atmosphere. In particular, snow water equivalent (SWE) is of primary importance for climatological and hydrological processes and is a good indicator of climate variability and change. Efforts to quantify SWE over land from spaceborne passive microwave measurements have been conducted since the 1980s but a more suitable method has yet to be developed for hemispheric-scale studies, and tools such as snow thermodynamic models allow a better understanding of the snow cover and can potentially significantly improve existing snow products at the regional scale. In this study, the use of three snow models (SNOWPACK, CROCUS and SNTHERM) driven by local and reanalysis meteorological data for the simulation of SWE is investigated temporally through three winter seasons and spatially over intensively sampled sites across Northern Québec. Results show that the SWE simulations are in agreement with ground measurements through three complete winter seasons (2004–2005–2005–2006 and 2007–2008) in southern Québec, with higher error for 2007–2008. The correlation coefficients between measured and predicted SWE values ranged between 0.72 and 0.99 for the three models and three seasons evaluated in southern Québec. In subarctic regions, predicted SWE driven with the North American Regional Reanalysis (NARR) data fall within the range of measured regional variability. NARR data allow snow models to be used regionally, and this paper represents a first step for the regionalization of thermodynamic multi-layered snow models driven by reanalysis data for improved global SWE evolution retrievals.

Rutter N. et al. (2009), **Evaluation of forest snow processes models (SnowMIP2)**, J. Geophys. Res., 114, D06111, doi:10.1029/2008JD011063.

**Abstract:** Thirty-three snowpack models of varying complexity and purpose were evaluated across a wide range of hydrometeorological and forest canopy conditions at five Northern Hemisphere locations, for up to two winter snow seasons. Modeled estimates of snow water equivalent (SWE) or depth were compared to observations at forest and open sites at each location. Precipitation phase and duration of above-freezing air temperatures are shown to be major influences on divergence and convergence of modeled estimates of the subcanopy snowpack. When models are considered collectively at all locations, comparisons with observations show that it is harder to model SWE at forested sites than open sites. There is no universal ‘best’ model for all sites or locations, but comparison of the consistency of individual model performances relative to one another at different sites shows that there is less consistency at forest sites than open sites, and even less consistency between forest and open sites in the same year. A good performance by a model at a forest site is therefore unlikely to mean a good model performance by the same model at an open site (and vice versa). Calibration of models at forest sites provides lower errors than uncalibrated models at three out of four locations. However, benefits of calibration do not translate to subsequent years, and benefits gained by models calibrated for forest snow processes are not translated to open conditions.
Wagnon, P., M. Lafaysse, Y. Lejeune, L. Maisincho, M. Rojas, and J. P. Chazarin (2009) : **Understanding and modeling the physical processes that govern the melting of snow cover in a tropical mountain environment in Ecuador**. Journal of Geophysical Research, 114, D19113.

**Abstract:** The ISBA/CROCUS coupled ground-snow model developed for the Alps and subsequently adapted to the outer tropical conditions of Bolivia has been applied to a full set of meteorological data recorded at 4860 m above sea level on a moraine area in Ecuador (Antizana 15 glacier, 0°28'S; 78°09'W) between 16 June 2005 and 30 June 2006 to determine the physical processes involved in the melting and disappearance of transient snow cover in nonglaciated areas of the inner tropics. Although less accurate than in Bolivia, the model is still able to simulate snow behavior over nonglaciated natural surfaces, as long as the modeled turbulent fluxes over bare ground are reduced and a suitable function is included to represent the partitioning of the surface between bare soil and snow cover. The main difference between the two tropical sites is the wind velocity, which is more than 3 times higher at the Antizana site than at the Bolivian site, leading to a nonuniform spatial distribution of snow over nonglaciated areas that is hard to describe with a simple snow partitioning function. Net solar radiation dominates the surface energy balance and is responsible for the energy stored in snow-free areas (albedo = 0.05) and transferred horizontally to adjacent snow patches by conduction within the upper soil layers and by turbulent advection. These processes can prevent the snow cover from lasting more than a few hours or a few days. Sporadically, and at any time of the year, this inner tropical site, much wetter than the outer tropics, experiences heavy snowfalls, covering all the moraine area, and thus limiting horizontal transfers and inducing a significant time lag between precipitation events and runoff.

Céron, J.-P., Tanguy, G., Franchistéguy, L., Martin, E., Regimbeau, F. and Vidal, J.-P. (2010), **Hydrological seasonal forecast over France: feasibility and prospects**, Atmospheric Science Letters, 11, 78-82

**Abstract:** This article presents a first evaluation of a hydrological forecasting suite at seasonal time scales over France. The hydrometeorological model SAFRAN-ISBA-MODCOU is forced by seasonal forecasts from the DEMETER project for the March-April-May period. Despite a simple downscaling method, the atmospheric forcings are reasonably well represented at the finest scale. The computed soil moisture shows some predictability with large regions of correlation above 0.3. Probabilistic scores for soil moisture and river flows for four different catchments are higher than that for atmospheric variables. These results suggest to go further for building an operational hydrological seasonal forecast system.

Douville H. (2010) **Relative contributions of soil and snow hydrology to seasonal climate predictability: a pilot study**. Climate Dyn., 34, 797-818, doi: 10.1007/s00382-008-0508-1.

**Abstract:** Land surface hydrology (LSH) is a potential source of long-range atmospheric predictability that has received less attention than sea surface temperature (SST). In this study, we carry out ensemble atmospheric simulations driven by observed or climatological SST in which the LSH is either interactive or nudged towards a global monthly re-analysis. The main objective is to evaluate the impact of soil moisture or snow mass anomalies on seasonal climate variability and predictability over the 1986-1995 period. We first analyse the annual cycle of zonal mean potential (perfect model approach) and effective (simulated vs. observed
climate) predictability in order to identify the seasons and latitudes where land surface initialization is potentially relevant. Results highlight the influence of soil moisture boundary conditions in the summer mid-latitudes and the role of snow boundary conditions in the northern high latitudes. Then, we focus on the Eurasian continent and we contrast seasons with opposite land surface anomalies. In addition to the nudged experiments, we conduct ensembles of seasonal hindcasts in which the relaxation is switched off at the end of spring or winter in order to evaluate the impact of soil moisture or snow mass initialization. LSH appears as an effective source of surface air temperature and precipitation predictability over Eurasia (as well as North America), at least as important as SST in spring and summer. Cloud feedbacks and large-scale dynamics contribute to amplify the regional temperature response, which is however, mainly found at the lowest model levels and only represents a small fraction of the observed variability in the upper troposphere.

Eckert, N., C. Coleou, H. Castebrunet, M. Dechatres, G. Giraud and J. Gaume (2010), **Cross-comparison of meteorological and avalanche data for characterising avalanche cycles : The example of December 2008 in the eastern part of the French Alps**. Cold Regions Science and Technology, 64(2), 119-136.

***Abstract :*** In December 2008, an intense avalanche cycle occurred in the eastern part of the southern French Alps. Southerly atmospheric fluxes that progressively evolved into an easterly return caused important snowfalls with return periods up to 10 years. Cold temperatures and drifting snow had important aggravating effects. The return period for the number of avalanches was above 50 years in two massifs and some of the avalanche had very long runouts that exceeded historical limits recorded in the French avalanche atlas. Using this case study, this paper illustrates and discusses how avalanche reports, snow and weather data and results from numerical modelling of the snow cover can be combined to analyse abnormal temporal clusters of snow avalanches. For instance, it is shown how statistical techniques developed in other fields can be used to test the significance of different explanatory factors, extract spatio-temporal patterns, compare them with previous cycles and quantify the magnitude/frequency relationship at different scales.

Peings Y., H. Douville (2010) **Influence of the Eurasian snow cover on the Indian summer monsoon variability in observations and CMIP3 simulations**. Climate Dyn., 34, 643-660, doi:10.1007/s00382-009-0565-0.

***Abstract :*** The present study is aimed at revisiting the possible influence of the winter/spring Eurasian snow cover on the subsequent Indian summer precipitation using several statistical tools including a maximum covariance analysis. The snow-monsoon relationship is explored using both satellite observations of snow cover and in situ measurements of snow depth, but also a subset of global coupled ocean-atmosphere simulations from the phase 3 of the Coupled Model Intercomparison Project (CMIP3) database. In keeping with former studies, the observations suggest a link between an east-west snow dipole over Eurasia and the Indian summer monsoon precipitation. However, our results indicate that this relationship is neither statistically significant nor stationary over the last 40 years. Moreover, the strongest signal appears over eastern Eurasia and is not consistent with the Blanford hypothesis whereby more snow should lead to a weaker monsoon. The twentieth century CMIP3 simulations provide longer timeseries to look for robust snow-monsoon relationships. The maximum covariance analysis indicates that some models do show an apparent influence of the Eurasian snow cover on the Indian summer monsoon precipitation,
but the patterns are not the same as in the observations. Moreover, the apparent
snow-monsoon relationship generally denotes a too strong El Niño-Southern
Oscillation teleconnection with both winter snow cover and summer monsoon rainfall
rather than a direct influence of the Eurasian snow cover on the Indian monsoon.

Peings Y., H. Douville, R. Alkama, B. Decharme (2010) **Snow contribution to springtime atmospheric predictability over the second half of the twentieth century. Climate Dyn.**, in press, doi: 10.1007/s00382010-0884-1.

***Abstract:*** A set of global atmospheric simulations has been performed with the ARPEGE-Climat model in order to quantify the contribution of realistic snow conditions to seasonal atmospheric predictability in addition to that of a perfect sea surface temperature (SST) forcing. The focus is on the springtime boreal hemisphere where the combination of a significant snow cover variability and an increasing solar radiation favour the potential snow influence on the surface energy budget. The study covers the whole 1950-2000 period through the use of an original snow mass reanalysis based on an off-line land surface model and possibly constrained by satellite snow cover observations. Two ensembles of 10-member AMIP-type experiments have been first performed with relaxed versus free snow boundary conditions. The nudging towards the monthly snow mass reanalysis significantly improves both potential and actual predictability of springtime surface air temperature over Central Europe and North America. Yet, the impact is confined to the lower troposphere and there is no clear improvement in the predictability of the large-scale atmospheric circulation. Further constraining the prescribed snow boundary conditions with satellite observations does not change much the results. Finally, using the snow reanalysis only for initializing the model on March 1st also leads to a positive impact on predicted low-level temperatures but with a weaker amplitude and persistence. A conditional skill approach as well as some selected case studies provide some guidelines for interpreting these results and suggest that an underestimated snow cover variability and a misrepresentation of ENSO teleconnections may hamper the benefit of an improved snow initialization in the ARPEGE-Climat model.

Rabier, F. et al. (2010), **The CONCORDIASI project in Antarctica**. Bull. Amer. Meteor. Soc.,91, 69-86.

***Abstract:*** The Concordiasi project is making innovative observations of the atmosphere above Antarctica. The most important goals of the Concordiasi are as follows:

1. To enhance the accuracy of weather prediction and climate records in Antarctica through the assimilation of in situ and satellite data, with an emphasis on data provided by hyperspectral infrared sounders. The focus is on clouds, precipitation, and the mass budget of the ice sheets. The improvements in dynamical model analyses and forecasts are used in chemical-transport models that describe the links between the polar vortex dynamics and ozone depletion, and to advance the understanding of the Earth system by examining the interactions between Antarctica and lower latitudes.

2. To improve our understanding of microphysical and dynamical processes controlling the polar ozone, by providing the first quasi-Lagrangian observations of stratospheric ozone and particles, in addition to an improved characterization of the 3D polar vortex dynamics. Techniques for assimilating these Lagrangian observations are being developed.
A major Concordiasi component is a field experiment during the austral springs of 2008–10. The field activities in 2010 are based on a constellation of 19 long-duration stratospheric super-pressure balloons (SPBs) deployed from the McMurdo station. Six of these balloons carried GPS receivers and in situ instruments measuring temperature, pressure, ozone, and particles. Thirteen of the balloons released more than 600 drop-sondes on demand for measuring atmospheric parameters. Lastly, radiosounding measurements were collected at various sites, including the Concordia station. It is shown in the accompanying Figure that the appropriate use of IASI data in retrievals allowed the analysis to retrieve surface temperature at Concordia, in agreement with in situ measurements.

Rousselot, M., Y. Durand, G. Giraud, L. Mérindol, L. Daniel (2010) : **Analysis and forecast of extreme new-snow avalanches : a numerical study of the avalanche cycle of February 1999 in France**. Journal of Glaciology, 56 (199), 758-770.

**Abstract** : Snow and weather conditions typical of exceptional cycles of fresh-snow avalanches in the northern Alps are investigated using the numerical avalanche-hazard forecasting procedure of Météo-France. Sensitivity tests are performed on the events of February 1999 in the Chamonix France region, and resulting snowpack instability modeled at the massif scale is compared using adapted new indices and maps. Our results complete conclusions of earlier observation-based studies by providing new insights into the snow and weather conditions of February 1999. The large avalanches mainly resulted from large and very unstable fresh-snow accumulations. Moreover, the snowpack instability was increased locally by wind transport of light and fresh snow in February. The mechanical weaknesses resulting from the weather conditions prior to February were a key factor in explaining the unusual volumes of these avalanches. This study suggests that the operational numerical SAFRAN/Crocus/ME'PRA (SCM) chain provides reliable forecasts of extreme new-snow avalanche situations at the massif scale, but that local-scale simulations are still needed to improve the efficiency of risk mitigation and civil protection policies.

Jacobi, H.-W., F. Domine, W.R. Simpson, T.A. Douglas, and M. Sturm, **Simulation of the specific surface area of snow using a one-dimensional physical snowpack model: Implementation and evaluation for subarctic snow in Alaska**, *The Cryosphere* 4, 35-51, 2010.

**Abstract** : The specific surface area (SSA) of the snow constitutes a powerful parameter to quantify the exchange of matter and energy between the snow and the atmosphere. However, currently no snow physics model can simulate the SSA. Therefore, two different types of empirical parameterizations of the specific surface area (SSA) of snow are implemented into the existing one-dimensional snow physics model CROCUS. The parameterizations are either based on diagnostic equations relating the SSA to parameters like snow type and density or on prognostic equations that describe the change of SSA depending on snow age, snowpack temperature, and the temperature gradient within the snowpack. Simulations with the upgraded CROCUS model were performed for a subarctic snowpack, for which an extensive data set including SSA measurements is available at Fairbanks, Alaska for the winter season 2003/2004. While a reasonable agreement between simulated and observed SSA values is obtained using both parameterizations, the model tends to overestimate the SSA. This overestimation is more pronounced using the diagnostic equations compared to the results of the prognostic equations. Parts of the SSA deviations using both parameterizations can be attributed to
differences between simulated and observed snow heights, densities, and temperatures. Therefore, further sensitivity studies regarding the thermal budget of the snowpack were performed. They revealed that reducing the thermal conductivity of the snow or increasing the turbulent fluxes at the snow surfaces leads to a slight improvement of the simulated thermal budget of the snowpack compared to the observations. However, their impact on further simulated parameters like snow height and SSA remains small. Including additional physical processes in the snow model may have the potential to advance the simulations of the thermal budget of the snowpack and, thus, the SSA simulations.

Domine, F., A. S. Taillandier, and W. R. Simpson (2007) : **A parameterization of the specific surface area of seasonal snow for field use and for models of snowpack evolution.** Journal of Geophysical Research-Earth Surface, 112(F2), F02031.

**Abstract:** The specific surface area (SSA) of snow is needed to model air-snow exchange of chemical species. SSA is related to many snow physical properties, such as albedo and permeability. However, it is not described in models of snowpack evolution, in part because it is difficult to measure. Snowpack models often predict snow grain shape and snow density, and the goal of this paper is to propose parameterizations of snow SSA, based on snow density and grain shape. SSA values of 345 snow samples from snowpacks of the Alpine, maritime, tundra and taiga types are presented. Samples are regrouped into three main types: fresh (F), recent (R), and aged (A) snows, with several subtypes referring to grain shapes. Overall, there is a clear inverse correlation between SSA and density, d. Empirical equations of the form SSA = A ln(d) + B are proposed for the F and R types. For aged snows, separate correlations are proposed for subtypes A1 (rounded grains), A2 (faceted crystals), A3 (depth hoar), and A4 (lightly melted snow). Within subtypes A1, A2, and A3, more elaborate classifications are made by considering the snowpack type (Alpine, taiga, or tundra). For A1, A2, and A3 types, different trends are related to different intensities of wind action, which increases in the order taiga, Alpine, and tundra. We finally propose three parameterizations of snow SSA with increasing sophistication, by correlating SSA to snow type, then to snow type and density, and finally to snow type, density, and snowpack type.

Domine, F., A.-S. Taillandier, A. Cabanes, T. A. Douglas, and M. Sturm (2009) : **Three examples where the specific surface area of snow increased over time.** The Cryosphere, 3(1), 31-39.

**Abstract:** Snow on the ground impacts climate through its high albedo and affects atmospheric composition through its ability to adsorb chemical compounds. The quantification of these effects requires the knowledge of the specific surface area (SSA) of snow and its rate of change. All relevant studies indicate that snow SSA decreases over time. Here, we report for the first time three cases where the SSA of snow increased over time. These are (1) the transformation of a melt-freeze crust into depth hoar, producing an increase in SSA from 3.4 to 8.8 m² kg⁻¹. (2) The mobilization of surface snow by wind, which reduced the size of snow crystals by sublimation and fragmented them. This formed a surface snow layer with a SSA of 61 m² kg⁻¹ from layers whose SSAs were originally 42 and 50 m² kg⁻¹. (3) The sieving of blowing snow by a snow layer, which allowed the smallest crystals to penetrate into open spaces in the snow, leading to an SSA increase from 32 to 61 m² kg⁻¹. We discuss that other mechanisms for SSA increase are possible. Overall, SSA increases are probably not rare. They lead to enhanced uptake of chemical compounds and to