precipitation point of view, yielding anomalous water balances (Valéry et al., 2010). Second, because of the seasonal presence of snow, hydrological models require an additional component (i.e. a snow accounting routine), which must also be parameterized.

In France, during the PUB decade, several parallel studies were designed to improve our spatial knowledge of precipitation over mountains: Météo-France kept improving the SAFRAN reanalysis (Quintana-Segui et al., 2008); Gottardi (2009) developed an improved interpolation approach called SPAZM over the main mountains; and Valéry (2010) also developed a scheme for interpolating precipitation and temperature based on elevation. Several studies used streamflow as an indicator that could be inverted to yield information on precipitation at high altitudes, as well as altitudinal gradients (Gottardi, 2009; Le Moine et al., 2013; Valéry et al., 2009).

As far as snow accounting routines are concerned, the study reported by Valéry et al. (2014) has shown that although they gain in being calibrated, they could also be applied with regional default parameters in a PUB perspective. Research at Météo-France on a more physically based approach (Boone, 2000) also has led to calibration-free snow accounting routines.

# 4 Conclusion

## 4.1 Boosting effect of PUB and future challenges

The PUB IAHS scientific decade has been very successful in setting a common agenda for hydrologists worldwide (Blöschl et al., 2013; Hrachowitz et al., 2013; Pomeroy et al., 2013), including France, as shown in this review. It helped frame local issues and study cases within a generic perspective, exchanging concepts and methods in a synchronized global effort and initiating comparative hydrology and international datasets. It has also reinforced the science-practice interface. Of course, benefitting from the forces of scientific progress and being aware of the wide variability of the processes and objects studied within the hydrology discipline across the world and across geographic scales, a number of studies reviewed under the PUB umbrella have been developed independently and would have been developed without this framework. By its agenda-setting nature, the PUB initiative has on some occasions been a priori provocative and is occasionally a posteriori synthesizing. This review within the French perspective is a contribution to this combined effort.

Although we have reached the end of the PUB decade, and although much progress has been made on issues relevant to ungauged basins, many challenges still lay ahead. Extending the approaches which have mainly concerned quantitative hydrology to water quality is a first formidable challenge. Better linking the specialities, which often tend to evolve towards distinct sciences (hydrology vs hydrogeology, water quantity vs water quality), is also an open gateway.

## 4.2 Perspectives for Panta Rhei

The IAHS Panta Rhei initiative and science plan were shaped in 2013 (Montanari et al., 2013) after a wide e-consultation of the hydrological community. Scientific questions raised therein are the science front of the major part of the hydrological discipline, in both its theoretical and practical aspects. They are also strongly linked to major issues of human security, water
variability and complex connex issues representing a wide range of threats and uncertainties, as acknowledged in Science-United Nations dialogues (Cudennec et al., 2015).

We believe that several follow-ups and lessons from the work and achievements reviewed in this article within the PUB perspective can be explored and valued to address the following questions that fit in the Panta Rhei agenda:

- What are the extrapolation capacities of models in changing conditions? They were analysed in space within the PUB framework and should be analysed along the retrospective and prospective time horizons within the Panta Rhei framework (Vaze et al., 2015);

- How can we design reference observation networks addressing space-time variabilities? (see e.g. Giuntoli and Renard, 2009)

- How can we study the co-evolution of hydrology, climate and society, with complex causal, temporal, scaling, additive and feedback effects?

- What are the key links of hydrological knowledge with other critical links in space and time, such as land, food, energy and biodiversity, as currently being addressed by the debate on the post-2015 Sustainable Development Goals? (https://sustainabledevelopment.un.org/topics/sustainabledevelopmentgoals);

- How can we assess the hydrological features of the acceleration of Anthropocene intervention, including the question of global boundaries (Steffen et al., 2015a; Steffen et al., 2015b)?

The French community will contribute to these challenges within international efforts to help find solutions in water resources and risk management for our common future.

# 5 Acknowledgements

The authors of this review acknowledge the tremendous efforts made by observers from French hydrometric and meteorological services, and thank all the institutions which provided access to large amounts of data on basins. Today this provides the opportunity for hydrologists to address the issues raised by PUB with invaluable databases. The authors also acknowledge that the studies presented herein may not give an exhaustive picture of all the contributions from French hydrologists to PUB.

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Section 7

Sciences physiques de l'océan
Physical Sciences of the Oceans

<figure><img src="image_30.png" /></figure>
# Recent progresses in biogeochemical regional modelling of the Eastern Boundary Current systems of the South Hemisphere

Dewitte B.¹, I. Montes², E. Gutknecht³, E. Machu⁴, A. Paulmier¹, K. Goubanova⁵ and V. Garçon¹

¹ Laboratoire d'Etudes en Géophysique et Océanopgraphie Spatiales, Toulouse, France

² Instituto Geofísico del Perú, Lima, Perú

³ Mercator Océan, Toulouse, France

⁴ Laboratoire de Physique des Océan, Brest, France

⁵ Centre Européen de Recherche et de Formation Avancée en Calcul Scientifique, Toulouse, France

It is now recognized that the so-called Eastern Boundary Upwelling System (EBUS) are key regions for the climate system due to the complexity of oceanic and atmospheric processes connecting to the open ocean and the troposphere, and to the fact that they host Oxygen Minimum Zones (OMZ), vast oceanic regions responsible for the world's largest fraction of water column denitrification. Of particular interest are the EBUS from the South Hemisphere, i.e. the Humboldt (Peru/Chile) and Benguela (Angola/Namibia) systems owed to their rich ecosystems and marine resources. Here we report on recent modeling studies on these EBUS, emphasizing on challenges for regional projections of climate change.

Progresses in our understanding of the oceanic circulation in these regions have been partly limited by the persistent biases in current generation global coupled models which do not account properly for regional air-sea interactions. In particular, feedbacks involving low-level clouds remain a primary cause of uncertainty in global models' climate sensitivity (Sherwood et al., 2014) and contribute maintaining a warm bias in these regions (Figure 1).

<figure><img src="image_31.png" /><figcaption>**Figure 1:** Difference in annual-mean SST between an ensemble of 39 CMIP5 models (historical experiment) and the observations (Reynolds data set, Reynolds et al., (2007)). The contour in thick blue line indicates the iso-contour of 0.2°C for the standard error (i.e. the dispersion among the models; inside the contour the standard error is larger than 0.2°C), whereas the green line indicates the limit of the OMZ (1mL/L iso-oxygen surface at 400m as derived from CARS data).</figcaption></figure>

The primary cause of such a bias is the underestimation of the upwelled waters due to the low-resolution of the global models that prevent the formation of a well-defined upwelling cell at the coast. In recent years, regional oceanic modelling has led to a quantitative step forwards in the realism of the oceanic simulations in these regions (cf. Penven et al. (2005); Montes et al. (2010) for the Peru upwelling system and Penven et al. (2001) for the Benguela system), allowing for integrative modelling studies. As a first step towards integration of the various components of the ecosystem, biogeochemical coupled modelling has been a keystone for designing Earth modelling systems. Most focus in the community in that field has been directed towards the understanding of the OMZ dynamics. In particular, these regions have been hypothesized to respond to climate change in a way that has drawn the societal concern. A long-term trend towards expansion and deoxygenation of the OMZ has been observed in the tropical oceans (Stramma et al., 2008), which might favour coastal hypoxia, environmental conditions that can lead to the deterioration of the structure and function of ecosystems. However, climate change projections from CMIP-class models are less conclusive
regarding OMZ trend (Cabré et al., 2015), calling for caution on results based from low-resolution global models.

Recent efforts have been thus directed towards tackling such issues based on regional coupled modelling, spurred by the need to provide more reliable projections of climate changes at regional scales, and the scarcity of long-term *in situ* data in these regions.

A prerequisite has been to produce atmospheric forcings suitable for regional modelling in EBUS considering strong limitations associated with the use of Reanalysis products and low-resolution global models outputs for oceanic downscaling (cf. Goubanova et al. (2011) and Cambon et al. (2013) for a discussion). Machu et al. (2015) used such an approach for their downscaling experiment of the IPSL model over the Benguela system with a regional coupled model based on the hydrodynamic ROMS model (Shchepetkin and McWilliams, 2003) and the biogeochemical model BioEBUS. BioEBUS is a nitrogen-based model derived from a N₂P₂Z₂D₂ model (Koné et al., 2005) in which the dissolved oxygen cycle was introduced. It was specifically designed to address the OMZ dynamics and nitrogen cycle off Namibia (see Gutknecht et al. (2013ab) for details and validation). Using this regional coupled model, Machu et al. (2015) show that among the most prominent changes brought by the regional downscaling compared to the global coupled model simulation are modifications of the structure of the lower trophic levels consisting in a shift of large offshore areas from autotrophic to heterotrophic regimes. Such a shift would have important consequences on ecosystem functioning (e.g. Duarte et al., 2013). The regional model also leads to a different distribution of phytoplankton and the nitrogen irrigation from the productive Benguela upwelling system towards the tropical/subtropical South Atlantic gyre (Figure 2).

<figure><img src="image_32.png" /><figcaption>**Figure 2:** Section at 8°E of crossshore annual flux of nitrate plus dissolved organic nitrogen (mmol N m⁻² s⁻¹) between 35° and 12°S for (a) the IPSL global model, (b) the regional simulation with ROMS-BioEBUS forced with IPSL atmospheric fluxes and c) the regional simulation with ROMS-BioEBUS forced with downscaled wind stress. Negative (positive) values indicate westward (eastward) flux (After Machu et al. (2015)).</figcaption></figure>

This modelling approach has been also applied to the Peru/Chile upwelling system that differs from the Benguela system in that it is more strongly connected to the equatorial circulation. Montes et
al. (2014) explore the sensitivity of the OMZ simulation by the same regional coupled model to the details of the equatorial circulation in the eastern Pacific. In particular, the equatorial undercurrent (EUC) connects to the Peru-Chile Undercurrent (PCUC) through a complex of pathways consisting in the extension of the EUC east of the Galapagos Island, and two branches further south, the primary and secondary Tsuchiya jets (Montes et al., 2010). Montes et al. (2014) show that a conduit by which the OMZ could vary at long timescales is through the secondary Tsuchiya jet that transports deoxygenated waters from the equatorial region to the latitude of ~10°S from which it flows eastward to connect to the PCUC. Their study also highlights the significant sensitivity of the OMZ simulation by the regional model to the characteristics of the boundary forcings (i.e. vertical and horizontal structures of currents), as well as the large non-linearity associated to biogeochemical processes (i.e. non-linear advection of O₂). Another issue arising from the societal concern in this region and that can be tackled with such a modelling approach, is how the interannual variations in the circulation in the equatorial region (e.g. El Niño) modify the OMZ at long timescales. In particular, whereas long-term trends in upwelling favourable winds are ubiquitous off Peru (K. Goubanova, pers. com), a SST cooling trend has been suggested from proxy data and the instrumental record (Gutierrez et al., 2011). It has been shown that such a cooling trend could be induced by changes in properties of the El Niño phenomenon (Dewitte et al., 2012). The similar model setting than Dewitte et al. (2012) is also able to produce a long-term deoxygenation of the upper part of the OMZ off Peru (Figure 3), indicative of likely rectified effect of O₂ variations into the mean OMZ.

<figure><img src="image_33.png" /></figure>

**Figure 3:** Long-term trend in dissolved oxygen (in microM/decade) at 12°S and over the period 1958-2008 as simulated by a regional coupled model in a configuration similar to Dewitte et al. (2012). The black thick line indicates the iso-oxygen surface at 1ml/L (i.e. oxycline depth) over the full period whereas the thick white line is for the period 2000-2008. Only the values significant at the 95% level are shown.

These modelling results demonstrate that long timescale of variability (decadal to climate change) can be addressed in EBUS through downscaling with high-resolution biogeochemical coupled models, which shall encourage further integration of the down-streamed components of the marine ecosystem. They also call for better understanding the details of processes controlling the rate of O₂ changes at different timescales and spatial scales. The interplay between O₂ eddy flux and O₂ transport by the mean current in controlling the OMZ boundaries and intensity has been a main focus in recent years within the community (cf. SOLAS program) and this on-going effort shall lead to a better understanding of the forced and internal OMZ dynamics. Besides relying on regional coupled modelling, such effort should be accompanied by dedicated observational campaigns. Off Peru, several projects have been launched in recent years, including the on-going AMOP international project (http://www.legos.obs-mip.fr/recherches/projets-en-cours/amop) that has provided invaluable *in-situ* data. Related activities will be also pursued over the Benguela system within the LMI ICEMASA (http://www.icemasa.org/), allowing for comparative studies.

*Acknowledgements:* We thank the EUR-OCEANS Consortium (Flagship EUR-OCEANS referenced as EOC/PFB/PC/2011.0013), the EC seventh framework program through the Marine Ecosystem Evolution in a Changing Environment (MEECE No. 212085), IRD and the ANR for support.
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# Zonal jets at 1000m in the tropics observed from ARGO floats'drifts

By S. Cravatte(1), F. Marin(1), W. S. Kessler(2)

¹IRD/LEGOS, Nouméa, New Calédonia

² PMEL/NOAA, Seattle, USA

## Introduction: about the intermediate circulation

The upper circulation of the equatorial oceans has been extensively observed and studied. In particular, the upper circulation of the Pacific Ocean has received much attention for its relation with the El Nino phenomenon. Surface drifters and satellite altimetry gave us a pretty good picture of the mean surface circulation and of its variability. Moorings and repeated hydrographic surveys provided estimates of current velocities across the upper tropical Pacific and Atlantic Oceans, in the first 500 meters or so. In situ observations are sparse below 500 meters, and our knowledge of the oceanic currents in the intermediate and deep equatorial oceans is very limited. However some synoptic deep sections revealed interesting features. Firing (1987) and Firing et al. (1998) first showed evidence in the western Pacific of a series of near-equatorial westward and eastward jets from 3°S to 3°N at intermediate depths, characterized by a large vertical extent, alternating every 1.5°-2° in latitude, with amplitudes of about 10 cm s⁻¹, the so-called "equatorial intermediate current system". Similar alternating zonal jets have also been evidenced in the equatorial Atlantic Ocean thanks to synoptic sections (Stramma and England, 1999; Bourlès et al., 2003), and their zonal coherence along at least 25 degrees in longitude was demonstrated by Ollitrault et al (2006) using acoustic and profiling floats.

Since the advance of the Argo program, and the presence in the global ocean of thousands of autonomous floats drifting at 1000m (or deeper), we now have a new way to observe the intermediate circulation. When compiled, Argo floats' drifts are very valuable tools to determine the absolute velocity at this depth on a global scale. 15 years ago, Firing et al. (1998) noted that "one would like to have a time series of sections covering at least 6°S-6°N, and preferably 10°S-10°N (...) to show whether the pattern of meridionally alternating mean zonal flows continues poleward." He also noted that "nothing is known about what happens to them at the boundaries". Argo floats' drifts compilation from 2003 to 2013 offers a sampling density sufficient to resolve the high-resolution meridional structure and zonal extent of the equatorial intermediate current system at 1000m, and makes it possible to answer in part these questions.

## Argo float subsurface drift computation, and the ANDRO dataset

Argo profiling floats are originally designed to provide vertical profiles of temperature and salinity every 10 days. They dive to the deepest depth of their profile (usually 2000m), rise to the surface, typically spend a few hours there where satellites determine their position, and dive back to their parking depth (usually 1000m) where they drift for about 9 days before diving again for their next profile. If done carefully, the positions and time of transmissions before and after each dive can be used to compute the subsurface Lagrangian velocities, averaged during the drift period.

We gathered all Argo floats that entered the 12°S-12°N region between 120°E and 70°W, from January 2003 to December 2013, and computed the Lagrangian subsurface velocity corresponding to their drift at their parking depth using their positions and times of transmission. There are several sources of uncertainties on this subsurface velocity computation; the float spends some time at the surface before its first transmission and after its last transmission, during which it drifts with the surface currents; it is also advected by the subsurface currents encountered during its ascent and descent. These errors were estimated and the median error was found to be small (less than 0.5 cm/s). The last important source of error is the parking depth: it is often erroneous in the metafile file (erroneously decoded, or constant even if the float grounded), especially for floats in the Atlantic Ocean. Each float's trajectory and the parameters of its dive were visually inspected to eliminate obvious bad values. Especially, the pressure at the parking depth was computed and inspected carefully. Most of the Argo floats have a parking depth around 1000 m. In the Tropical Pacific Ocean, there are enough floats drifting at 1500m to get a picture of the mean circulation at that depth in some areas, even if large regions remain unsampled. The subsurface velocities at 1000m and 1500m were
thus mapped to produce a mean monthly climatological field on a 1° longitude 0.25° latitude grid, using an optimal objective analysis method. The details of the methods used, of the error estimations and of the gridding procedure are described in Cravatte et al. (2012).

Ollitrault and Rannou (2013) recently produced the ANDRO product, a very useful quality-controlled subsurface displacement dataset for the global ocean from all Argo floats till 31 December 2009, with validated parking pressures. We checked that this dataset was consistent with our results in the tropical Pacific.

## Mean circulation at 1000m and 1500m in the tropical Pacific

The mean zonal and meridional currents in the Tropical Pacific are shown in Figures 1 and 2. The zonal velocity by far dominates the meridional velocity in most locations, except near the western boundary. Several features stand out:

a-The zonal velocity at 1000 m is dominated by alternating westward and eastward jets with a meridional scale of 1.5° and speeds about 5 cm/s. Strikingly, these jets are zonally coherent across the entire basin, confirming and extending the synoptic observations of Firing et al. (1998). Moreover, these jets are observed at least from 12°S to 12°N, a wider latitudinal range than shown by previous observations.

b-The jets are strong in the western and central parts of the basin but weaken and eventually disappear in the east (near 110°W at the equator, farther west poleward).

c-The jets are stronger in the Southern Hemisphere, where six to seven jets can be identified at 1000 m. They are weaker (except in the west) in the Northern Hemisphere, where seven to eight jets are also identified.

d- In the western Pacific, the 1000-m zonal jets appear to slant slightly poleward from east to west.

Figure 2 also shows the mean zonal and meridional velocities at 1500m. Even if some regions remain largely unsampled, it gives interesting clues about the deeper extension of the zonal jets. Alternating zonal jets are also present at 1500 m and appear, at first glance, to have a similar geographical distribution than the ones observed at 1000 m. This suggests that the jets have a large vertical extension. It is worth noting that currents at 1500 m are westward along the equator, as at 1000 m. This is surprising, because it is known that, at these depths, equatorial deep jets alternating in the vertical with a wavelength of several hundred meters are present in a narrow equatorial band (Firing et al. 1998, their Fig. 1). Thus, the similarity between 1000m and 1500m zonal currents at the equator is probably coincidental and needs to be further investigated.

<figure><img src="image_34.png" /></figure>

Figure 1: Mean zonal currents (upper) and mean meridional currents (lower) (cm/s) at 1000m from optimal interpolation. Topography shallower than 1000-m depth is shaded in dark gray. Updated from Cravatte et al. 2012.
<figure><img src="image_35.png" /></figure>

Figure 2: Mean zonal currents (upper) and mean meridional currents (lower) (cm/s) at 1500m from optimal interpolation. Topography shallower than 1000-m depth is shaded in dark gray. Updated from Cravatte et al. 2012.

## What happens to them at the boundaries?

Near the western coast of the South Pacific delimited by Solomon Islands chain and Papua New Guinea, the zonal jets originate (or terminate) very close to the coast, and appear to be connected to adjacent westward or eastward jets by meridional velocities (not shown). A northward boundary current is also seen flowing along the Solomon Island chain at 1000m. In the northern Pacific, the situation is analogous, though vigorous eddies dominate the mean circulation, and these recirculations are less clear.

## Seasonal variability

It is not possible to resolve the full temporal structure of the jets' variability, given the sampling and the short record spanning only a few years. However, it is possible to characterize their seasonal variability. Figure 3 shows the 1000m zonal velocity anomalies at the equator and at 4°N and 4°S. At the three latitudes, there is a very clear annual cycle of zonal velocity. Current anomalies are out of phase between the equator and 4° and propagate westward at a speed of about 0.43 m/s. This meridional structure and the phase speed correspond to the vertical propagation of the annual Rossby wave described previously based on subsurface temperature changes (e.g. Kessler and McCreary 1993). Interestingly, the zonal currents' seasonal variability exhibits a meridional scale quite different from that of the background mean jets, with a meridional wavelength of 7°–9° instead of 3°. The amplitude of the seasonal variability, which is stronger close to the equator, implies that the most equatorial jets may transiently reverse, as noticed by Gouriou et al. (2006) and explained by Marin et al. (2010). The zonal currents' annual variability is thus superimposed on the mean zonal jets, but the scale difference suggests that the seasonal dynamics are distinct from the background zonal jets' dynamics (Figure 3, middle and right panels).

## In the tropical Atlantic and Indian Oceans

As in the Pacific, we analyzed the Argo float drifts at 1000m between 12°S and 12°N to characterize the mean velocity and its seasonal variability. We used the ANDRO dataset until December 2009 and updated the subsurface velocities until December 2013. Ubiquitous zonal jets are shown to extend across the tropical Atlantic, in agreement with the findings of Ollitrault et al. (2006) and Ollitrault and Colin de Verdière (2014), but they are weaker than in the Pacific, with larger amplitude in the northern hemisphere. Their meridional scale is about 400km, slightly larger than in the Pacific. They are also weak in the east, and slant poleward in the west. In the Indian Ocean, there are no coherent zonal features emerging from the mean circulation pattern, although the seasonal
variability (mainly semiannual) seems to be well resolved (not shown). The mean zonal currents may be hidden by a stronger variability. Understanding the dynamical origin of these differences will be the goal of future studies.

<figure><img src="image_36.png" /></figure>

dashed lines). (Right) Mean zonal current $u$ at 1000 m in cm/s, zonally averaged in the 160°E–120°W band in the Southern Hemisphere and in the 120°E–120°W band in the Northern Hemisphere. Adapted from Cravatte et al., 2012.

Figure 3: (left) Seasonal anomalies (mean removed) of 1000-m zonal currents (cm/s) averaged between (top) 3°–5°N, (middle) 2°S–2°N, and (bottom) 3°–5°S. (Middle) Zonal velocity anomalies averaged along the paths shown in the left panels (in black lines), between 160°E and 160°W, for 1000-m current anomalies (thin black) and their first annual harmonic (dashed black). The same calculations have been made for zonal current anomalies at 1500-m currents, and 1500-m zonal currents anomalies and their first annual harmonic have been averaged along similar Rossby paths, shifted one month in advance (red thin and

<figure><img src="image_37.png" /><figcaption>Figure 4: Mean zonal</figcaption></figure>

Left:

currents from Argo floats drifts (cm/s) at 1000m in the Pacific (upper) and Atlantic Ocean (lower) (cm/s). Right: Mean zonal current at 1000 m in cm/s, zonally averaged in the 160°E–120°W band in the Southern Hemisphere and in the 120°E–120°W band in the Northern Hemisphere in the Pacific, and in the 60°W–15°W band in the Atlantic Ocean.

## Conclusion

The analysis of ARGO floats drifts at 1000 m depth indicates that the mean intermediate circulation in the tropical Pacific is dominated by the presence of alternating zonal jets, that had
previously only been sporadically observed from synoptic hydrographic cruises at specific longitudes. These jets are coherent over the whole basin width, from Solomon islands to about 2000 km of the American continent, and they are present from 12°S to 12°N. These extra-equatorial jets flow zonally either from West to East, or from East to West, at a speed of about 5-10 cm/s, alternating regularly every 1.5° in latitude. Their hydrological and biogeochemical properties are largely unknown, along with the physical processes responsible for their existence. They are also present in the Atlantic Ocean, but could not be evidenced in the Indian Ocean. Their representation in oceanic models, even at high resolution, is very poor.

The aim of the ZEBRE project funded by LEFE/IMAGO and LEFE/GMMC (PI, S. Cravatte, 2013-2015) is to better document, and understand, the dynamics of these deep extra-equatorial jets in the Pacific ocean and in the other tropical oceans, and to identify their possible link with the deep alternating jets that are observed in the midlatitudes, from the combined analyses of in situ observations (ADCP currents, three-dimensional products of geostrophic velocities, ARGO float drifts) and of numerical simulations. In the context of this project, the CASSIOPEE cruise (2015, PI F. Marin) will acquire a unique set of synoptic in situ observations of the ocean circulation from the surface to the ocean bottom in the Southwest Equatorial Pacific Ocean. The objectives of this cruise are to describe the fine scale meridional and vertical structure of these deep extra-equatorial jets, the hydrological and biogeochemical properties of the water masses they advect, and their possible interaction with the western boundary circulation.

## Acknowledgements

This study strongly relies on Argo data subsurface drifts. Argo float data were collected and made freely available by the International Argo Program and the national programs that contribute to it. The Argo Program is part of the Global Ocean Observing System. We would like to thank Michel Ollitrault for his comments and for providing the ANDRO dataset.

## References

* Bourlès, B., et al., 2003, The deep currents in the eastern equatorial Atlantic Ocean, Geophys. Res. Lett., 30(5), 8002, doi:10.1029/2002GL015095.

* Cravatte, S., W. S. Kessler, and F. Marin, 2012: Intermediate zonal jets in the tropical Pacific Ocean observed by Argo floats. J. Phys. Oceanogr., 42, 1475–1485.

* Firing, E., 1987: Deep zonal currents in the central equatorial Pacific. J. Mar. Res., 45, 791–812.

* Firing, E. S. E. Wijffels, and P. Hacker, 1998: Equatorial subthermocline currents across the Pacific. J. Geophys. Res., 103 (C10), 21 413–21 423.

* Gouriou, Y., T. Delcroix, and G. Eldin, 2006: Upper and intermediate circulation in the western equatorial Pacific Ocean in October 1999 and April 2000. Geophys. Res. Lett., 33, L10603, doi:10.1029/2006GL025941.

* Kessler, W. S., and J. P. McCreary, 1993: The annual wind-driven Rossby wave in the subthermocline equatorial Pacific. J. Phys. Oceanogr., 23, 1192–1207.

* Marin, F., E. Kestenare, T. Delcroix, F. Durand, S. Cravatte, G. Eldin, and R. Bourdalle-Badie, 2010: Annual reversal of the equatorial intermediate current in the Pacific: Observations and model diagnostics. J. Phys. Oceanogr., 40, 915–933.

* Ollitrault, M., and J. P. Rannou, 2013: ANDRO: An Argo-based deep displacement dataset. J. Atmos. Oceanic Technol., 30, 759–788.

* Ollitrault, M., M. Lankhorst, D. Fratantoni, P. Richardson, and W. Zenk, 2006: Zonal intermediate currents in the equatorial Atlantic Ocean. Geophys. Res. Lett., 33, L05605, doi:10.1029/2005GL025368.

* Ollitrault, M. and A. Colin de Verdière, 2014: The Ocean General Circulation near 1000-m Depth. J. Phys. Oceanogr., 44, 384–409. doi: http://dx.doi.org/10.1175/JPO-D-13-030.1

* Stramma, L., and M. England, 1999, On the water masses and mean circulation of the south Atlantic Ocean, J. Geophys. Res., 104(C9), 20,863 – 20,883.
# SPICE SOUTHWEST PACIFIC OCEAN CIRCULATION

A. Ganachaud (PI), S. Cravatte (co-PI), G. Eldin, T. Delcroix, L. Gourdeau, F. Durand, F. Marin, C. Maes, J. Fefevre, E. Kestenare

Laboratoire d'études en géophysique et océanographie spatiale (LEGOS), Université de Toulouse; (OMP-PCA)/IRD/CNRS/CNES 14 Av. Edouard Belin, F-31400 Toulouse, France.

## Mission

### Objectifs :

Pour étudier la circulation océanique du Pacifique sud-ouest et les alimentations des eaux équatoriales dans un contexte climatique, le programme international SPICE a été lancé en 2008. La contribution française se concentre sur l'étude de la Mer de Corail et de la Mer des Salomon, point d'étranglement où les courants de bord sont intenses. Les objectifs sont les suivants:

* Décrire la circulation interne de ces mers, compliquée par les effets d'île, courants de bord et détroits

* Mesurer et modéliser les transports depuis la Mer de Corail vers l'équateur via la mer des Salomon et leurs variations aux échelles saisonnières et interannuelles ;

* Evaluer le mélange entre les masses d'eau dans cette région et son rôle dans la variabilité climatique.

Le transport et les caractéristiques des eaux issues du gyre du Pacifique Sud et entrant en Mer de Corail, puis en Mer des Salomon avant d'atteindre l'équateur sont des éléments importants du système climatique, notamment liés à la dynamique du phénomène El Niño. Une description et analyse correctes du transport de ces eaux et de l'évolution de leurs propriétés sont importantes pour pouvoir améliorer les prévisions océaniques et à terme climatiques. Les forts courants de bord combinés avec une topographie compliquée et des détroits étroits entre les îles rendent la modélisation difficile pour les échelles à considérer. SPICE est un programme international collaboratif entériné par CLIVAR et animé par le LEGOS. Le LEGOS a apporté, et apporte une contribution majeure au programme, avec des travaux sur les mers de Corail, des Salomon ainsi que sur les cheminements grande échelle entre l'entrée Mer de Corail et l'équateur. Cette contribution se décline sous la forme d'un important programme de mesures in situ, de modélisation numérique (collaboration LGGE) et d'analyses de données altimétriques. Ce dispositif expérimental vise à appréhender la variabilité intrasaisonnière, saisonnière et interannuelle des transports et des propriétés des masses d'eau à l'entrée et aux sorties des mers de Corail et des Salomon.

## Stratégie

L'étude est basée sur une approche modèle-observation. Un important programme de mesures in situ a été mené.

En Mer de Corail, 10 campagnes en mer et trois déploiements de mouillages ont permis de documenter les courants et les masses d'eaux et sont analysées en combinaison avec les données historiques (e.g. S-ADCP retraités). Un glider a effectué 4 transects depuis la Nouvelle-Calédonie. Un monitoring expérimental du transport du Courant Equatorial Sud par des déploiements de flotteurs Argo et de sondes XBT/XCTD à partir d'un navire commercial a aussi été mis en place (voir la fiche « Mer de Corail »).

<figure><img src="image_38.png" /></figure>

Topographie du Pacifique sud-ouest et dispositif expérimental SPICE. La zone rouge délimite les mesures, ou contributions réalisées par OLVAC.
En Mer des Salomon, qui était jusqu'alors quasi-inexplorée par les océanographes, nous avons accomplis un important effort de mesure à travers des campagnes, l'installation de mouillages dans les détroits, et de modélisation haute résolution au 1/12° et 1/36° de résolution, notamment pour mesurer la variabilité des flux vers l'équateur et les transformations de masses d'eau lors de leur transit.

<figure><img src="image_39.png" /></figure>

## Deux résultats majeurs & prospectives

### Grande échelle : connexion à l'équateur.

Pour quantifier les provenances des eaux du sous-courant équatorial (EUC), une simulation océanique (1/4°) a été analysée à l'aide d'un outil Lagrangien. Les eaux des courants de bord ouest représentent 87% du transport de l'EUC, avec une contribution du New Guinea Coastal Undercurrent (NGCU) qui atteint 50%.

Trajectoires Lagrangienes des eaux nourrissant l'EUC. Les différentes couleurs indiquent les provenances des eaux, en particulier celles passant par les (2011)

### Petite échelle : Jets, courants de bord et sous méso-échelle.
Dans cette région très dynamique, le courant équatorial sud se divise à la rencontre des îles, puis suit les côtes sous forme d'un courant de bord (ici le Gulf of Papua Current, GPC) qui, en Mer des Salomon, se renforce et se divise pour rejoindre l'équateur par différentes routes. En parallèle avec l'examen de données historiques, nous avons mis en place des simulations numériques très haute résolution donnant accès aux fines échelles générées par la forte énergie des courants.

<figure><img src="image_40.png" /></figure>

<figure><img src="image_41.png" /></figure>

(haut) : Courants moyens en Mer des Salomon issus des données courantométriques (ADCP) des instruments de bord des navires de recherche transitant par la région (Cravatte et al. 2011). Les transports issus de la campagne Flusec 2007 sont indiqués en vert (Gasparin et al. 2012).

(gauche) : Instantané de la salinité de surface dans une simulation numérique à très haute résolution (1/36°) réalisée en Mer des Salomon (Djath et al. 2014a,b)

**Futur:** Au cours des dix dernières années, des efforts considérables ont été consacrés à améliorer la compréhension de cette région, par des observations in situ, la modélisation, ainsi que la télédétection et des analyses des données historiques. De nombreux aspects ont été abordés par le LEGOS: transports de masse, de chaleur et variabilité; propriétés et dynamique des courants de bord et des jets et transformations des masses d'eau. Les projets futurs incluent l'approfondissement de certains aspects (thèses en cours), ainsi que l'exploration du mélange, de la dynamique turbulente de sous-mésoéchelle et des courants en lien avec les changements climatiques.
Website: http://spice.legos.obs-mip.fr

## Partenariat et financements

SPICE est un programme international impliquant la France, les Etats-Unis, l'Australie, le Japon et les îles du Pacifique. SPICE ne dispose pas de financement propre. Les partenariats et financements spécifiques sont mentionnés dans les fiches "Solwara" et "Mer de Corail".
# SOLWARA Solomon Sea circulation and geochemistry

S. Cravatte (PI), A. Ganachaud (co-PI), C. Jeandel (co-PI), G. Eldin (co-PI), L. Gourdeau, J. Lefevre, P. Van Beek, F. Lacan, M. Souhaut, C. Pradoux

*Laboratoire d'études en géophysique et océanographie spatiale (LEGOS), Université de Toulouse;
(OMP-PCA)/IRD/CNRS/CNES 14 Av. Edouard Belin, F-31400 Toulouse, France*

**Objectives :**

The Solwara project aims at better understanding the inflow, the outflow and the circulation within the Solomon Sea, as well as geochemical sources and water mass transformations along this pathway to the equator. The objectives are:

* To measure, model and understand the total transport from the Coral Sea to the equator and the partitioning in the transport between the different Solomon Straits on seasonal to interannual time scales;

* To evaluate the partition between the western boundary current and the direct flow from the SEC to the east Solomon Sea;

* To describe in details the inside Solomon Sea circulation and flows between trenches and islands;

* To evaluate water mass transformation and mixing through the Solomon Sea, with a special focus on the geochemical sources from the Solomon and PNG margins.

* To estimate the intensity and time scales of element exchanges along the Solomon Sea margins.

* To estimate the geographic origins, pathways and transit times of these water masses, between the Coral Sea and the equatorial region.

**Main tasks**

The oceanic circulation in the South Pacific redistributes waters from the large subtropical gyre toward the equator through a subtle journey, as suggested by model results and meridional gradients of trace elements. These waters are first transported from the central South Pacific, westward, in the South Equatorial Current and eventually join the equator after transiting in the western boundary current system of the Coral and Solomon Seas. This circulation is also associated with the equatorward supply of trace elements that controls biological activity in the equatorial area. The Solwara project aims at improving our physical and geochemical knowledge of the oceanic features of the Solomon Sea, as well as the development of numerical modeling tools and optimal monitoring systems to adequately sample the time-variability. Improving the realism of the oceanic circulation in this region in climate models may improve the realism of decadal variability, and thus permit a better understanding of its mechanisms. The "Solwara" project will be an important contribution to SPICE (spiceclivar.org; fiche-SPICE) and GEOGRACES (geotraces.org) international programs.

**Strategy**

<figure><img src="image_42.png" /></figure>

Intrusion of the South Pacific High salinity as simulated by a 1/12° résolution model embedded in a global ¼°. Melet et al. (2010a).

We base our approach on a combination of very high resolution modeling (1/12° to 1/36°) and observations. Models needed to be examined and improved over this region because of the complicated flows due to boundary currents, narrow straits and sharp shelves, which generate large uncertainties. At the same time, historical current-meter (ADCP) data were compiled to provide the first basic description of the observed circulation; and an important collaborative in situ observation program was carried out, including LEGOS cruises / moorings (below) and gliders (Scripps). Besides, altimetric data, including specific along-track products, were used to provide an adequate description of the
mesoscale variability. Tracer supplies from continental margins/ocean exchange, as well as their pathways were explored using Lagrangian modeling tools associated with in situ geochemical observations.

## Two major results & prospective

**Pandora** A multidisciplinary cruise (Pandora, 40 days on l'Atalante) was done in July 2012 through a France-USA collaboration. It completed a 2007 survey (Flusec, Alis) along the south boundary of the Solomon Sea, and was followed by a 30 day cruise in 2014 (RV Thompson) to renew the moorings and repeat measurements on a different season.

<figure><img src="image_43.png" /><figcaption>Pandora cruise trajectory (2012) as indicated by measured currents from the ship current meter (ADCP). The main currents are indicated in blue; mooring locations by the green circles. Eldin et al. (2013)</figcaption></figure>

Pandora provided a quasi-synoptic description of surface and subsurface circulation and strait transports and allowed to deploy 11 moorings to measure the transport temporal variability. The data will document water mass transformations and mixing, ocean-margins exchanges on a wide range of space and time scales through the combined analysis of physical and geochemical data. Four LEGOS moorings were re-deployed in the three northern straits in 2014. These cruises

provide an unprecedented "snapshot" description of the circulation, the few previous surveys being either low resolution or low coverage.

## Trace Element Isotopes and Lagrangian Pathways

An original approach coupling real geochemical tracer data -measured in the framework of EUCFe and Solwara projects- with a modelled Lagrangian circulation allowed a very fine description of the dynamical tracer transports and a precise quantification of the external sources of micro-nutrients that are enriching the water masses. We demonstrated that the *lower* thermocline only is naturally fertilized by these inputs as it flows along the Papua New Guinea coast.

Circulation and Nd parameter evolution in the Bismarck Sea: (upper) Upper thermocline; (lower) Lower thermocline. Modelled pathways and transports (coloured lines and associated Sv) coupled to discrete measurements of Nd parameters (ε and concentrations; coloured points). This figure does not illustrate the whole outflow of the straits but only the fraction that finally flows along the Papua New Guinea coast, around 3°S, 144°E. Large coloured arrows represent the net exchange flux of Nd between the continental margin and the seawater, in tons of Nd per year. Adapted from Grenier et al. 2013.

<figure><img src="image_44.png" /></figure>

<figure><img src="image_45.png" /></figure>

**Future:** The tremendous data set and numerical simulation generated by the recent cruises and models will be a large research focus on the next 3 years, with at least three LEGOS theses. Substantial efforts are required to accomplish the basic analysis (geochemical lab measurements and CTD/Nutrient treatments). Future developments will include understanding the role of the Solomon Sea transports in the tropical climate system.

**Website:** http://solomonsea.oceanography.org. (Numerical model simulations http://thredds.sedoo.fr/solwara/).
## Partnership and funding

**Partners:** Scripps Institute of Oceanography (SIO); Pacific Marine Environmental Laboratory (NOAA/PMEL); University of the South Pacific (USP, Fiji); University of Papua New Guinea (UPNG); LGGE, Grenoble, GET, Toulouse, LOCEAN, Paris, MIO, Marseille, CSIRO, Australia.

**Funding:** INSU/LEFE, INSU/DT, IRD, ANR
# Vertical stratification of the upper ocean: documenting the specific role of the salinity in the Tropics.

Christophe Maes

Laboratoire de Physique des Océans (LPO), CNRS, Ifremer, IRD, UBO, Plouzané, France

Observational studies in the western Pacific Ocean since the mid-1980s have suggested that the barrier layer resulting from the salinity stratification within the mixed layer could influence significantly ocean-atmosphere interactions, and consequently the variability of the El Nino/Southern Oscillation (ENSO) phenomenon. In addition, despite considerable progress in the representation of the tropical mean state and climate variability by coupled ocean-atmosphere general circulation models, several studies have concluded that it is not yet possible to say whether ENSO activity will be enhanced or damped by global warming. In the Pacific Ocean, a stringent test for models is a correct representation of the mean zonal gradient of sea surface temperature (SST) along the equator and also a correct position of the salinity front separating the Western Pacific Warm Pool (WPWP) from the central cold waters. Analyzing 19 models from the World Climate Research Program Coupled Model Intercomparison Project Phase 5 (CMIP5), Brown et al. (2014) have demonstrated that it remains difficult for these coupled models to simulate the mean longitude, the interannual excursions, and the zonal convergence of ocean currents associated with the WPWP. It remains also difficult in some cases to discriminate the physical mechanisms, as for example, the warm pool displacements result from a net heating or cooling rather than a zonal advection of warm water, and to relate them to the intrinsic interannual variability simulated by the model itself. One major ingredient that needs some improvements is the simulation and the generation of the salinity barrier layer that is a key element of the whole Pacific ocean-atmosphere coupled system (Maes and Belamari 2011).

While the sea surface salinity (SSS) and barrier layer variabilities in the equatorial Pacific have been investigated in previous studies, using oceanographic cruise data, reconstructed salinity profiles, and Voluntary Observing Ship thermosalinograph measurements, it has been possible only recently to combine the SSS measurements from satellite missions with the vertical structure at depth as observed by the Argo floats, in order to monitor salinity variations with accuracy within the Pacific basin (Qu et al. 2014). The availability of these unique data sets also raises some new questions about the conceptual view of the ocean vertical structure that is based on a definition of several layers within which the water properties exhibit different degrees of vertical uniformity. Maes and O’Kane (2014a) propose a new perspective for describing the impact of the salinity on the static stratification of the oceanic upper layers, using a methodology keyed to the Brunt-Vaisala frequency, $N^2$. This perspective recognizes that the stabilizing effect of the salinity operates near the bottom of the mixed layer but that its effect could be expanded downward to the main pycnocline and to regions where both the salinity and temperature are mixed over the same depth. Significant seasonal variations in $N^2$ profiles are found to be limited to the upper 300 m depth, and in several regions of the tropics, the salinity part contributes 40–50% to $N^2$, approximately equal to the thermal stratification contribution (see the figure 1). The study that achieved these results was undoubtedly made possible by the expansion of the ocean observing system through the Argo float network, which offers concomitant temperature and salinity profiles at depth and at the global scale. When the same diagnostics are applied to the pre-Argo period, differences as large as 30% in terms of amplitude are observed in the seasonal variability of the salinity signature in the Tropical areas (Maes and O’Kane, 2014b). These results newly emphasize the challenge of investigating and exploring the role of the salinity stratification in seasonal prediction systems and long-term climate variability.
<figure><img src="image_46.png" /><figcaption>Figure 1: Annual mean of (top) the OSS (Ocean Salinity Stratification) defined as the stabilizing effect (positive) due to the haline part of $N^2$ (in cph) and of (bottom) the ratio (in %) of $N^2$(S) to $N^2$(T, S). The contour lines represent the 34.8 (continuous) and 35.8 (dash) of SSS. The mean fields are based on the seasonal climatology described by Maes and O'Kane (2014a).</figcaption></figure>

## References

* Brown, J., C. Langlais, and C. Maes, Zonal Structure and Variability of the Western Pacific Dynamic Warm Pool edge in CMIP5, *Clim. Dyn.*, 42:3061–3076, doi:10.1007/s00382-013-1931-5, 2014.

* Maes, C., and S. Belamari, On the impact of salinity barrier layer on the Pacific Ocean mean state and ENSO, *SOLA*, 7, 97-100, 2011.

* Maes, C. and T. J. O'Kane, Seasonal variations of the upper ocean salinity stratification in the Tropics, *J. Geophys. Res. Oceans*, 119, 1706-1722, doi:10.1002/2013JC009366, 2014a.

* Maes C. and T. J. O'Kane, Upper ocean salinity stratification in the Tropics as derived from $N^2$, the buoyancy frequency, *MERCATOR OCEAN/CORIOLIS newsletter*, no50, 15-19, 2014b.

* Qu, T., Y. T. Song and C Maes, Sea surface salinity and barrier layer variability in the equatorial Pacific as seen from Aquarius and Argo, *J. Geophys. Res. Oceans*, 119, 15–29, doi:10.1002/2013JC009375, 2014.
# Sea Surface Salinity from SMOS satellite mission: a synthesis of the main 2010-2015 oceanic results in France

J. Boutin (1), N. Reul (2), C. Maes (3), G. Reverdin (1), T. Delcroix (4), F. Gaillard (3)

(1) Laboratoire d'Océanographie et du Climat : Expérimentation et Approches Numériques (LOCEAN), CNRS, IRD, UPMC, MNHN, Paris, France

(2) Laboratoire d'Océanographie Spatiale (LOS), Ifremer, Plouzané, France

(3) Laboratoire de Physique des Océans (LPO), CNRS, Ifremer, IRD, UBO, Plouzané, France

(4) Laboratoire d'Etudes en Géophysique et Océanographie Spatiales (LEGOS), IRD, CNES, CNRS, UPS, Toulouse, France

Ocean salinity is an important driver of ocean circulation and represents a key indicator of changes in the global water cycle. Recent advances in observing sea surface salinity (SSS) from space have provided an unprecedented capability to study the influence of salinity on ocean circulation and its relations to climate variability and the water cycle (Reul et al. 2014a). The Soil Moisture and Ocean Salinity (SMOS) satellite mission has been launched in 2009 under the auspices of the European Spatial Agency (ESA). It is the first satellite mission that provided a global monitoring of sea surface salinity (SSS) from space. Since 2010, SMOS provides a quasi-synoptic monitoring of the global ocean every 3 to 5 days. In tropical and subtropical areas, the precision of monthly SSS anomalies at 50 to 100km resolution has been determined to be close to 0.2 psu. Several French teams have been strongly involved in the physics of measurements of the L-band radiometry, in the calibration and validation of the SMOS salinity measurements and in their use for studying ocean processes. Here we review the scientific results achieved by the french teams thanks to these novel measurements.

Large scale interannual SSS anomalies in the Indian Ocean, linked to the Indian Ocean Dipole (Durand et al. 2013), and in the western tropical Pacific, linked to the 2010-11 La Niña (Hasson et al. 2014), have been detected by SMOS with a spatio-temporal resolution very complementary to Argo and ships of opportunity. These anomalies are related to freshwater fluxes as well as to ocean advection anomalies. In addition, tropical instability waves in the equatorial Pacific Ocean were observed to vary interannually (Yin et al. 2014). Combining SMOS SSS with ship SSS and an ocean model, Hasson et al. (2013) identified decadal trend of the longitudinal displacement of the salinity maximum in the South-Eastern tropical Pacific Ocean. Along the equator, SMOS measurements reveal the signature of the equatorial upwelling with fine spatial scales associated with the thermohaline signature of the dense waters (Maes et al. 2014). Another great advance with the SMOS data set has been to conduct unprecedented studies on the small scales of SSS at typical 50 to 100 km. Reul et al (2014b) show for instance that the eddies resulting from the meandering of the Gulf Stream region could be followed by SSS during the summer when the warming of the mixed-layer is blurring the SST signature of eddies and meanders. In the same context, Maes et al. (2013) revealed the importance of the eddies stirring in the Coral Sea and Kolodziejczyk et al. (2015) focused on the seasonal variability of the surface horizontal thermohaline structure in the subtropical Atlantic Ocean. Near-coastal studies, especially dedicated to the main riverine inputs in the ocean were also conducted (Hopkins et al. 2013; Reul et al. 2014a, 2014c; Fournier et al. 2015). The peculiar seasonal dynamics of SSS off Panama, in the so-called eastern Pacific fresh pool, was carefully analyzed with complementary and sporadic in situ data, and with SMOS, which provided the unprecedented large scale context (Alory et al. 2012). Feedbacks of barrier-layers and salt-driven stratification in freshwater pools on local air-sea interactions, such as the favored intensification of hurricanes as they cross the Amazon river plume as been identified and demonstrated (Grodsky et al. 2014; Reul et al. 2014c). It is important to note that most of these studies have explored new possibilities of monitoring these signals and would represent the roots for further analyses in the context of climate change.

In the tropical convergence zones, spatio-temporal variability of SMOS SSS was demonstrated to be strongly related to rain events (Boutin et al. 2013, 2014). Given that satellite radiometry senses the salinity in the top centimeter of the ocean while most in situ systems measure it at a few meter depths, this pleads for an improved description and understanding of the penetration of the fresh water in the upper ocean. Last, using SSS as a tracer of water masses, SMOS SSS brought new insights on the processes driving variability of biogeochemical parameters. Fournier et al. (2015) revisited the relationships between SSS and upper water column optical properties in the Amazon
plume, and Land et al. (2015) discussed the general possibility of better monitoring the short-term, small-spatial scales of the carbonate variability in the ocean thanks to the synergistic use of satellite SSS, SST and ocean color. Brown et al. (2015) highlighted the dominant roles of the upwelling and freshwater fluxes in driving the air-sea CO2 flux variability in the northeastern tropical Pacific Ocean.

A new era of SSS observing system from Space has started with the launch of SMOS. It was followed by the launch of Aquarius in 2011. Both missions provide complementary observations, SMOS (~50km, 3-5days) providing a better spatial resolution and revisit time than Aquarius (~150km, 7days), but with large regions blurred by radio frequency interferences and experiencing larger systematic biases. The recently launched Soil Moisture Active Passive (SMAP) mission is expected to consolidate and complement these new data sets, likely by providing high resolution SSS monitoring in contrasted areas.

Acknowledgements: We acknowledge major contributions of Gael Alory, Séverine Fournier, Audrey Hasson, Nicolas Kolodziejczyk and Xiaobin Yin. Most of these studies have been supported through CNES/TOSCA and ESA grants.

<figure><img src="image_47.png" /><figcaption>Figure 1: Small scale (~50km) SSS variability monitored by SMOS in the Gulf Stream region (left; Reul et al. 2014b) and in the Azores front region (right; Kolodziejczyk et al. 2014) and their relationship to mesoscale surface currents.</figcaption></figure>

<figure><img src="image_48.png" /><figcaption>Figure 2: SMOS SSS in the tropical Pacific Ocean: on left, Seasonal dynamics of SSS off Panama, in the far Eastern Pacific Fresh Pool (Alory et al. 2012); on right, (top) signature of tropical instability waves (Yin et al. 2014), and (bottom) temporal variability at 110W between 8°N and 8°S (Maes et al. 2014).</figcaption></figure>
<figure><img src="image_49.png" /><figcaption>SSS Averaged from Sep 17 through Sep 27</figcaption></figure>

<figure><img src="image_50.png" /></figure>

Figure 3: SSS variability related to freshwater fluxes: (left) in the Amazon plume in Sept. 2010 (see further details in Reul et al. 2014a and Fournier et al. 2015) and (right) under rainfall (Boutin et al. 2014)

References

Brown, C. W., J. Boutin, and L. Merlivat (2015), New insights of pCO2 variability in the tropical eastern Pacific Ocean using SMOS SSS, *Biogeosciences Discuss.*, 12(6), 4595-4625, doi:10.5194/bgd-12-4595-2015.

Fournier S., Chapron B., Salisbury J., Vandemark D., Reul N. (2015), Comparison of spaceborne measurements of sea surface salinity and colored detrital matter in the Amazon plume. *Journal of Geophysical Research* - *Oceans*. http://dx.doi.org/10.1002/2014JC010109, in press.

Kolodziejczyk, N., O. Hernandez, J. Boutin, and G. Reverdin (2015), SMOS salinity in the subtropical north Atlantic salinity maximum. Part II: Horizontal thermohaline variability, *J. Geophys. Res. Oceans*, 120, 972–987, doi:10.1002/2014JC010103.

Land P., J. Shutler, H. Findlay, F. Girard-Ardhuin, R. Sabia, N. Reul, J.F. Piolle, B. Chapron, Y. Quilfen, J. Salisbury, D. Vandemark, R. Bellerby, P. Bhadury (2015), Salinity from space unlocks satellite-based assessment of ocean acidification. *Environmental Science & Technology*, 49(4), 1987-1994. http://dx.doi.org/10.1021/es504849s

Boutin, J., N. Martin, G. Reverdin, S. Morisset, X. Yin, L. Centurioni, and N. Reul (2014), Sea surface salinity under rain cells: SMOS satellite and in situ drifters observations, *Journal of Geophysical Research: Oceans*, 119(8), 5533–5545, doi: 10.1002/2014JC010070.

Grodsky S., G. Reverdin, J. Carton and V. Coles (2014), Year-to-year salinity changes in the Amazon Plume: contrasting 2011 and 2012 Aquarius/SACD and SMOS satellite data. *Remote Sensing of Environment*, 140, 14–22, doi: 10.1016/j.rse.2013.08.033.

Hasson, A., T. Delcroix, J. Boutin, R. Dussin, and J. Ballabrera-Poy (2014), Analyzing the 2010–2011 La Niña signature in the tropical Pacific sea surface salinity using in situ data, SMOS observations, and a numerical simulation, *J. Geophys. Res. Oceans*, 119(6), 3855–3867, doi:10.1002/2013JC009388.

Maes C., N. Reul, D. Behringer, and T. O’Kane (2014), The salinity signature of the equatorial Pacific cold tongue as revealed by the satellite SMOS mission, **Geoscience Letters**, 1:17 doi: 10.1186/s40562-014-0017-5

Reul N., S. Fournier, J. Boutin, O. Hernandez, C. Maes, B. Chapron, G. Alory, Y. Quilfen, J. Tenerelli, S. Morisset, Y. Kerr, S. Mecklenburg and S. Delwart (2014a), Sea Surface Salinity Observations from Space with SMOS satellite: a new tool to better monitor the marine branch of the water cycle, **Surv in Geophys**, 35:681–722, doi:10.1007/s10712-013-9244-0.

Reul, N., B. Chapron, T. Lee, C. Donlon, J. Boutin, and G. Alory (2014b), Sea surface salinity structure of the meandering Gulf Stream revealed by SMOS sensor, *Geophys. Res. Lett.*, 41, doi:10.1002/2014GL059215

Reul, N., Y. Quilfen, B. Chapron, S. Fournier, V. Kudryavtsev, and R. Sabia (2014c), Multisensor observations of the Amazon-Orinoco river plume interactions with hurricanes, *Journal of Geophysical Research: Oceans*, 119(12), 8271-8295, doi: 10.1002/2014JC010107.

Yin, X., J. Boutin, G. Reverdin, T. Lee, S. Arnault, and N. Martin (2014), SMOS Sea Surface Salinity signals of tropical instability waves, *J. Geophys. Res. Oceans*, 119(11), 7811-7826, doi:10.1002/2014JC009960.
Boutin, J., N. Martin, G. Reverdin, X. Yin and F. Gaillard (2013), Sea surface freshening inferred from SMOS and ARGO salinity: Impact of rain, *Ocean Science*, **9**, 183-192, doi: 10.5194/os-9-183-2013.

Durand F., Alory G., Dussin R., Reul N. (2013), SMOS reveals the signature of Indian Ocean Dipole events. *Ocean Dynamics*, 63:1203–1212, DOI 10.1007/s10236-013-0660-y.

Hasson, A., T. Delcroix, and J. Boutin (2013), Formation and variability of the South Pacific Sea Surface Salinity maximum in recent decades, *J. Geophys. Res. Oceans*, 118(10), 5109-5116, doi:10.1002/jgrc.20367.

Hopkins, J., M. Lucas, C. Dufau, M. Sutton, J. Stum, O. Lauret, C. Channelliere (2013), *Detection and variability of the Congo River plume from satellite derived sea surface temperature, salinity, ocean colour and sea level*. *Remote Sensing of Environment*, 139, 365-385. 10.1016/j.rse.2013.08.015

Maes C., B. Dewitte, J. Sudre, V. Garçon, and D. Varillon (2013), Small-scale features of temperature and salinity surface fields in the Coral Sea, *Journal of Geophysical Research: Oceans*, 118, 5426–5438, doi: 10.1002/jgrc.20344.

Grodsky, S. A., N. Reul, G. Lagerloef, G. Reverdin, J. A. Carton, B. Chapron, Y. Quilfen, V. N. Kudryavtsev, and H.-Y. Kao (2012), Haline hurricane wake in the Amazon/Orinoco plume: AQUARIUS/SACD and SMOS observations, *Geophysical Research Letters*, 39(20), L20603, doi:10.1029/2012GL053335.

Alory, G., C. Maes, T. Delcroix, N. Reul, and S. Illig (2012), Seasonal dynamics of sea surface salinity off Panama: The Far Eastern Pacific fresh pool, *J. Geophys. Res.*, 117, C4, doi:10.1029/2011JC007802.
# Ocean waves in Earth system sciences : towards a broad-band understanding

Fabrice Ardhuin,

Laboratoire de Physique des Océans, Brest, France

Several branches of geosciences have led to a more complete understanding of ocean wave dynamics and their connection with the atmosphere and ocean. Waves are particularly important for air-sea interactions, remote sensing, seismology, and coastal oceanography including natural hazards with coastal flooding or extreme waves. This synergy of efforts was the main topic of the IOWAGA projet. (http://wwz.ifremer.fr/iowaga/)

## Numerical modelling and ocean wave forecasting

The most widely used form of numerical wave models uses a phase-averaged spectral representation of the wave action evolution in space and time. This approach leads to a set of coupled transport equation with source terms, one for each spectral component, i.e. of the order of 1000. The main difficulties and sources of errors of this approach are the definition of parametrizations used in the source terms (e.g. Leckler et al. 2013), the quality of the forcing fields (winds, currents, sea ice ... which can be modified by wave feedback in coupled models), and the numerical method to integrate the wave action equation (WAE). Two main approaches have been used for solving the WAE, either a splitting technique, akin to the barotropic / baroclining splitting of most ocean circulation models primitive equation models (e.g. WAMDI 1988), or an implicit integration of the WAE (e.g. Booij et al. 1999). A review of these techniques and compromises particular to coastal applications is given by Roland and Ardhuin (2014), together with an update on parameterization relevant for coastal areas, in particular bottom friction. These developments have led to a major improvement in forecast accuracy, with the implementation of parameterization based on Ardhuin et al. (2010) in operational forecasting systems at Meteo-France in 2012 and NOAA/NCEP in 2013. Investigations on the impact of these parameterizations in a coupled wave-atmosphere context are under way, in order to make a smooth operation possible also at ECMWF. Improvements in coastal wave forecasts required the use of several "new" ingredients. In particular it was shown that the forcing by tidal currents could have several important effects (e.g. figure 1), and taking into account the nature of bottom sediments was important to define the bottom roughness.

<figure><img src="image_51.png" /></figure>

**Figure 1:** (a) Typical time series of wave heights at the buoy 62069 (also known as « Pierres Noires » located 20 km west of the entrance to the Bay of Brest, in 50 m depth. The observed values are represented by the black solid line. Two model results are shown, one including forcing by tidal currents (semitransparent blue), and the other without currents (red). (b) Modeled water level at the buoy, indicating the phase of the tide. Taken from Ardhuin et al. (2012).
## Coupled circulation and waves in three dimensions

Following the theoretical foundation laid in the previous decade (McWilliams et al. 2004, Ardhuin et al. 2008), the investigation of wave-induced currents in three dimensions has entered the age of practical and even operational applications. This effort has been driven by needs for predicting pollutant transport either in deep water, where the near-surface shear is dominated by the Stokes drift, all the way to the beach with water quality issues related to fecal bacteria or harmful algae. Particular difficulties remain for the formulation of a mixing scheme for phase-averaged waves and currents (e.g. Teles et al. 2013). Still, coupled 3D models have been successfully applied to coastal areas (Michaud et al. 2012) and a semi-closed stratified bays (Delpey et al. 2014). In that latter case, it was found that wave-induced momentum and mass fluxes had a strong influence on the flushing time of the 1-km diameter bay of Saint-Jean de Luz, with very different responses of the freshwater budget in 2D and 3D.

## Infra-gravity waves

Although the details of the generation processes of infragravity waves at the shore at strongly debated, the investigation of water levels at small scale and the puzzle of the seismic hum sources have led Ardhuin et al. (2014) to propose a robust parameterization for free wave generation at the shoreline. Using this, the model WAVEWATCH III (Tolman et al. 2014) was extend to low frequencies and was used to demonstrate that infragravity wave bursts off the coast of Japan originate on the U.S. West coast (Rawat et al. 2014). The same model was also used to show that the propagation of infragravity waves over a sloping bottom can explain the measured low frequency seismic noise known as hum (Ardhuin et al. 2014). This is illustrated in figure 2.

<figure><img src="image_52.png" /></figure>

Figure 2: the March 11 event is the second most energetic infragravity wave event for 2008 in the North Atlantic. The (short) wave heights at the coast exceeded 11 m with peak periods up to 25 s. This event occurred during an “earthquake-free” window and is thus clearly identified in seismic records throughout western Europe.

## Short wave properties and remote sensing

One of the difficult problems in the solutions of the WAE is to arrive at a proper balance in the short wave region (around 1 m wavelength) both in terms of energy and also in terms of its directional distribution. Parameterizations available today either assume the energy level, say proportional to the inverse fifth power of the frequency, or make assumptions on the dissipation processes. These
generally fail to produce a proper energy level, and generally produce a too narrow spectral
distribution. Since these short waves carry most of the wind stress, a better reproduction of the short
wave spectrum is necessary to arrive at a consistent definition of the air-sea momentum flux. Also,
these short waves respond most strongly to current gradients, both divergence and shear (e.g.
Kudryavtsev et al. 2005, Rascle et al. 2014), and they produce patterns in images of the sun glitter or
in radar backscatter images. Understanding these responses thus offers a unique method to measure
surface currents at near-kilometer resolution. A series of in situ experiments in strong current shears is
under way to test the shear-to-wave-gradient transfer function. A first deployment was performed in
September
2014.

<figure><img src="image_53.png" /><figcaption>Figure 3: Piece of a SAR image acquired by Sentinel-1A on September 1st, 2014. White regions correspond to stronger backscatter (higher roughness). Regions with no wind at all or with slicks appear in black. The strong tidal currents around the island of Ouessant produce various patterns in surface roughness: roughness fronts to the west of the island, and a nice "roughness mushroom" in the region of strong currents to the east of the island.</figcaption></figure>

Wave-ice interactions

Another area of intense research is the interaction of waves with sea ice, which defines the marginal
ice zone, with specific ice rheology properties that can shelter the pack ice from the wave-induced
breaking of the ice. Until recently, the quantitative analysis of wave properties in the ice required
expensive and difficult in situ missions (e.g. Kohout et al. 2014). The analysis of SAR data from
ESA's latest Sentinel 1A satellite has shown that it was possible to measure wave heights in the ice
from space, with a very high sensitivity (Ardhuin et al. 2015). The combination of this technique
with planned in situ experiments will certainly provide a wealth of data to better understand the
dissipation and scattering of waves in the ice, allowing a more accurate modelling of the marginal ice
zones.
References

Ardhuin F., E. Rogers, A. Babanin, J.-F. Filipot, R. Magne, A. Roland, A. van der Westhuysen, P. Quefeulou, J.-M. Lefevre, L. Aouf, and F. Collard, "Semi-empirical dissipation source functions for wind-wave models: part I, definition, calibration and validation," J. Phys. Oceanogr., vol. 40, p. in press, 2010.

Ardhuin, F., Dumas, F., Bennis, A.-C., Roland, A., Sentchev, A., Forget, P., Wolf, J., Girard, F., Osuna, P., and Benoit, M., “Numerical wave modeling in conditions with strong currents: dissipation, refraction and relative wind,” J. Phys. Oceanogr., 42, 2101–2120, 2012.

Ardhuin, F., Lavanant, T., Obrebski, M., Mari', ein finite depth," J. Fluid Mech., 716, 316-348, 2013.
Ardhuin, F., et al. , "A numerical model for ocean ultra low frequency noise: wave-generated acoustic- gravity and Rayleigh modes," J. Acoust. Soc. Amer., 134, 4, 3242-3259, 2013.

Ardhuin, F., Rawat, A., and Aucan, J., “A numerical model for free infragravity waves: Definition and validation at regional and global scales,” Ocean Modelling, 77, 20–32, 2014.

Ardhuin, F., Gualtieri, L., and Stutzmann, E., “How ocean waves rock the earth: two mechanisms explain seismic noise with periods 3 to 300 s,” Geophys. Res. Lett., 42, 765–772, 2015.

Ardhuin, F., Collard, F., Chapron, B., Girard-Ardhuin, F., Guitton, G., Mouche, A., and Stopa, J., “Estimates of ocean wave heights and attenuation in sea ice using the sar wave mode on sentinel-1a,” Geophys. Res. Lett., 42, 2015.

Booij, N., Ris, R. C., and Holthuijsen, L. H., “A third-generation wave model for coastal regions. 1. model description and validation,” J. Geophys. Res., 104, 7,649–7,666, Apr. 1999

Kohout, A. L., Williams, M. J. M., Dean, S. M., and Meylan, M. H., “Storm-induced sea-ice breakup and the implications for ice extent,” Nature, 509, 604–607, 2014.

Leckler, F., Ardhuin, F., Filipot, J.-F., and Mironov, A., “Dissipation source terms and whitecap statistics,” Ocean Modelling, 70, 9, 62–74, 2013.

Rascle, N., Chapron, B., Ponte, A., Ardhuin, F., and Klein, P., “Surface roughness imaging of currents shows divergence and strain in the wind direction,” J. Phys. Oceanogr., 44, 2153–2163, 2014.

Rawat, A., Ardhuin, F., Ballu, V., Crawford, W., Corela, C., and Aucan, J., “Infra-gravity waves across the oceans,” Geophys. Res. Lett., 41, 7957–7963, 2014.

Roland, A. and Ardhuin, F., “On the developments of spectral wave models: numerics and parameterizations for the coastal ocean,” Ocean Dynamics, 64, 6, 833–846, 2014.

WAMDI Group, “The WAM model - a third generation ocean wave prediction model,” J. Phys. Oceanogr., 18, 1775–1810, 1988.
# EPIGRAM, understanding the ocean dynamics along the french coasts

Y. Morel

Laboratoire d'études en géophysique et océanographie spatiale (LEGOS), Université de Toulouse;
(OMP-PCA)/IRD/CNRS/CNES 14 Av. Edouard Belin, F-31400 Toulouse, France

<figure><img src="image_54.png" /></figure>

Sea surface temperature in june (result from a realistic numerical model).

<figure><img src="image_55.png" /></figure>

Operation (SEASOAR) on board the Beautemps-Beaupré oceanographic vessel.

<figure><img src="image_56.png" /></figure>

vertical section of temperature and salinity across the Ushant front (MOUTON/FROMVAR campaigns).

## Scientific objectives and society challenges

The scientific objectives of the EPIGRAM project are :

* the realisation of campaigns at sea and the collection of data in the bay of Biscay and the Manche (english Channel);

* the scientific analysis of the collected data (furniture of diagnostics for selected processes).

* the validation of realistic numerical models of the area ;

* improving our understanding of the major physical processes of the area.

The results obtained during the project are of major importance for the scientific community: the project allowed to gather observations at sea over an area where the evolution is driven by many different physical mechanisms. Previous campaigns of such importance over the region date back to 20 years ago. In addition, EPIGRAM permitted to develop, validate and improve realistic numerical model used for operational applications, such as :

* the drift of objects, oil spills or other pollutions;

* search and rescue operations at sea;

* the production of energy from marine origin (tide, waves, currents, thermal, ...);

* the coupling of the physics with biogeochemistry for the survey of marine ecosystems, and the development of data assimilation techniques to improve the realism of the systems.

## General organisation

The project is organized in 5 major scientific themes, linked with the major physical mechanisms driving the dynamics of the bay of Biscay and the Manche. About 30 specific studies have
been identified and carried out concerning :

* the effect of tides;

* internal tides;

* seasonnal processes for the dynamics at large scale and the exchanges of water masses between the coastal and the open ocean;

* influence of the atmospheric forcings and of river plumes on the shelf dynamics;

* influence of waves on the coastal circulation.

**Scientific results and applications**

The project was carried out between january 2009 and december 2013. It yielded :

six major campaigns at sea, some of which resumed for several years, cumulating more than 100 days at sea for data collecting and technological studies;

all identified physical processes have been observed and studied with an imporved general understanding of the dynamics in the area;

* the organization of the scientific community around a common objective;

* realistic models, with pre-operationnal tests and applications (PREVIMER, MyOcean, french Navy operational system, immersion alert system), ready to be exploited;

* 40 scientific publications in international scientific journals and 7 PhD thesis.

<figure><img src="image_57.png" /></figure>
## Participation and funding

EPIGRAM is a fundamental research project which allowed a better understanding of the dynamics in the bay of Biscay and the Manche. It gathered 13 public laboratories publics depending on 5 institutions (CNRS, Ifremer, MétéoFrance, SHOM, Universities). The project was funded by ANR and received 490.000,00€. The total amount of the project is evaluated around 7.000.000,00€.
# Spontaneous emergence of low-frequency variability in the GLOBAL EDDYING Ocean

Thierry Penduff<fnref n="1" />, Guillaume Sérazin<fnref n="1" />,<fnref n="2" />, Sandy Grégorio<fnref n="1" />, Stéphanie Leroux<fnref n="1" />, Laurent Bessières<fnref n="2" />, Jean-Marc Molines<fnref n="1" />, Bernard Barnier<fnref n="1" />, Laurent Terray<fnref n="2" />.

## Abstract

Oceanographers are studying the ability of the global turbulent ocean to generate chaotic variability at climate scales (1-90 years) spontaneously, i.e. without atmospheric variability beyond the annual timescale. Building on previous idealized studies, high-resolution global ocean/sea-ice simulations reveal in a realistic context the intensity and complex structure of this phenomenon, raising the question of its origin, its possible impact on climate, and the actual constraint exerted by the atmosphere on the oceanic variability.

## Introduction

So-called mesoscale eddies are ubiquitous in the ocean, and are well-known examples of small-scale (from a few tens or hundreds of kilometers, from weeks to months) intrinsic ocean variability: they emerge from the instability of the main currents without any atmospheric variability. This mesoscale variability is highly chaotic: just as their atmospheric counterparts, oceanic cyclones and anticyclones are turbulent, which makes their evolution difficult to predict. Idealized ocean simulations have also shown that under certain conditions, the non-eddying general ocean circulation may develop intrinsic variability at much larger scales (up to multiple decades, at basin-scale), under constant or purely seasonal atmospheric forcing (e.g. Dijkstra & Ghil, 2005; Sushama et al, 2007; Shimokawa & Matsuura, 2010). Ongoing research making use of state-of-the-art eddying ocean general circulation models shows that the low-frequency variability of the ocean circulation (at timescales of years to decades) also has a partially chaotic (i.e. stochastic) character, at spatial scales reaching thousands of kilometers.

## Low-Frequency Intrinsic Variability (LFIV)

A numerical model of the global ocean and sea-ice, developed by the DRAKKAR consortium, was used in 2008 to simulate the 3-dimensional evolution of the general circulation and mesoscale during 327 years at high resolution (¼° horizontal mesh, i.e. 10-25 kilometers). This simulation was forced climatologically: it was driven every year by exactly the same atmospheric mean annual cycle (solar radiation, wind, air temperature and humidity, etc). Despite the absence of any atmospheric fluctuation slower than one year, this virtual ocean spontaneously produced a strong variability of currents and water masses, with timescales reaching several decades. The imprint on sea-surface height of this oceanic Low-Frequency Intrinsic Variability (LFIV) was described in the global realistic setup by Penduff et al (2011), focusing on interannual timescales and on comparisons with altimeter observations. Various features of this phenomenon were consistent with previous idealized studies.

Figure 1 summarizes a few results from subsequent studies (Sérazin et al, 2015; Grégorio et al, 2015) conducted within the CHAOCEAN project supported by CNES and NASA; the estimated percentage of low-frequency oceanic variance due to LFIV is shown for 4 variables. When mesoscale turbulence is simulated (horizontal resolutions of ¼° or 1/12°), this percentage locally reaches 80-100% for sea-surface height and temperature (50-80% for large scales), in particular around Antarctica and mid-latitudes western boundary currents. The low-frequency variability of the Atlantic Meridional Overturning Circulation (AMOC) intensity and of the associated heat transport is also substantially impacted by LFIV around 30°S (around 50% and 25% of their variance, respectively). This AMOC LFIV has scales reaching several decades and the size of the whole Atlantic basin. These results are consistent with idealized simulations and provide new information about the LFIV's actual intensity

<footnote n="1">¹MEOM-LGGE (UMR5183), Grenoble, France</footnote>

<footnote n="2">² SUC-CERFACS (URA1857), Toulouse, France.</footnote>
and structure in a realistic context. In regions where this percentage exceeds 50%, the nonlinear oceanic processes resolved in ¼° and 1/12° simulations thus account more than the atmospheric low-frequency fluctuations in the production of low-frequency variability. The processes at work are currently being studied in collaboration with dynamicists (e.g. Huck et al, 2015; Arbic et al, 2014).

The possible "climatic" relevance of these results may be summarized in two points. The LFIV is virtually nonexistent when mesoscale is not resolved, such as in 2°-resolution laminar ocean models (see the red line corresponding to the AMOC, Figure 1) that were largely used in recent climate projections. Moreover, the strong LFIV emerging in eddying ocean models with climatological forcing has an irregular and intermittent behavior; this low-frequency oceanic "noise" has an imprint on large-scale surface temperatures (Figure 1) in regions where air-sea fluxes are strongest in Nature, and may therefore influence the atmosphere and climate in future coupled climate projections (which will use ¼° ocean models). This potential impact remains to be assessed, but the presence of oceanic LFIV will likely increase the complexity and richness of future projections.

<figure><img src="image_58.png" /><figcaption>Figure 1: Percentage of interannual variance (1 to 11 year timescales) due to intrinsic processes. Maps are derived from ¼° simulations and correspond to sea-level (SLA, top) and sea-surface temperature (SST, bottom), considering all resolved spatial scales (left) and scales larger than 1000 km (right). Right panels show the same percentage in the Atlantic for the overturning circulation intensity (AMOC) and the associated heat transport (MHT); AMOC results are given for 2°, ¼° and 1/12° global simulations (Sérazin et al, 2015; Grégorio et al, 2015).</figcaption></figure>

## Ensemble simulations

These results raise oceanographic questions as well: is the emergence of LFIV in the turbulent regime able to make the low-frequency oceanic variability less deterministic than we thought? May small perturbations or uncertainties in initial oceanic conditions grow, evolve and organize themselves over yearly or decadal timescales? What then are the respective imprints of chaotic LFIV and atmospheric variability on the ocean variability measured by satellites and in-situ instruments? To address these questions and anticipate the analysis of future coupled simulations, oceanographers and climatologists from LGGE and CERFACS are preparing a turbulent ocean/sea-ice ensemble simulation, the first of its kind. In 2015, fifty ¼° global ocean/sea-ice simulations will be performed in parallel over the period 1958 to 2014, after a slight perturbation of their initial states. The main objectives of this ANR project (OCCIPUT) supported by a PRACE computational allocation, is to examine in detail the structure and evolution of the ensemble mean (deterministic) and ensemble dispersion (chaotic) of key quantities, and to identify the variables and regions where the LFIV dominates that forced by the atmosphere.

A prototype of this global 57-year simulation, consisting of 10 members integrated between 1993 and 2012 in the North Atlantic, has recently provided original results. Figure 2 shows the sea-
surface temperature (SST) interannual variability (1993-2000) forced by the atmosphere (left), generated by intrinsic variability in 2000 (middle), and the ratio of these two fields (right). Like Figure 1, this third panel shows that along the Gulf Stream extension, near Newfoundland and Norway, chaotic intrinsic processes generate more interannual SST variability than the atmospheric interannual variability does. Ongoing analyzes indicate that these contributions depend on depth, on the variable considered, and fluctuate temporally. The 50-member ensemble simulation planned in 2015 is presented in more detail in Penduff et al (2014): it will allow us to quantify more precisely the impact of this chaotic phenomenon in the global ocean, up to decadal scales, and its modulation by the low-frequency forcing.

<figure><img src="image_59.png" /><figcaption>Figure 2: Atmospherically-forced and intrinsic interannual variability of sea-surface temperature (SST) from the OCCIPUT regional prototype (10 ensemble members). Colors show the interannual standard deviation of the SST ensemble mean over 1993-2000 (variability forced by the atmosphere or "signal", left); the ensemble standard deviation computed from the 10 SST fields averaged in 2000 (LFIV in the middle); the signal-to-noise ratio in decibels (right; the interannual variability is mostly intrinsic in yellow and red areas). Contours: SST ensemble mean averaged over 1993-2000 (left panel) and in 2000 (middle and right panels).</figcaption></figure>

## Acknowledgements

The climatological ¼° and 1/12° simulations were performed by DRAKKAR at CINES thanks to a GENCI allocation, and as part of the 2013 GENCI/CINES *Grand Challenges*, respectively. This article is a contribution to the CHAOCEAN and OCCIPUT projects. CHAOCEAN is supported by the Centre National d'Etudes Spatiales (CNES) through the Ocean Surface Topography Science Team (OST/ST). OCCIPUT is supported by the Agence Nationale de la Recherche (ANR, contract ANR-13-BS06-0007-01, through GENCI and PRACE allocations (Project RA2531).

## References

Arbic, B.K., M. Müller, J.G. Richman, J.F. Shriver, A.J. Morten, R.B. Scott, G. Sérazin, and T. Penduff, 2014: Geostrophic turbulence in the frequency-wavenumber domain: Eddy-driven low-frequency variability. J. Phys. Oceanogr., 44, 2050–2069. doi:10.1175/JPO-D-13-054.1.

Grégorio, S., T. Penduff, G. Sérazin, J. Le Sommer, J.-M. Molines, and B. Barnier, 2015: Intrinsic variability of the Atlantic Meridional Overturning Circulation at interannual-to-multidecadal timescales. J. Phys. Oceanogr., to appear.

Huck, T., O. Arzel, and F. Sévellec, 2015. Multidecadal Variability of the Overturning Circulation in Presence of Eddy Turbulence. J. Phys. Oceanogr., Jan. 2015, Vol. 45, No. 1: pp. 157-173. DOI: 10.1175/JPO-D-14-0114.1

Penduff, T., M. Juza, B. Barnier, J. Zika, W.K.Dewar, A.-M. Treguier, J.-M. Molines, and N. Audiffren, 2011: Sea-level expression of intrinsic and forced ocean variabilities at interannual time scales. J. Climate, 24, 5652–5670. doi: 10.1175/JCLI-D-11-00077.1.

Penduff, T., B. Barnier, L. Terray, L. Bessières, G. Sérazin, S. Grégorio, J.-M. Brankart, M.-P. Moine, J.-M. Molines, and P. Brasseur, 2014: Ensembles of eddying ocean simulations for climate. CLIVAR
Exchanges, Special Issue on High Resolution Ocean Climate Modelling, 65, Vol 19 No.2, July 2014.
[Link](http://dx.doi.org/10.1175/JCLI-D-14-00554.1)

Sérazin, G., T. Penduff, S. Grégorio, B. Barnier, Jean-Marc Molines, and L. Terray 2015 : Intrinsic variability of sea level from global 1/12° ocean simulations: spatio-temporal scales. J. Climate, 28, 4279-4292, http://dx.doi.org/10.1175/JCLI-D-14-00554.1

Shimokawa, S. and Matsuura, T., 2010: Chaotic behaviors in the response of a qua- sigeostrophic oceanic double gyre to seasonal external forcing. Journal of Physical Oceanography, 40(7):1458–1472.

Sushama, L., M. Ghil, and K. Ide, 2007: Spatio-temporal variability in a mid-latitude ocean basin subject to periodic wind forcing. Atmos.–Ocean, 45, 227–250.
# Dansgaard-Oeschger cycles

O. Arzel and A.Colin de Verdière

Laboratoire de Physique des Océans, Brest, France

Measurements of oxygen isotope ratio in air bubbles trapped in Greenland ice cores have revealed the presence of large climate shifts, known as Dansgaard-Oeschger events (DO hereafter), that are ubiquitous during the past 8 glacial cycles (Loulergue et al. 2008). These dramatic events are associated with strong (8-16°C) and rapid (decadal) warming of the high latitudes climate of the North Atlantic (Fig. 1). The sequence of the events starts with an abrupt warming followed by a plateau phase (interstadial) which lasts several centuries, and an abrupt decline back to cold (stadial) conditions. Then the climate slowly warms until a new abrupt warming event occurs to complete a cycle in a period of about 1,500 years. Interglacial periods by contrast are more stable and do not exhibit such abrupt climate shifts. Of course, observations remain too sparse both in time and space to provide a quantitative understanding of the physical mechanisms at play. Nevertheless, the relatively long time scales involved in these climate variations suggest a potential role of the deep ocean circulation: if we divide the volume $V=4\times10^{17}$ m$^3$ of the Atlantic basin by the strength $\Psi=15$ Sv of the Atlantic Meridional Overturning Circulation (AMOC), an advective timescale of about 1000 years emerges. However a number of physical mechanisms allowing quasi-periodic transitions between different modes of operation of the AMOC have been proposed. Among them one must distinguish between those where abrupt millennial climate shifts result from changes in external forcing (e.g freshwater cycle, solar cycle) from those where either internal instabilities of the large-scale ocean circulation or nonlinear sea ice - ocean - ice sheet interactions play a fundamental role.

<figure><img src="image_60.png" /><figcaption>**Fig. 1** Time-evolution of GRIP ice core $\delta^{18}$O. The peaks represent DO events. Warmer climates are associated with higher $\delta^{18}$O.</figcaption></figure>

External causes mostly point to calving due ice-sheet instabilities that can deliver massive amounts of meltwater to the North Atlantic ocean, and therefore strongly perturb the ocean circulation and ultimately the northward heat transport that contributes to the maintenance of the climate. It is however not clear whether these massive icebergs discharges are a cause or a consequence of changes in the AMOC. Following this idea, abrupt transitions between warm interstadials and cold stadials have first been obtained in ocean climate models in response to imposed periodic fluctuations in freshwater forcing, but only when the magnitude of the forcing is large enough (Ganopolski and Rahmstorf, 2001). The origin of the 1,500 yr periodicity was left unspecified but it was suggested that it might be related to solar forcing as suggested by Braun et al. (2005) for instance. Muscheler and Beer (2006) did not however find any convincing persistent solar influence on DO cycles. An alternative mechanism called stochastic resonance, a process that results from the combination of a small periodic forcing and noise, was proposed to explain the origin of DO cycles (see Monahan et al. (2008) for a critical review of this theory). On different grounds, Timmermann et al. (2003) argued that the absence of abrupt millennial oscillations during interglacial periods is intimately linked to the absence of the Laurentide ice-sheet during those periods. They suggested that DO events are the joint result of stochastic forcing and Heinrich events through the coherence resonance mechanism, and
offered thus a dynamical basis to explain the observation that Heinrich events are followed by a series
of DO cycles. Interactions between the large-scale atmospheric circulation and ice-sheet topography
has also been proposed as a candidate for DO events (Wunsch 2006).

By contrast, a number of studies conducted with models varying in complexity from
conceptual to intermediate have revealed internal instabilities of the AMOC on millennial timescales
without the need for either time-varying external forcing, atmospheric noise or ice sheet-ocean
interaction. It is thus on this specific ground that A. Colin de Verdière, O. Arzel and co-authors have
developed a new theory, published in a series of 6 papers between 2006 and 2013, that has contributed
to elucidate the question as to why the AMOC during cold glacial periods is less stable than during
warm interglacials. In this context, the authors propose that the origin of DO cycles is intimately
linked to the coolness of the climate which ultimately reduces the oceanic heat transport through the
temperature stratification. In what follows, a brief history of the mechanisms upon which this idea has
been developed is presented.

Winton (1993) was the first to obtain abrupt millennial variability using an idealized ocean
model. The “deep-decoupling” oscillations thus simulated feature the sawtooth shape profile
reminiscent of DO cycles and are characterized by a deep-coupled (advective) phase with strong
meridional circulation and heat transport, and a deep-decoupled (diffusive) phase with reduced
meridional circulation and heat transport. In this scenario, the heat accumulated for centuries in the
deep ocean during the weak phase of the oscillation is released on a short time period to the
atmosphere through a convective instability of the polar water column. In such a mechanism, which
requires both advective and convective processes, the circulation oscillates between two *unstable*
states. This is very different from oscillations resulting from periodic changes in freshwater forcing
(e.g. Ganopolski and Rahmstorf, 2001) that require only advective processes, and can as such be easily
reproduced in the Stommel (1961) box model where only advective processes are present and the
circulation oscillates between two *stable* states. When convection is added to the Stommel box model,
DO type oscillations emerge naturally and Colin de Verdière (2006, 2007) was able to extract and
rationalize the bifurcation sequence of these deep-decoupling oscillations with the freshwater forcing
as the main control parameter. If the freshwater forcing is strong enough and polar precipitation low
enough, the circulation experiences abrupt climate shifts on millennial timescales. Since climate
reconstructions suggest weaker hydrological cycles during glacial periods, in agreement with what is
expected from the Clausius-Clapeyron equation, this theory implies that the stability of glacial
climates would be higher than that of interglacials. However glacial climates are punctuated by major

<figure><img src="image_61.png" /><figcaption>**Fig. 2** Time series of the maximum strength of the MOC simulated by the UVic model (idealized geometry) when both the atmospheric CO2 concentration and the land ice edge in the Northern Hemisphere (black line) slowly evolve, over a 100,000-yr years period, from warm (CO2=280 ppm; no land ice) to cold climate conditions (CO2=200 ppm; land ice edge at 50°N). The red line shows the MOC response when the atmospheric CO2 concentration is fixed to its modern value, while the land ice edge slowly moves southward from the northern domain boundary (i.e., 84°N) to 50°N. The blue line shows the AMOC response when land ice is absent and when the atmospheric CO2 concentration slowly decreases from its modern to its glacial value. From Arzel et al. (2010).</figcaption></figure>
reorganizations on millennial timescales while warmer interglacials, such as the actual Holocene, are characterized by a remarkable placidity. Therefore if deep-decoupling oscillations are to be the key to observed DO oscillations, another source of instability than the freshwater forcing per se must necessarily be at play. A first possibility emerged when Winton (1997) showed that cold climates are indeed less stable than warm climates. The nonlinearity of the seawater equation of state was shown to be the source of the instability: a weaker thermal expansion coefficient at lower temperatures implies a stronger sensitivity of polar waters to freshwater forcing. By contrast, using a linear equation of state, Loving and Vallis (2005) reproduced DO type oscillations in an idealized ocean-atmosphere-sea ice model for sufficiently cold background conditions. Here the control parameter was atmospheric infrared emissivity rather than freshwater forcing.

The origin of the oscillations obtained by Loving and Vallis (2005) was unraveled by Colin de Verdière and te Raa (2010) with a help of a simple 2D model. It was shown that it is the weakening of the oceanic heat transport, which is the major regulator of the circulation, under cold background conditions that is the key to the existence of such instabilities. A weaker northward heat transport is achieved because of the weaker temperature stratification induced by surface cooling, the temperature of the deep ocean remaining close to that of polar surface waters lying at proximity of the sea ice edge. Imagine that the circulation weakens, then the sea surface temperature gradient increases to bring the circulation back on track. The ability of the circulation to resume therefore depends on the ability of the AMOC to transports heat northwards. Climates with weaker Atlantic northward heat transport tends therefore to be associated with less stable AMOC, and are therefore prone to abrupt millennial variability. This is particularly obvious when the climate slowly cools from warm to cold background conditions (Fig. 2). This experiment, conducted with the UVic coupled (ocean-atmosphere-sea ice) model set-up in an idealized Atlantic geometry, further indicates that the existence of millennial oscillations primarily depends on the land ice extent (ice-sheet), with the CO2 levels playing a secondary role (Arzel et al. 2010). Climbing a step further up in the model hierarchy, Arzel et al. (2012) reach the same conclusions using a realistic geometry of the UVic coupled model (Fig. 3). The comparison of the bifurcation structure of the AMOC between climates using different background conditions clearly indicate that the existence of DO-type oscillations (Fig. 4) in the model is intimately related to the strength of the northward oceanic heat transport in the Atlantic basin (Fig. 5). More recently, Arzel and England (2013) showed that wind-stress feedback is an efficient amplifier of abrupt millennial variability, through ocean-atmosphere-sea ice interactions. Of course, which physical mechanism lies at the heart of the existence of glacial variability is unknown. Much modeling effort will be needed to determine if any of the mechanisms discussed here could present any relevance for glacial climate variability. Testing the robustness of the newly developed theory in climate models of increasing complexity would certainly be very informative.
<figure><img src="image_62.png" /><figcaption>**Fig. 3** Range of background climate states for which abrupt millennial-scale climate transitions between two *unstable* states (indicated by crosses) of the AMOC. Glacial climates used ice-sheet topography at 19 kyr BP. The stability of the circulation is assessed in terms of CO2 levels and strength of the surface freshwater forcing (a globally-compensated freshwater anomaly is superimposed to the simulated freshwater forcing in the 20-50°N latitude band over the Atlantic ocean). Both the preindustrial and the warmest glacial climates are stable, while the other ones exhibit DO-type oscillations in a range of freshwater forcing. From Arzel et al. (2012).</figcaption></figure>

<figure><img src="image_63.png" /><figcaption>**Fig. 4** Timeseries of the strength of the AMOC (Sv) in a numerical simulation of the UVic model showing abrupt millennial oscillations (left) and surface air temperature change during an abrupt event (right). The relatively weak changes in sea ice extent during a stadial-interstadial transition could explain the relatively weak SAT change compared to those observed. This could be (at least partly) due to the absence of change in surface wind-stress in the model. This aspect has been explored by Arzel et al. (2013). From Arzel et al. (2012).</figcaption></figure>
<figure><img src="image_64.png" /></figure>

**Fig. 5** Comparison of Atlantic northward heat transports between different climates (the same freshwater forcing is used, i.e. -0.29 Sv, see Fig. 3). The upper right panel highlights an abrupt transition in the maximum heat transport. This transition marks the limit between climate states exhibiting DO-type oscillations and those that are stable (see Fig. 3). From Arzel et al. (2012).

**References**

Arzel, O. and Colin de Verdière, A. and M. H. England (2010) The role of oceanic heat transport and wind-stress forcing in abrupt millennial-scale climate transition, J. Clim, 23, 2233-2256

Arzel, O. et al. (2012) Abrupt millennial variability and interdecadal-interstadial oscillations in a global coupled model: sensitivity to the background climate state, Clim. Dyn., 39, 259-275

Arzel, O. and M. H. England (2013): Wind-stress feedback amplification of abrupt millennial-scale climate changes, Clim. Dyn., 40, 983-995

Braun, H. et al. (2005) Possible solar origin of the 1,470-year glacial climate cycle demonstrated in a coupled model, Nature, 438, 208-211

Colin de Verdière, A (2006) Bifurcation structure of thermohaline millennial oscillations, J. Clim., 19, 5777-5795

Colin de Verdière, A (2007) A simple model of millennial oscillations of the thermohaline circulation, J. Phys. Oceanogr., 37, 1142-1155

Colin de Verdière, A. and L. te Raa (2010) Weak oceanic heat transport as a cause of the instability of glacial climates, Clim. Dyn., 35, 1237-1256

Ganopolski, A. and S. Rahmstorf (2001) Rapid changes of glacial climate simulated in a coupled climate model, Nature, 409, 153-158

Loulergue, L. et al (2008) Orbital and millennial-scale features of atmospheric CH4 over the past 800,000 years, Nature, 453, 383-386

Loving, J. L. and G. K. Vallis (2005) Mechanisms for climate variability during glacial and interglacial periods, Paleoceanogr., 20, doi:10.1029/2004PA001113

Monahan, A. H. and J. Alexander and A. Weaver (2008) Stochastic models of the meridional overturning circulation: time scales and patterns of variability, Philo. Trans. Royal. Soc., 366, 2527-2544

Muscheler, R. and J. Beer (2006) Solar forced Dansgaard/Oeschger events ? Geophys. Res. Letters, 33, 10.1029/2006GL026779

Stommel, H. (1961) Thermohaline convection with two stable regimes of flow, Tellus, 13, 224-230

Timmermann, A. et al. (2003) Coherent resonant millennial-scale climate oscillations triggered by massive meltwater pulse, J. Clim., 16, 2569-2585

Winton (1993) Thermohaline oscillations induced by strong steady salinity forcing of ocean general circulation models, J. Phys. Oceanogr., 23, 1389-1410
Winton (1997) The effect of cold climate upon North Atlantic Deep Water formation in a simple ocean-atmosphere model, J. Clim., 10, 37-51

Wunsch, C. (2006) Abrupt climate change: an alternative view, Quaternary Research, 65, 191-203
# On the Reference Level Problem in Oceanography

A. Colin de Verdière, M. Ollitrault

Laboratoire de Physique des Océans, Brest, France

As is well known, the determination of the large scale geostrophic circulation from hydrology (temperature-salinity distribution) is incomplete and requires the knowledge of the velocity field or geopotential at one level, the so called level of reference. Lacking this at ocean basin scales, a number of assumptions have been used in the past to provide inverse solutions such as minimum kinetic energy at the reference level, quasi-adiabatic flows, Sverdrup balance, conservation of large scale potential vorticity to name just a few (see Wunsch, 1996). Nowadays adjoint methods allow to integrate data from very different nature into fluid mechanics model equations producing time dependent three dimensional large scale oceanic flows (see .e.g. Mazloff et al., 2010 with results from the ECCO MIT model).

However with the increasing space-time coverage of Argo float displacements, new direct methods are now possible. Indeed a high quality data base of float displacements, named ANDRO has been generated by Ollitrault and Rannou, 2013 from which it is possible to infer the time mean circulation of the World Ocean on 1° x 1° squares around 1000 dbar with unprecedented details (see Ollitrault and Colin de Verdière, 2014 for an extended description). Now to use that information to provide a reference level and obtain the full three dimensional circulation with the help of the temperature-salinity distribution has required a novel approach. The question is to find out how to obtain the geopotential from the time mean velocity fields. Away from the tropics, both are linked by the geostrophic relation:

$$ 
\begin{aligned}
-fv &= -\frac{1}{\arccos\theta} \frac{\partial\Phi}{\partial\lambda} & (a) \\
+fu &= -\frac{1}{a} \frac{\partial\Phi}{\partial\theta} & (b)
\end{aligned}
\qquad (1) $$

where $\Phi$ is the geopotential of the relevant pressure surface (here typically 1000 db), $\lambda$ is longitude, $\theta$ is latitude, $a$ is the earth radius assumed constant ($a = 6370$ km) and $f = 2 \Omega \sin\theta$ is the Coriolis parameter. In this isobaric formulation the derivatives on the right are carried out at constant pressure. Taking the curl of (1) gives:

$$ \frac{\partial(fu)}{\partial\lambda} + \frac{\partial(fv \cos\theta)}{\partial\theta} = 0 \quad \text{or} \quad \nabla_H \cdot (f \cdot u) = 0 \qquad (2) $$

with the horizontal divergence in spherical coordinates given by:

$$ \nabla_H \cdot (...) = \frac{1}{\arccos\theta} \left[ \frac{\partial}{\partial\lambda}(...) + \frac{\partial}{\partial\theta}(\cos\theta...) \right] $$

The naïve use of equation (1a) to get $\Phi$ by integrating zonally the v velocity introduces a constant of integration function of $\theta$. To get that function of $\theta$, equation (1b) is used in turn but for this method to work, a compatibility condition is required which is precisely that (2) be satisfied, but unfortunately it is not... Why? Although individual float displacement which represent ten days nominal average of the velocities are geostrophic to a good approximation, there is little hope that the time mean velocities satisfy this consequence of geostrophy represented by equation (2). There are two reasons for this: first (2) is a vorticity equation which has an accuracy inferior to the Rossby number accuracy of (1) and second the number of float days in each square box to compute the time averages is inhomogeneous as it varies from a few months to several years. Because of this underlying inhomogeneity, the divergence of the ANDRO mean velocity fields is $O(U/\Delta x)$ while the divergence from (2) is smaller of $O(U/\text{earth radius})$ only. It is therefore not possible to use the equations (1) as they stand to get $\Phi$.
To obtain the geopotential $\phi$ from the velocity fields, the solution developed by Ollitrault and Colin de Verdière, 2014 has been to take the divergence of equations (1). By differentiating (1a) $\times$ cos $\theta$ with respect to $\lambda$ and (1b) with respect to $\theta$, one obtains:

$$ \frac{\partial^2 \Phi}{\partial \lambda^2} + \frac{\partial^2 \Phi}{\partial \theta^2} = a \left[ \frac{\partial(fv \cos \theta)}{\partial \lambda} - \frac{\partial(fv)}{\partial \theta} \right] \quad (3) $$

This equation is nothing but a special form of the divergence of the horizontal momentum equation which is at the heart of quasi-geostrophic inversions from vorticity to stream function. An important difference with quasi-geostrophy, however, is that here the geopotential is not a streamfunction since the flow is horizontally divergent [see (2)]. Equation (3) is the equation of balance for planetary geostrophic dynamics. It has been mentioned first by Charney, 1955 in the context of numerical weather prediction. The regimes of validity of this equation have been discussed by Gent and McWilliams (1983) who show that it conserves the Rossby number accuracy of the momentum equations. This Poisson equation requires boundary conditions at the edges of the domain under consideration. Near solid boundaries, the geostrophic relation becomes a poor approximation for the velocity component normal to the coast. Since it goes to zero at the coast, there is no Coriolis force left to balance the along shore pressure gradients and non linear terms must be considered. On the other hand the velocity along the coast can still be assumed geostrophic to a good approximation. Indeed the Florida and Gulf Stream transports have been estimated early using cross-shore hydrographic sections (Wüst 1924 e.g.). Thus if $\mathbf{n}$ is the outward unit normal at the edge of the domain and $\mathbf{s}$ the tangential vector such that $\mathbf{s} = \mathbf{k} \times \mathbf{n}$, (with $\mathbf{k}$ the upward unit vector), the appropriate geostrophic boundary condition to be used in conjunction with (3) reads:

$$ \tilde{\nabla} \Phi \cdot \mathbf{n} = \mathbf{u}^* \cdot \mathbf{s} \quad (4) $$

with $a \cdot \tilde{\nabla} = (\frac{\partial}{\partial \lambda}, \frac{\partial}{\partial \theta})$ and $\mathbf{u}^* = (\text{fu}, \text{fv} \cos \theta)$.

The condition (5) is a Neumann type boundary condition for the elliptic Poisson problem (3). Observed along coast velocities are used to find the normal pressure gradient that is imposed to the interior. Imposing instead a Dirichlet condition, that is a constant geopotential would assume that geostrophy remains valid for velocities normal to the coast, which is generally not true. The solution is defined up to an arbitrary constant provided a consistency relation exists between the forcing of (3) and the boundary condition (4). This consistency relationship is obtained by integrating (3) over the domain $D$. In the two dimensional $(\lambda, \theta)$ Cartesian space (3) can be rewritten as:

$$ \tilde{\nabla}^2 \Phi = \mathbf{k} \cdot \tilde{\nabla} \times \mathbf{u}^* \quad (5) $$

Using the divergence theorem on the left hand side and Green's theorem on the right hand side, the domain integral $\iint_D$ (5) $d\lambda d\theta$ becomes:

$$ \int_{\partial D} \tilde{\nabla} \Phi \cdot \mathbf{n} ds = \int_{\partial D} \mathbf{u}^* \cdot \mathbf{s} ds \quad (6) $$

Use of the boundary condition (4) therefore guarantees that the compatibility condition (6) is satisfied. In practice the interior of the domain $D$ will be defined by those points where the Cartesian vorticity $\tilde{\nabla} \times \mathbf{u}^*$ has been estimated, which are adjacent either to a solid boundary or to a data hole. Note that the problem has no singularity at the equator and can therefore be solved directly for the World Ocean. Of course as the equator is approached, the Coriolis acceleration vanishes and so does the pressure gradient. The geopotential $\Phi$ ($\text{m}^2 \text{s}^{-2}$) so estimated on the mean 1000 dbar surface is given below as $\delta z = 100 \Phi / g_c$ i.e. in cm height (with $g_c = 9.80665 \text{ m s}^{-2}$).
<figure><img src="image_65.png" /></figure>

Note the strong inertial recirculations of the subtropical gyres, with the Agulhas gyre being the strongest, the powerful Antarctic circumpolar current with its western boundary current type meridional excursions downstream of New Zealand and Drake Passage and finally the strong anticyclonic Zapiola gyre in the Argentinian basin. There is also a weak cyclonic cell in the region of the Mediterranean water plume. Note finally that the velocity data alone is able to show that the Pacific (and Indian) ocean is higher than the Atlantic.

Once the geopotential is found, geostrophic velocities are found from (1) and now these velocities satisfy the vorticity equation (2). The whole operation amounts to project the velocity data on the geostrophic mode thereby filtering out the unwanted divergent motions. It is now possible to obtain the geopotential and the geostrophic velocities at all pressures using hydrostatics $\partial\Phi/\partial p = -1/\rho$ where $\rho$ is obtained from a temperature-salinity climatology. The absolute mean topography of the ocean surface shown below has been obtained with the help of the World Ocean Atlas 2009 (Locarnini et al. 2010).

Mean absolute geopotential height (cm) at 0 m

<figure><img src="image_66.png" /></figure>
References

Charney, J., 1955: The use of the primitive equations of motions in numerical prediction, Tellus, VII, 22-26.

Gent, P. and J. C.McWilliams, 1983: Regimes of validity for balanced models, *Dyn. Atmos. and Oceans*, **7**, 167-183.

Mazloff M. R., P. Heimbach, and C. Wunsch, 2010: An eddy permitting Southern Ocean Estimate, *J. Phys. Oceanogr.*, **40**, 880-899.

Locarnini, R. A., A. V. Mishonov, J. I. Antonov, T. P. Boyer, H. E. Garcia, O. K. Baranova, M. M. Zweng, and D. R. Johnson, 2010. World Ocean Atlas 2009, Volume 1: Temperature. S. Levitus, Ed. NOAA Atlas NESDIS 68, U.S. Government Printing Office, Washington, D.C., 184 pp.

Ollitrault, M. and J.P. Rannou, 2013: ANDRO: An Argo-based deep displacement dataset. *J. Atmos. Oceanic Technol.*, **30**,759-788.

Ollitrault, M., A. Colin de Verdière, 2014, The Ocean General Circulation near 1000m depth, *J. of Phys. Oceanogr.* **44**, 384-409.

Wunsch C, 1996: The ocean circulation inverse problem, Cambridge University Press.

Wüst, G. 1924: Florida und Antillenstrom, eine hydrodynamische untersuchung. *Veröff. Inst. Meeresk. Univ. Berl.* (A) 12, 5-49.
An international initiative:
# South Atlantic Meridional Overturning Circulation (SAMOC)
## Years 2011–2014

Sabrina Speich LPO-IUEM (1011-2013) & LMD-IPSL (since 2014); Silvia Garzoli (NOAA, AOML, USA); Alberto Piola (Nat. Hydrog. Institute, Argentina); Edmo Campos (Brazil)

Participating Laboratories: LPO, France; LMI ICEMASA (France South Africa), Univ. of Cape Town, South Africa; CSIR, Cape Town, South Africa; NOAA, USA; Univ. of San Paolo, Brazil; AWI, Germany; Hydrographic Institut, Buenos Aires, Argentina.

Contribution to CLIVAR, SAMOC, ARGO International, SOCCO (South Africa), SOCLIM (BNP-ParisBas)

Funding sources: ANR, Ifremer, INSU (France), FAPESP (Brazil), NOAA (USA), SANAP & DEA (South Africa), National Hydrographic Institute, Argentina.

**Objectives:**

Earth's climate, responding to the different thermo-dynamic properties of the land and ocean surfaces, is sensitive to the continental configuration. Indeed, because of the ocean- land configuration, each ocean basin is exposed to different atmospheric forcing, taking on correspondingly distinct property and circulation characteristics. These in turn provide feedback to the climate system through their effect on Sea Surface Temperature (SST) distribution, heat and freshwater fluxes and ocean overturning. Similarity between oceans is inhibited by their varied degrees of isolation from one another, and thus the coupled ocean-atmosphere system is influenced by the efficiency of interocean exchanges that link the ocean basins. The Southern Ocean and the South Atlantic are critical nexus for the global ocean circulation and climate. They connect efficiently North Atlantic and Arctic water masses with the rest of the world ocean. Limited observations suggest that the Southern Ocean and South Atlantic are changing. They are warming more rapidly than the global ocean average. However, the short and incomplete nature of existing data means that the causes and consequences of observed changes are difficult to assess. Sustained observations are required to detect, interpret and respond to changes. Within the South Atlantic Meridional Overturning Circulation (SAMOC) international initiative we have initiate a monitoring array experiment devoted to assess the capability of deep moored arrays together with Argo profiling floats and remote sensed data to 1) estimate the Meridional Overturning Circulation in the South Atlantic; 2) infer regional water masses transformation, pathways and variability 3) apprehend dynamical processes driving water masses exchanges and transformations. The profiling Argo floats obtained under the LEFE GMMC-Coriolis programme have been deployed within SAMOC to achieve such objectives. In addition, because of the knowledge on regional dynamics and water masses acquired since the Clivar GoodHope programme, we were able to start implement new method for Argo data validation and calibration.

**Main results**

Variations in the Meridional Overturning Circulation (MOC) are known to have global implications to the climate system, however until recently most MOC observing programs have been focused in the North Atlantic. Recent model and data analyses have suggested that critical water mass changes to the upper and lower limbs of the MOC occur in the South Atlantic, and only limited latitudinal coherence has been found to date between the MOC observations made by the North Atlantic observing systems at different latitudes. As a result, a priority for a the ocean science community involved in field experiments has been the establishment of a MOC observing system in the South Atlantic, and recently the International CLIVAR panel endorsed a South Atlantic MOC ("SAMOC") Initiative to both strengthen existing programs seeking to study the MOC in the South Atlantic and to encourage further expansion of the MOC observing system in the region. The MOC is a primary mechanism for the transport and storage of heat, freshwater and carbon by the ocean and therefore has a large impact on climate variability and change. The MOC is a three-dimensional
circulation pattern that links each of the basins and spans the full-depth of the global oceans. Given the complex, multibasin nature of the MOC, achieving a more complete understanding of its behaviour and changes requires a more comprehensive observing system, one that extends across neighbouring ocean basins as the one we are developing for the South Atlantic within the CLIVAR SAMOC initiative.

Within the MOC, the South Atlantic Ocean plays a key role as a nexus for water masses formed elsewhere and en-route to remote regions of the global ocean. Because of this important interbasin exchanges, the South Atlantic Ocean is the only major ocean basin that transports heat from the pole towards the equator. However, the South Atlantic is not merely a passive conduit for remotely formed water masses. Indeed, within this basin water masses are significantly altered by local air-sea interactions and diapycnal/isopycnal fluxes, particularly in regions of intense mesoscale activity and steep topography. These contributions have been shown to have a crucial role in the strength of the MOC in paleoceanographic and modelling studies.

Here we will briefly summarize some of the results we obtained during these years. We will introduce first the preliminary results on estimates of the daily MOC strength and depth at 35°S during a ~20 month long pilot array of mooring as well as Argo data and model outputs. Then, we will summarize studies that focalized on water masses exchanges and transformation and related processes within the region as well as new developments in the use of the Argo data set and in the validation-calibration processing of Argo data. Last, we used the Argo data float to calibrate and characterize the 200 stations undertaken by TaraOceans during its journey across the world ocean basins.

### 35°S Meridional Overturning Circulation strength and variability (Meinen et al. 2013)

We obtained preliminary estimates of the daily MOC strength at 34.5°S during a ~20 month long pilot array of mooring. Data from two boundary arrays deployed along 34.5°S and from Argo are combined to produce the first continuous in situ time series observations of the basin-wide MOC in the South Atlantic.

The MOC variability show to be as large as that at 26N, with both eastern and western boundary flows contributing equally to the variance. Data Daily estimates of the MOC between March 2009 and December 2010 range between 3 Sv and 39 Sv (1 Sv = $10^6 m^3 s^{-1}$) after a 10-day low-pass filter is applied. Much of the variability in this ~20 month record occurs at periods shorter than 100 days. Approximately two-thirds of the MOC variability is due to changes in the geostrophic (baroclinic plus barotropic) volume transport, with the remainder associated with the direct wind-forced Ekman transport. When low-pass filtered to match previously published analyses in the North Atlantic, the observed temporal standard deviation at 34.5°S matches or somewhat exceeds that observed by time series observations at 16°N, 26.5°N and 41°N. The higher standard deviation may be due to opposite-signed variability unobserved by the array inshore of the 1000 m isobath (e.g. partially observed eddies). For periods shorter than 20 days the basin-wide MOC variations are most strongly influenced by Ekman flows, while at periods between 20 and 90 days the geostrophic flows tend to exert slightly more control over the total transport variability of the MOC. The geostrophic shear variations are roughly equally controlled by density variations on the western and eastern boundaries at all time scales captured in the record. The observed time-mean MOC vertical structure and temporal variability agree well with the limited independent observations available for confirmation (Meinen et al., 2013).

### Three-dimensional fields from the Argo Array (work in progress in collaboration with E. Rusciano and B. Blanke; Blanke et al. 2014; Rusciano et al. 2015 in prep)

We developed a procedure to obtain three-dimensional climatological fields of T, S and absolute geostrophic velocities from Argo data available in the South Atlantic from 2004 to 2013. The 3-D velocity field extends from the surface to 2000 dbar with a vertical resolution of 10 dbar. These fields are derived by combining on a 1° by 1° spatial grid a total of more than 600 000 individual profiles of temperature and salinity located in the South Atlantic Ocean. By deriving the dynamic height from floats and combining the derived relative geostrophic velocity with the ANDRO world atlas (Argo New Displacements Rannou and Ollitrault) of deep displacements (Ollitrault and Rannou, 2013), we computed the absolute geostrophic velocity and transports down to 2000 m. We are now
using this field to achieve various studies. The first one concerns the description of Intermediate Waters properties and circulation in the region. The second one try to apply Lagrangian estimates to the 3D climatological field we derived to investigate on the path and along path transformations of the upper 2000 m water column in the South Atlantic. While the first study is ready for submission, the latter is under development.

**Intermediate water properties and circulation in the South Atlantic (Rusciano et al. 2012; Rusciano et al. 2015)**

We combined the Argo measured variables and the ANDRO velocity data set to infer the absolute geostrophic velocities and to estimate the evolution of the dynamical properties of South Atlantic intermediate waters within the isoneutral layer ($27.1 < \gamma_n < 27.6$). We found four different regional types of Intermediate Waters converging in the area south of 15°S: Three Antarctic Intermediate Water (AAIW) varieties (the Atlantic, A-AAIW, characterized by $S \le 34.2$; the Indian, I-AAIW, $34.3 \le S \le 34.4$; and the recently identified Indo-Atlantic, IA-AAIW, $34.2 < S < 34.3$), and a saltier Tropical South Atlantic Intermediate Water (TSA-IW, $34.4 < S \le 34.6$).

Across 17°S two veins of intermediate waters flows northward. One along the western boundary carrying 2.3 Sv of modified Indian AAIW (I-AAIW). The other along the eastern boundary carrying 4.5 Sv of tropical intermediate waters (TSA-IW). South, between the Subantarctic Front and 27°N, intermediate waters circulate around the South Atlantic subtropical gyre. The saltiest AAIW circulate at the northern edge of the gyre while the core is made of AAIW derived from a mixing of Indian and Atlantic AAIW. This mixing happens essentially at depth in the Cape Basin (Southeastern Atlantic) where the complex dynamics of the Agulhas Retroflection and Agulhas ring injections, the strong interaction with steep topography and the coming across of waters of remote origins (South African slope and the South Atlantic Current) enhance water masses stirring and therefore impact heat and salt exchanges. No fresh A-AAIW is observed to penetrate north of 30°S.

**The thermohaline variability of the Southeast Atlantic sector of the Southern Ocean between 1992-2012 using an altimetry-based Gravest Empirical Mode (Master thesis of L. Hutchinson; Hutchinson et al. 2015 in prep)**

By taking advantage of the numerous repeat hydrographic CTD sections along the GoodHope line, the large number of Argo floats deployed in the area, and the available satellite altimetry data, an Altimetry Gravest Empirical Mode (AGEM) is developed for the ACC south of Africa. The AGEM has improved precision to comparable proxies and offers an ideal technique to investigate the thermohaline variability of the upper 2000dbar of the water column over the past two decades. In order to assess and attribute changes in ocean dynamics and water masses, we separate the diabatic and adiabatic components of the reconstructed trends. Integrated over the whole top 2000dbar of the ACC south of Africa, results show adiabatic changes of $0.016 \pm 0.010^\circ\text{C}.\text{yr}^{-1}$ and $3.86 \times 10^{-4} \pm 1.30 \times 10^{-3} \text{yr}^{-1}$, and diabatic trends of $8.29 \times 10^{-4} \pm 9.20 \times 10^{-3}^\circ\text{C}.\text{yr}^{-1}$ and $-5.72 \times 10^{-4} \pm 1.0 \times 10^{-3} \text{yr}^{-1}$ for temperature and salinity respectively. By combining the original AGEM fields with the diabatic differences, a new AGEM (AD-AGEM) is created rendering mean property changes of $0.012 \pm 0.011^\circ\text{C}.\text{yr}^{-1}$ and $-5.47 \times 10^{-4} \pm 1.60 \times 10^{-3} \text{yr}^{-1}$. The study focuses on the temporal evolution of the Antarctic Intermediate Water (AAIW), finding mean trends of $-0.015 \pm 0.096^\circ\text{C}.\text{yr}^{-1}$ and $-2.8 \times 10^{-3} \pm 1.33 \times 10^{-2} \text{yr}^{-1}$ for the layer within the Subantarctic zone, and $0.029 \pm 0.12^\circ\text{C}.\text{yr}^{-1}$ and $7.17 \times 10^{-4} \pm 5.90 \times 10^{-3} \text{yr}^{-1}$ for the Polar Frontal Zone. The results expose the uniqueness of the ACC south of Africa in its response to climate change.

**Evolution of properties of Agulhas rings in the South Atlantic (Master2 thesis of Rémi Laxenaire; Laxenaire et al. 2015 in prep)**

The Indo-Atlantic exchange achieved by mesoscale eddies formed in the region of the Agulhas Current Retroflection is investigated by combining daily satellite altimetry data, profiling floats. The eddy tracking algorithm developed by Alexis Chaigneau and Cori Pegliasco (Chaigneau and Pegliasco 2013 -not yet published-) is used and further developed in our study. This algorithm takes into account eddy shapes, merging and, thanks to this study, splitting events. The method is validated in a region characterized by a particularly high eddy activity, and where eddy formation, strong eddy-eddy interactions and influence of the bottom topography are observed.
The collocation of identified eddies and Argo profiles allows us to study isolated records of
the internal structure of eddies in the water column.

The thermal contents anomalies associated with the eddies, and computed with the Argo
profiles, are mapped over the area of study. The anticyclones (cyclones) show positive (negative)
anomalies that vary from +10¹⁰ J/m² (-10¹⁰ J/m²) south of Africa to +4.10⁹ J/m² (-4.10⁹ J/m²) in the
western part of the domain, in the South Atlantic. A continuous gradient exists between these two
extremes. These Eulerian mappings highlight the importance of both the cyclones and anticyclones for
heat content.

One Agulhas ring is sampled on the vertical by 19 Argo profiles between the Cape Basin and the western part of the domain in the South Atlantic. We show the presence of a homogeneous surface layer down to 300 m that subducts when the Walvis Ridge is crossed. A reminiscence of this layer is found in the region of the Mid-Atlantic Ridge, but it is no more detectable west of it. An eddy with a similar behaviour was detected in the simulation. This comparative Lagrangian study highlights that Agulhas Rings can present a warm and salty layer up to the sea surface during more than 1 year, during which exchanges with atmosphere can occur, before the anticyclones subduct. Moreover, we show that the anticyclones can advect Indian water masses to the western part of the South Atlantic.

Delayed mode analysis of Argo floats: improvements of the method in the Southern Ocean (in collaboration with C. Cabanes and J. Lepesquer; Cabanes et al. 2014)

Argo floats conductivity sensors can be subject to drift and offset due to bio-fouling or other
technical problems. Several approaches have been proposed to correct the float salinity measurements
in delayed time. The method developed by Owens and Wong 2009 (OW, 2009) is now widely used by
the Argo community. However, our experience has shown that it is sometimes difficult to detect a
float salinity drift or offset in the Southern Ocean around southern Africa only on the basis of the
results from OW method and it can be necessary to use complementary approaches. In our study we
have shown that some modifications of the standard OW method are necessary to gain confidence in
the results proposed by the method and to make it easier for the PI of the float to decide whether a
salinity correction is necessary. We have followed Cabanes et al. (2014) and slightly modified the OW
method in order to better take into account the large variability of the salinity field, which is assumed
to be constant in the standard method. Indeed, this high variability is caused, in the Southern Ocean,
by a complex ocean dynamics characterized by intense mesoscale features and various water masses
varying at the same scale.

To take into account these complex variations, additional modifications have been
implemented to better take into account the presence of fronts and large eddy variability. Particularly,
in this area, we implemented a new methodology that takes into account in the comparison with
historical data water-masses properties such as variable theta-related depths and the dynamic height of
the Argo profile. The work is still in progress, however we hope to finalise this improvements to the
Argo data delayed mode analyses this year.

## Future of the project :

The SAMOC international initiative is continuing. In particular, SAMOC has received much attention lately from the European Commission, USA and Canada within the Galway Agreement on Atlantic observing implementation. The participation of SAMOC community in the H2020 EU AtlantOS project will foster collaborations with Northern Hemisphere countries and the capability of the observing system will be evaluated and possibly enhanced. Due to the new appointment of the French PI, Sabrina Speich, to the "Laboratoire de Meteorologie Dynamique" (LMD) of the "Institute Pierre-Simon Laplace" (IPSL) in Paris, there is a need to break off momentarily the activity of SAMOC in terms of Coriolis Argo floats deployment.
<figure><img src="image_67.png" /></figure>

Figure 1: Transport time series of the MOC at 34.5°S (red line) with estimated daily error bars (gray shading) as derived in Meinen et al. 2013. Black vertical error bar at left illustrates the estimated bias accuracy. Also shown are five MOC estimates determined from trans-basin XBT sections where the horizontal length of the bar illustrates the start and end times of each trans-basin cruise.

<figure><img src="image_68.png" /></figure>

Figure 2: Isopycnic potential vorticity ($10^{-11} \text{ m-1s}^{-1}$) computed in the neutral density isopycnal layer ($27.1 < \gamma_n < 27.6$).

## Bibliographic references linked with the project

Blanke, B., S. Speich, and E. Rusciano, 2014: Lagrangian water mass tracing from pseudo-Argo, model-derived salinity, tracer and velocity data: An application to Antarctic Intermediate Water in the South Atlantic Ocean. Ocean Modelling. 11/2014; DOI: 10.1016/j.ocemod.2014.11.004.

Cabanes, C., V. Thierry, J. Lepesquer, S. Speich, C. Lagadec, 2014: Delayed mode analysis of argo floats: improvements of the method in the north atlantic and southern ocean. Coriolis Quarterly Newsletter - Special Issue, 50, 3-11

Meinen, C. S., S. Speich, R. C. Perez, S. Dong, A. R. Piola, S. L. Garzoli, M. O. Baringer, S. Gladyshev, and E. J. D. Campos, Temporal variability of the Meridional Overturning Circulation at 34.5°S: Results from two pilot boundary arrays in the South Atlantic, J. Geophys. Res., 118 (12), 6461-6478, doi:10.1002/2013JC009228, 2013

Messager, C., S. Speich, and E. Key, 2012: Marine atmospheric boundary layer over some Southern Ocean fronts during the IPY BGH 2008 cruise, Ocean Sci., 8, 1001-1023, 2012

Rimaud, J., S. Speich, B. Blanke, and N. Grima, 2012: The exchange of Intermediate Water in the southeast Atlantic: Water mass transformations diagnosed from the Lagrangian analysis of a regional ocean model, J. Geophys. Res., 117, C08034, doi:10.1029/2012JC008059.

Rusciano, E. and S. Speich, 2015: Antarctic Intermediate Water properties and spreading in the South Atlantic. To be submitted shortly to J. Geophys. Res.

Rusciano, E., S. Speich and M. Ollitrault, 2012: Antarctic Intermediate Water dynamics, budget and fluxes. Interocean exchanges South of Africa. J. Geophys. Res. 117, C10010, doi:10.1029/2012JC008266.

Speich, S., M. Arhan, E. Rusciano, V. Faure, M. Ollitrault, A. Prigent, S. Swart, 2012: Use of ARGO floats to study the ocean dynamics south of Africa: What we have learned from the GoodHope project and what we plan within the SAMOC International Programme. Mercator Ocean—Coriolis Quarterly Newsletter - Special Issue, 45, 21-27.

von Schuckmann, K., Sallée, J.-B., Chambers, D., Le Traon, P.-Y., Cabanes, C., Gaillard, F., Speich, S., and Hamon, M.: Consistency of the current global ocean observing systems from an Argo perspective, Ocean Sci., 10, 547-557, doi:10.5194/os-10-547-2014, 2014
# The ACC at Drake Passage: volume transport and meridional heat flux

Christine Provost, Young-Hyang Park, Nathalie Sennéchael, Zoé Koenig and Ramiro Ferrari

LOCEAN, UMR 7159, CNRS/UPMC/MNHN/IRD, Université Pierre Marie Curie, Paris, France

Many observational studies have sought to establish how the transport of the ACC varies with time and space. Most of these have focused on the flow through Drake Passage (DP), since all the transport through that section contributes to the circumpolar transport, and complications with additional flows are safely avoided. The landmark International Southern Ocean Studies (ISOS) program with intensive field experiments in DP (i.e. the Drake 79 mooring array) provided fundamental insights into the spatial and temporal structure of the flow and led to the canonical value of total ACC transport of 134 Sv with a standard deviation of 11 Sv for a year-long record.

The satellite altimetry opened new perspectives to the monitoring of the ACC circulation and its transport (i.e Barré et al., 2011). To monitor transport using altimetry, hydrographic data and current meter time series were collected during three years (2006-2009) along the ground track 104 of Jason altimetry satellite as part of the DRAKE project (Provost et al., 2011) (Figure 1).

<figure><img src="image_69.png" /><figcaption>**Figure 1.** Bathymetry in the Drake Passage in meters. In red, Jason satellite ground track 104 where an asterisk indicates a mooring location (M1 through M10). In green, the SR1b annually repeated hydrography line. OB, Ona Basin; YB, Yaghan Basin; SFZ, Shackleton Fracture Zone; WSR, West Scotia Ridge; PAR, Phoenix Atlantic Ridge. The black lines represent the mean location of the fronts : SAF, Subantarctic Front; PF, Polar Front; SACCF, Southern ACC Front.</figcaption></figure>

The Drake *in situ* data were used to evaluate the altimetry products. Both hydrographic data and velocity time series showed that altimetric data, and in particular the multi-satellite gridded products provided accurate information on surface geostrophic velocity variations (Barré et al., 2011; Renault et al., 2011; Ferrari et al., 2012; Ferrari et al., 2013). A 21-year (1992-2013) ACC transport time series with a 7-day resolution was produced from the combination of information from 3 years of
in situ velocity data and satellite altimetry observations along the Jason ground track 104 (Koenig et al., 2014). Time- and space-varying transfer functions relating altimetry-derived surface currents and velocity profiles in the water column derived from the current meter mooring data were constructed. The transport time series was then extended over the altimetric era with a time resolution of 7 days. This time series was decomposed into three transport components: total transport (TT), baroclinic transport (BC, calculated relative to the zero bottom velocity), and the barotropic transport (BT, calculated by integrating the bottom velocity over the entire water column) (Fig. 2). The internal consistency and the robustness of the method were carefully assessed and comparisons with independent data spanning the 20 years (such as SR1b annually repeated hydrography line) demonstrated that the accuracy of the method and periods larger than 30 days are reliable (Koenig et al., 2014).

<figure><img src="image_70.png" /><figcaption>**Figure 2:** Time series of the total (in black), baroclinic (in red), and barotropic (in magenta) transport from 0 to bottom, with their smoothed time series (respectively green, blue, and light blue) obtained with a 1 month running mean. Total mean: 141 Sv, std: 13 Sv. Barotropic mean: 5 Sv, std: 16 Sv. Baroclinic mean: 136 Sv; std: 11 Sv. Note that the red right axis to the right refers to the baroclinic transport only.</figcaption></figure>

While the time-mean whole-track TT (141 Sv) is largely determined by the BC component (136 Sv), with a minor contribution from the BT component (5 Sv), its temporal variability shows the opposite tendency. In fact, the strongest transport variability is associated with BT (16 Sv std) and the least variability with BC (11 Sv std), with the TT variability (13 Sv std) lying in between the two, indicating that the BC variability is partly compensating the BT variability, consistent with a significant anti-correlation (r = -0.6) between the BT and BC.

In terms of the frequency content, the intraseasonal variability of periods of 1 to 5 months constitutes the majority of the variability for all three transport components. Common to both BT and TT are two energy-containing frequency bands, one for periods of 1-2 months and the other for 3-5 months. These contain significant peaks centered at 28, 36 and 50 days in the 1-2 month band and distinctive (though not significant at the 95% level) peaks at 99 and 142 days in the 3-5 month band. Among these, the 50-day peak constitutes the most energetic signal. The spectral energy decreases sharply with increasing periods with significant low-frequency signals only at annual and biannual timescales.

The lagged SLA regressions onto the three transport components reveal different projections in space and time according to each component. The SLA patterns significantly correlated with BT are confined entirely within the Yaghan Basin, with a representative intraseasonal timescale of about 2 months. In contrast, the BC-regressed SLA patterns project much more extensively in the upstream area from DP as slowly propagating wave-trains annually recurrent at the entrance to DP on one hand,