bluuebunny/crossref_metadata_2025 huggingface dataset:
Controlling atmospheric forcing parameters of global ocean models: sequential assimilation of sea surface Mercator-Ocean reanalysis data
Authors: Skandrani C. , Brankart J.-M. , Ferry N. , Verron J. , Brasseur P. , Barnier B.
Published: April 2010

Abstract. In the context of stand alone ocean models, the atmospheric forcing is generally computed using atmospheric parameters that are derived from atmospheric reanalysis data and/or satellite products. With such a forcing, the sea surface temperature that is simulated by the ocean model is usually significantly less accurate than the synoptic maps that can be obtained from the satellite observations. This not only penalizes the realism of the ocean long-term simulations, but also the accuracy of the reanalyses or the usefulness of the short-term operational forecasts (which are key GODAE and MERSEA objectives). In order to improve the situation, partly resulting from inaccuracies in the atmospheric forcing parameters, the purpose of this paper is to investigate a way of further adjusting the state of the atmosphere (within appropriate error bars), so that an explicit ocean model can produce a sea surface temperature that better fits the available observations. This is done by performing idealized assimilation experiments in which Mercator-Ocean reanalysis data are considered as a reference simulation describing the true state of the ocean. Synthetic observation datasets for sea surface temperature and salinity are extracted from the reanalysis to be assimilated in a low resolution global ocean model. The results of these experiments show that it is possible to compute piecewise constant parameter corrections, with predefined amplitude limitations, so that long-term free model simulations become much closer to the reanalysis data, with misfit variance typically divided by a factor 3. These results are obtained by applying a Monte Carlo method to simulate the joint parameter/state prior probability distribution. A truncated Gaussian assumption is used to avoid the most extreme and non-physical parameter corrections. The general lesson of our experiments is indeed that a careful specification of the prior information on the parameters and on their associated uncertainties is a key element in the computation of realistic parameter estimates, especially if the system is affected by other potential sources of model errors.
Adjustment of the basin-scale circulation at 26° N to variations in Gulf Stream, deep western boundary current and Ekman transports as observed by the Rapid array
Authors: Bryden H. L. , Mujahid A. , Cunningham S. A. , Kanzow T.
Published: April 2010

Abstract. The Rapid instrument array across the Atlantic Ocean along 26° N provides unprecedented monitoring of the basin-scale circulation. A unique feature of the Rapid array is the combination of full-depth moorings with instruments measuring temperature, salinity, pressure time series at many depths with co-located bottom pressure measurements so that dynamic pressure can be measured from surface to bottom. Bottom pressure measurements show a zonally uniform rise (and fall) of bottom pressure of 0.015 dbar on a 5 to 10 day time scale, suggesting that the Atlantic basin is filling and draining on a short time scale. After removing the zonally uniform bottom pressure fluctuations, bottom pressure variations at 4000 m depth against the western boundary compensate instantaneously for baroclinic fluctuations in the strength and structure of the deep western boundary current so there is no basin-scale mass imbalance resulting from variations in the deep western boundary current. After removing the mass compensating bottom pressure, residual bottom pressure fluctuations at the western boundary just east of the Bahamas balance variations in Gulf Stream transport. Again the compensation appears to be especially confined close to the western boundary. Thus, fluctuations in either Gulf Stream or deep western boundary current transports are compensated in a depth independent (barotropic) manner very close to the continental slope off the Bahamas. In contrast, compensation for variations in wind-driven surface Ekman transport appears to involve fluctuations in both western basin and eastern basin bottom pressures, though the bottom pressure difference fluctuations appear to be a factor of 3 too large, perhaps due to an inability to resolve small bottom pressure fluctuations after removal of larger zonal average, baroclinic, and Gulf Stream pressure components. For 4 tall moorings where time series dynamic height (geostrophic pressure) profiles can be estimated from sea surface to ocean bottom and bottom pressure can be added, there is no general correlation between surface dynamic height and bottom pressure. Dynamic height on each mooring is strongly correlated with sea surface height from satellite observations and the variability in both dynamic height and satellite sea surface height decrease sharply as the western boundary is approached.
The multifractal structure of satellite sea surface temperature maps can be used to obtain global maps of streamlines
Authors: Turiel A. , Nieves V. , Garcia-Ladona E. , Font J. , Rio M.-H. , Larnicol G.
Published: April 2010

Abstract. Nowadays Earth observation satellites provide information about many relevant variables of the ocean-climate system, such as temperature, moisture, aerosols, etc. However, to retrieve the velocity field, which is the most relevant dynamical variable, is still a technological challenge, specially in the case of oceans. New processing techniques, emerged from the theory of turbulent flows, have come to assist us in this task. In this paper, we show that multifractal techniques applied to new Sea Surface Temperature satellite products opens the way to build maps of ocean currents with unprecedented accuracy. With the application of singularity analysis, we show that global ocean circulation patterns can be retrieved in a daily basis. We compare these results with high-quality altimetry-derived geostrophic velocities, finding a quite good correspondence of the observed patterns both qualitatively and quantitatively; and this is done for the first time on a global basis, even for less active areas. The implications of this findings from the perspective both of theory and of operational applications are discussed.
Problems with estimation and interpretation of oceanic heat transport – conceptual remarks for the case of Fram Strait in the Arctic Ocean
Authors: Schauer U. , Beszczynska-Möller A.
Published: April 2010

Abstract. While the concept of oceanic heat transport – or rather heat transport divergence – is well known, it is sometimes applied inaccurately. Often so-called "heat transports" are computed across a partial section which means that the volume flow through such a section is not zero. In this case the "heat transports" depend entirely on the choice of the temperature scale. The consequences of such arbitrariness are demonstrated with a simple calculation exercise for the passages to the Arctic Ocean. To circumvent the arising difficulties for the Fram Strait in the Arctic we propose a stream tube concept to define a net zero volume flow section which can, with coarse assumptions, be used to determine oceanic heat transport by the portion of Atlantic water flow that passes through Fram Strait. Weaknesses of this approach and consequences for observational strategies are discussed.
Sensors for observing ecosystem status
Authors: Kröger S. , Parker E. R. , Metcalfe J. D. , Greenwood N. , Forster R. M. , Sivyer D. B. , Pearce D. J.
Published: April 2010

Abstract. This paper aims to review the availability and application of sensors for observing marine ecosystem status. It gives a broad overview of important ecosystem variables to be investigated, such as biogeochemical cycles, primary and secondary production, species distribution, animal movements, habitats and pollutants. Some relevant legislative drivers are listed, as they provide one context in which ecosystem studies are undertaken. In addition to literature cited within the text the paper contains some useful web links to assist the reader in making an informed instrument choice, as the authors feel that the topic is so broad, it is impossible to discuss all relevant systems or to provide appropriate detail for those discussed. It is therefore an introduction to how and why ecosystem status is currently observed, what variables are quantified, from what platforms, using remote sensing or in-situ measurements, and gives examples of useful sensor based tools. Starting with those presently available, to those under development and also highlighting sensors not yet realised but desirable for future studies.
Surface circulation in the Eastern Mediterranean using drifters (2005–2007)
Authors: Gerin R. , Poulain P.-M. , Taupier-Letage I. , Millot C. , Ben Ismail S. , Sammari C.
Published: April 2010

Abstract. Within the framework of the EGITTO/EGYPT programs, the spatial structure and the temporal variability of the surface circulation in the eastern basin of the Mediterranean Sea was studied with satellite-tracked drifters. A total of 97 drifters drogued to 15-m nominal depth were released between September 2005 and March 2007, regularly along ship-of-opportunity routes in the Sicily Channel and within specific structures during dedicated campaigns in the Levantine sub-basin. This study complements previous ones in the former and in the Ionian, but it is the first one in the latter. After editing and low-pass filtering, the drifter trajectories were used to estimate pseudo-Eulerian statistics: mean current, variance ellipses, mean and eddy kinetic energies. A statistical analysis was also performed dividing the dataset in two extended seasons (winter and summer). A branching behaviour of the surface water after passing through the Sicily Channel is evidenced, together with a seasonal variability inducing a reversal of the circulation in the southern part of the Ionian. In the Levantine, the surface circulation describes an eastward flow along the Libyan and Egyptian slopes (the Libyo-Egyptian Current: LEC) that continues in a cyclonic circuit along the Middle East and Turkish slopes. This general alongslope circuit can be perturbed locally and temporally by the numerous anticyclonic eddies that co-exist in the Levantine, mainly created by the instability of the LEC in the south (Libyo-Egyptian Eddies: LEEs), but also by the wind (Ierapetra and Pelops), and by the topography (over the Eratosthenes Seamount and off Latakia). The most frequent perturbation is the entrainment of part of the flow seaward: the LEEs close to the slope can interact with the LEC, which then spreads more or less around them, so that a series of contiguous LEEs (paddle-wheel effect) can possibly result in an eastward offshore transport (the so-called Mid Mediterranean Jet). Additionally, when LEEs are close enough to the slope, most of the surface flow is spread seaward. Along the slope the current is then induced by the LEEs southern side which results in a westward current. Locally and temporally the circulation along the slope can thus be reversed.
Observed and simulated estimates of the meridional overturning circulation at 26.5° N in the Atlantic
Authors: Baehr J. , Cunnningham S. , Haak H. , Heimbach P. , Kanzow T. , Marotzke J.
Published: April 2010

Abstract. Daily timeseries of the meridional overturning circulation (MOC) estimated from the UK/US RAPID/MOCHA array at 26.5° N in the Atlantic are used to evaluate the MOC as simulated in two global circulation models: (I) an 8-member ensemble of the coupled climate model ECHAM5/MPI-OM, and (II) the ECCO-GODAE state estimate. In ECHAM5/MPI-OM, we find that the observed and simulated MOC have a similar variability and time-mean within the 99% confidence interval. In ECCO-GODAE, we find that the observed and simulated MOC show a significant correlation within the 99% confidence interval. To investigate the contribution of the different transport components, the MOC is decomposed into Florida Current, Ekman and mid-ocean transports. In both models, the mid-ocean transport is closely approximated by the residual of the MOC minus Florida Current and Ekman transports. As the models conserve volume by definition, future comparisons of the RAPID/MOCHA mid-ocean transport should be done against the residual transport in the models. The similarity in the variance and the correlation between the RAPID/MOCHA, and respectively ECHAM5/MPI-OM and ECCO-GODAE MOC estimates at 26.5° N is encouraging in the context of estimating (natural) variability in climate simulations and its use in climate change signal-to-noise detection analyses. Enhanced confidence in simulated hydrographic and transport variability will require longer observational time series.
Mediterranean Forecasting System: forecast and analysis assessment through skill scores
Authors: Tonani M. , Pinardi N. , Fratianni C. , Pistoia J. , Dobricic S. , Pensieri S. , de Alfonso M. , Nittis K.
Published: April 2010

Abstract. This paper describes the first evaluation of the quality of the forecast and analyses produced at the basin scale by the Mediterranean ocean Forecasting System (MFS) (http://gnoo.bo.ingv.it/mfs). The system produces short-term ocean forecasts for the following ten days. Analyses are produced weekly using a daily assimilation cycle. The analyses are compared with independent data from buoys, where available, and with the assimilated data before the data are inserted. In this work we have considered 53 ten days forecasts produced from 16 August 2005 to 15 August 2006. The forecast skill is evaluated by means of root mean square error (rmse) differences, bias and anomaly correlations at different depths for temperature and salinity, computing differences between forecast and analysis, analysis and persistence and forecast and persistence. The Skill Score (SS) is defined as the ratio of the rmse of the difference between analysis and forecast and the rmse of the difference between analysis and persistence. The SS shows that at 5 and 30 m the forecast is always better than the persistence, but at 300 m it can be worse than persistence for the first days of the forecast. This result may be related to flow adjustments introduced by the data assimilation scheme. The monthly variability of SS shows that when the system variability is high, the values of SS are higher, therefore the forecast has higher skill than persistence. We give evidence that the error growth in the surface layers is controlled by the atmospheric forcing inaccuracies, while at depth the forecast error can be interpreted as due to the data insertion procedure. The data, both in situ and satellite, are not homogeneously distributed in the basin; therefore, the quality of the analyses may be different in different areas of the basin.
The density of seawater as a function of salinity (5 to 70 g kg−1) and temperature (273.15 to 363.15 K)
Authors: Millero F. J. , Huang F.
Published: April 2010

Abstract. New seawater density measurements were made as a function of temperature T=(273.15 to 363.15) K and salinity (5 to 70 g kg−1). The measurements (N=230) from T=273.15 to 313.15 K and Practical Salinity (S) from 0 to 40 were found to be in good agreement (σ=0.0036 kg m−3) with the equation of state of seawater (Millero and Poisson, 1981) made on samples with a known chlorinity (Cl). These results indicate that the Practical Salinities (S) are in agreement to within ±0.003 kg m−3 with the values calculated from the Chlorinity, SCl=1.80655 Cl. The measurements from 298.15 to 363.15 were used to extend the equation of state to high temperatures and salinities. All the densities were made relative to pure water (ρ−ρ0 where ρ0 is the density for pure water) were fitted to equations of the form (ρ−ρ0)/(kg m−3)=A SA+B SA1.5+C SA2 where A, B, and C are functions of temperature and SA (g kg−1) is the absolute salinity, SA=(35.16504/35) S g kg−1. The fitted results from SA=0 to 50 g kg−1 and T=273.15 to 313.15 K (N=242) gave standard errors of 0.0037 kg m−3. The fitted results from 298.15 to 363.15 K (N=280) gave standard errors of 0.0063 kg m−3 and all the results (N=522) from 273.15 to 363.15 K gave standard errors of 0.0063 kg m−3. The earlier density measurements (Millero et al., 1976b; Poisson et al.,1980) used to determine the equation of state of seawater (Millero and Poisson, 1981) were combined to derive equations that are valid from 273.15 to 313.15 K and 273.15 to 363.15 K. The standard errors of these fits are respectively, 0.0038 kg m−3 (N=713) and 0.0063 kg m−3 (N=962). These new measurements expand the equation of state of seawater to a wider range of temperature (273.15 to 363.15) K and absolute salinity (0 to 70 kg m−3.
Transformation of an Agulhas eddy near the continental slope
Authors: Baker-Yeboah S. , Flierl G. R. , Sutyrin G. G. , Zhang Y.
Published: April 2010

Abstract. The transformation of Agulhas eddies near the continental slope of southern Africa and their subsequent self-propagation are analyzed in both observational data and numerical simulations. Self-propagation results from a net dipole moment of a generalized heton structure consisting of a surface-intensified anticyclonic eddy and deep cyclonic pattern. Such Agulhas vortical structures can form near the retroflection region and further north along the western coast of southern Africa. We analyze nonlinear topographic wave generation, vortex deformations, and filament production as an important part in water mass exchange. Self-propagating structures provide a conduit for exchange between the deep ocean and shelf regions in the Benguela upwelling system.
The timescale and extent of thermal expansion of the global ocean due to climate change
Authors: Marčelja S.
Published: April 2010

Abstract. With recently improved instrumental accuracy, the change in the heat content of the oceans and the corresponding contribution to the change of the sea level can be determined from in situ measurements of temperature variation with depth. Nevertheless, it would be favourable if the same changes could be evaluated from just the sea surface temperatures because the past record could then be reconstructed and future scenarios explored. Using a single column model we show that the average change in the heat content of the oceans and the corresponding contribution to a global change in the sea level can be evaluated from the past sea surface temperatures. The calculation is based on the time-dependent diffusion equation with the known fixed average upwelling velocity and eddy diffusivity, as determined from the steady-state limit. In this limit, the model reduces to the 1966 Munk profile of the potential temperature variation as a function of depth. There are no adjustable parameters in the calculation and the results are in good agreement with the estimates obtained from the in situ data. The method allows us to obtain relevant timescales and average temperature profiles. The evaluation of the thermosteric sea level change is extended back to the beginning of accurate sea surface temperature records. The changes in sea surface temperature from 1880 until the present time are estimated to have produced a thermosteric sea level rise of 35 mm. Application to future IPCC scenarios gives results similar to the average prediction of more complex climate models.
Pre-operational short-term forecasts for Mediterranean Sea biogeochemistry
Authors: Lazzari P. , Teruzzi A. , Salon S. , Campagna S. , Calonaci C. , Colella S. , Tonani M. , Crise A.
Published: April 2010

Abstract. Operational prediction of the marine environment is recognised as a fundamental research issue in Europe. We present a pre-operational implementation of a biogeochemical model for the pelagic waters of the Mediterranean Sea, developed within the framework of the MERSEA-IP European project. The OPATM-BFM coupled model is the core of a fully automatic system that delivers weekly analyses and forecast maps for the Mediterranean Sea biogeochemistry. The system has been working in its current configuration since April 2007 with successful execution of the fully automatic operational chain in 87% of the cases while in the remaining cases the runs were successfully accomplished after operator intervention. A description of the system developed and also a comparison of the model results with satellite data are presented, together with a measure of the model skill evaluated by means of seasonal target diagrams. Future studies will address the implementation of a data assimilation scheme for the biogeochemical compartment in order to increase the skill of the model's performance.
The role of atmosphere and ocean physical processes in ENSO in a perturbed physics coupled climate model
Authors: Philip S. Y. , Collins M. , van Oldenborgh G. J. , van den Hurk B. J. J. M.
Published: April 2010

Abstract. We examine the behaviour of the El Niño – Southern Oscillation (ENSO) in an ensemble of global climate model simulations with perturbations to parameters in the atmosphere and ocean components respectively. The influence of the uncertainty in these parametrisations on ENSO are investigated systematically. The ensemble exhibits a range of different ENSO behaviour in terms of the amplitude and spatial structure of the sea surface temperature (SST) variability. The nature of the individual feedbacks that operate within the ENSO system are diagnosed using an Intermediate Complexity Model (ICM), which has been used previously to examine the diverse ENSO behaviour of the CMIP3 multi-model ensemble. Unlike in that case, the ENSO in these perturbed physics experiments is not principally controlled by variations in the mean climate state. Rather the parameter perturbations influence the ENSO characteristics by modifying the coupling feedbacks within the cycle. The associated feedbacks that contribute most to the ensemble variations are the response of SST to local wind variability and damping, followed by the response of SST to thermocline anomalies and the response of the zonal wind stress to those SST anomalies. Atmospheric noise amplitudes and oceanic processes play a relatively minor role.
Thermodynamic properties of sea air
Authors: Feistel R. , Wright D. G. , Kretzschmar H.-J. , Hagen E. , Herrmann S. , Span R.
Published: April 2010

Abstract. Very accurate thermodynamic potential functions are available for fluid water, ice, seawater and humid air covering wide ranges of temperature and pressure conditions. They permit the consistent computation of all equilibrium properties as, for example, required for coupled atmosphere-ocean models or the analysis of observational or experimental data. With the exception of humid air, these potential functions are already formulated as international standards released by the International Association for the Properties of Water and Steam (IAPWS), and have been adopted in 2009 for oceanography by IOC/UNESCO. In this paper, we derive a collection of formulas for important quantities expressed in terms of the thermodynamic potentials, valid for typical phase transitions and composite systems of humid air and water/ice/seawater. Particular attention is given to equilibria between seawater and humid air, referred to as "sea air" here. In a related initiative, these formulas will soon be implemented in a source-code library for easy practical use. The library is primarily aimed at oceanographic applications but will be relevant to air-sea interaction and meteorology as well. The formulas provided are valid for any consistent set of suitable thermodynamic potential functions. Here we adopt potential functions from previous publications in which they are constructed from theoretical laws and empirical data; they are briefly summarized in the appendix. The formulas make use of the full accuracy of these thermodynamic potentials, without additional approximations or empirical coefficients. They are expressed in the temperature scale ITS-90 and the 2008 Reference-Composition Salinity Scale.
Reality checks on microbial food web interactions in dilution experiments: Responses to the comments of Dolan and McKeon
Authors: Landry M. R. , Calbet A.
Published: April 2010

Abstract. Microzooplankton grazing rate estimates by the dilution approach have recently been criticized as systematically biased in the direction of being overestimates of actual rates in nature, and particularly in the open oceans. This argument is based on observed mortality responses of ciliated protozoa to reduced food in several coastal experiments and a global extrapolation which assumes that all grazing in all ocean systems scales to the abundance of ciliates. We suggest that these conclusions are unrealistic on several counts: they do not account for community differences in open ocean and coastal systems; they ignore experimental direct evidence supporting dilution rate estimates in the open oceans, and they discount dilution effects on mortality as well as growth in multi-layered, open-ocean food webs. High microzooplankton grazing rates in open-ocean systems are consistent with current views on export fluxes and trophic transfers. More importantly, significantly lower rates would fail to account for the efficient nutrient recycling requirements of these resource-limited and rapid-turnover communities.
Formulation of an ocean model for global climate simulations
Authors: Griffies S. M. , Gnanadesikan A. , Dixon K. W. , Dunne J. P. , Gerdes R. , Harrison M. J. , Rosati A. , Russell J. L. , Samuels B. L. , Spelman M. J. , Winton M. , Zhang R.
Published: April 2010

Abstract. This paper summarizes the formulation of the ocean component to the Geophysical Fluid Dynamics Laboratory's (GFDL) coupled climate model used for the 4th IPCC Assessment (AR4) of global climate change. In particular, it reviews elements of ocean climate models and how they are pieced together for use in a state-of-the-art coupled model. Novel issues are also highlighted, with particular attention given to sensitivity of the coupled simulation to physical parameterizations and numerical methods. Features of the model described here include the following: (1) tripolar grid to resolve the Arctic Ocean without polar filtering, (2) partial bottom step representation of topography to better represent topographically influenced advective and wave processes, (3) more accurate equation of state, (4) three-dimensional flux limited tracer advection to reduce overshoots and undershoots, (5) incorporation of regional climatological variability in shortwave penetration, (6) neutral physics parameterization for representation of the pathways of tracer transport, (7) staggered time stepping for tracer conservation and numerical efficiency, (8) anisotropic horizontal viscosities for representation of equatorial currents, (9) parameterization of exchange with marginal seas, (10) incorporation of a free surface that accomodates a dynamic ice model and wave propagation, (11) transport of water across the ocean free surface to eliminate unphysical "virtual tracer flux" methods, (12) parameterization of tidal mixing on continental shelves.
Detecting changes in Labrador Sea Water through a water mass analysis of BATS data
Authors: Henry-Edwards A. , Tomczak M.
Published: April 2010

Abstract. A new water mass analysis technique is used to analyse the BATS oceanographic data set in the Sargasso Sea of 1988-1998 for changes in Labrador Sea Water (LSW) properties. The technique is based on a sequential quadratic programming method and requires careful definition of constraints to produce reliable results. Variations in LSW temperature and salinity observed in the Labrador Sea are used to define the constraints. It is shown that to minimize the residuals while matching the observed temperature and salinity changes in the source region the nitrate concentration in the Labrador Sea has to be allowed to vary as well. It is concluded that during the period of investigation nitrate underwent significant variations in the Labrador Sea.
Distributions of mixed layer properties in North Pacific water mass formation areas: comparison of Argo floats and World Ocean Atlas 2001
Authors: Bingham F. M. , Suga T.
Published: April 2010

Abstract. Winter mixed layer characteristics in the North Pacific Ocean are examined and compared between Argo floats in 2004 and 2005 and the World Ocean Atlas 2001 (WOA01) climatology for a series of named water masses, North Pacific Tropical Water (NPTW), Eastern Subtropical Mode Water (ESTMW), North Pacific Subtropical Mode Water (NPSTMW), Light Central Mode Water (LCMW) and Dense Central Mode Water (DCMW). The WOA01 is found to be in good agreement with the Argo data in terms of water mass volumes, average temperature-salinity (T-S) properties, and outcrop areas. The exception to this conclusion is for the central mode waters, especially DCMW, whose outcropping is shown to be much more intermittent than is apparent in the WOA01 and whose T-S properties vary from what is shown in the WOA01. Distributions of mixed layer T-S properties measured by floats are examined within the outcropping areas defined by the WOA01 and show some shifting of T-S characteristics within the confines of the named water masses. In 2005, all the water masses were warmer than climatology on average, with DCMW being highest at about 1°C. Similar results were found for the 2004 Argo data except ESTMW and DCMW which were slightly cooler than climatology. Differences between float data and climatology were examined for the entire North Pacific in order to put the above results into context. This analysis showed the winter North Pacific mixed layer to be warmer and fresher than climatology in both 2004 and 2005, with magnitudes of about 0.3–0.4°C and 0.06–0.07. This warming and freshening was apparent throughout a large area of the tropics and northeastern North Pacific, but in the mode water formation areas the trends were less clear.
Observing The Mediterranean Sea from space: 21 years of Pathfinder-AVHRR Sea Surface Temperatures (1985 to 2005). Re-analysis and validation
Authors: Marullo S. , Buongiorno Nardelli B. , Guarracino M. , Santoleri R.
Published: April 2010

Abstract. The time series of satellite infrared AVHRR data from 1985 to 2005 has been used to produce a daily series of optimally interpolated SST maps over the regular grid of the operational MFSTEP OGCM model of the Mediterranean basin. A complete validation of this OISST (Optimally Interpolated Sea Surface Temperature) product with in situ measurements has been performed in order to exclude any possibility of spurious trends due to instrumental calibration errors/shifts or algorithms malfunctioning related to local geophysical factors. The validation showed that satellite OISST is able to reproduce in situ measurements with a mean bias of less than 0.1°C and RMSE of about 0.5°C and that errors do not drift with time or with the percent interpolation error.
Inferring the zonal distribution of measured changes in the meridional overturning circulation
Authors: de Boer A. , Johnson H. L.
Published: April 2010

Abstract. Recently, hydrographic measurements have been used to argue that the meridional overturning circulation at 25° N has decreased by 30% over the last 50 years. Here, we show that the most likely interpretation consistent with this approach (i.e., with the dynamic method together with a level-of-no-motion assumption and Ekman dynamics), is that any decrease in strength of the deep western boundary current must have been compensated, not by a basin-wide increase in upper layer southward flow, but by changes in the nonlinear region, immediately outside of the Florida Straits.
Tidal modulation of two-layer hydraulic exchange flows
Authors: Frankcombe L. M. , Hogg A. McC.
Published: April 2010

Abstract. Time-dependent, two layer hydraulic exchange flow is studied using an idealised shallow water model. It is found that barotropic time-dependent perturbations, representing tidal forcing, increase the baroclinic exchange flux above the steady hydraulic limit, with flux increasing monotonically with tidal amplitude (measured either by height or flux amplitude over a tidal period). Exchange flux also depends on the non-dimensional tidal period, γ, which was introduced by Helfrich (1995). Resonance complicates the relationship between exchange flux and height amplitude, but, when tidal strength is characterised by flux amplitude, exchange flux is a monotonic function of γ.
Operational ocean models in the Adriatic Sea: a skill assessment
Authors: Chiggiato J. , Oddo P.
Published: April 2010

Abstract. In the framework of the Mediterranean Forecasting System project (MFS) sub-regional and regional numerical ocean forecasting systems performance are assessed by mean of model-model and model-data comparison. Three different operational systems have been considered in this study: the Adriatic REGional Model (AREG); the AdriaROMS and the Mediterranean Forecasting System general circulation model (MFS model). AREG and AdriaROMS are regional implementations (with some dedicated variations) of POM (Blumberg and Mellor, 1987) and ROMS (Shchepetkin and McWilliams, 2005) respectively, while MFS model is based on OPA (Madec et al., 1998) code. The assessment has been done by means of standard scores. The data used for operational systems assessment derive from in-situ and remote sensing measurements. In particular a set of CTDs covering the whole western Adriatic, collected in January 2006, one year of SST from space born sensors and six months of buoy data. This allowed to have a full three-dimensional picture of the operational forecasting systems quality during January 2006 and some preliminary considerations on the temporal fluctuation of scores estimated on surface (or near surface) quantities between summer 2005 and summer 2006. In general, the regional models are found to be colder and fresher than observations. They eventually outperform the large scale model in the shallowest locations, as expected. Results on amplitude and phase errors are also much better in locations shallower than 50 m, while degraded in deeper locations, where the models tend to have a higher homogeneity along the vertical column compared to observations. In a basin-wide overview, the two regional models show some dissimilarities in the local displacement of errors, something suggested by the full three-dimensional picture depicted using CTDs, but also confirmed by the comparison with SSTs. In locations where the regional models are mutually correlated, the aggregated mean-square-error has been found to be lower, which is a useful outcome of having several operational systems in the same region.
The Sicily Channel Regional Model forecasting system: initial boundary conditions sensitivity and case study evaluation
Authors: Gaberšek S. , Sorgente R. , Natale S. , Ribotti A. , Olita A. , Astraldi M. , Borghini M.
Published: April 2010

Abstract. The Sicily Channel Regional Model forecasting system was tested using an optimization package for the initial and lateral boundary conditions. Spurious high frequency oscillations during the spin-up time were successfully reduced both in duration and magnitude by optimizing the time tendency of the free surface elevation using the Variational Initialization and Forcing Platform method developed in the framework of the Mediterranean Forecasting System Towards the Environmental Prediction project. The effect of optimization was most profound for the free surface elevation, where all oscillations with periods shorter than 4 h were suppressed. The overall forecast skill was assessed on a 5 day case study starting on 6 April 2005, characterized by a fast passage of a deepening atmospheric low-pressure field with strong winds and marked wind direction change. We compared the predicted variables with in-situ and remotely sensed data. The forecasts of temperature, including the sea surface temperature, and salinity were quite successful, while the forecasted currents, especially within the surface layer, were not in good agreement with the measurements.
Atmospheric forcing by ALADIN/MFSTEP and MFSTEP oriented tunings
Authors: Brožková R. , Derková M. , Belluš M. , Farda F.
Published: April 2010

Abstract. ALADIN/MFSTEP is a configuration of the numerical weather prediction (NWP) model ALADIN run in a dedicated real-time mode for the purposes of the MFSTEP Project. A special attention was paid to the quality of atmospheric fluxes used for the forcing of fine-scale oceanographic models. This paper describes the novelties applied in ALADIN/MFSTEP initiated by the MFSTEP demands, leading also to improvements in general weather forecasting.
Improved near real time surface wind resolution over the Mediterranean Sea
Authors: Bentamy A. , Ayina H.-L. , Queffeulou P. , Croize-Fillon D.
Published: April 2010

Abstract. Several scientific programs, including the Mediterranean Forecasting System Toward Environmental Predictions (MFSTEP project), request high space and time resolutions of surface wind speed and direction. The purpose of this paper is to focus on surface wind improvements over the global Mediterranean Sea, based on the blending near real time remotely sensed wind observations and ECMWF wind analysis. Ocean surface wind observations are retrieved from QuikSCAT scatterometer and from SSM/I radiometers available at near real time at Météo-France. Using synchronous satellite data, the number of remotely sensed data available for each analysis epoch (00:00 h; 06:00 h; 12:00 h; 18:00 h) is not uniformly distributed as a function of space and time. On average two satellite wind observations are available for each analysis time period. The analysis is performed by optimum interpolation (OI) based on the kriging approach. The needed covariance matrixes are estimated from the satellite wind speed, zonal and meridional component observations. The quality of the 6-hourly resulting blended wind fields on 0.25° grid are investigated trough comparisons with the remotely sensed observations as well as with moored buoy wind averaged wind estimates. The blended wind data and remotely wind observations, occurring within 3 h and 0.25° from the analysis estimates, compare well over the global basin as well as over the sub-basins. The correlation coefficients exceed 0.95 while the rms difference values are less than 0.30 m/s. Using measurements from moored buoys, the high-resolution wind fields are found to have similar accuracy as satellite wind retrievals. Blended wind estimates exhibit better comparisons with buoy moored in open sea than near shore.
"Scale oriented" embedded modeling of the North-Western Mediterranean in the frame of MFSTEP
Authors: Estournel C. , Auclair F. , Lux M. , Nguyen C. , Marsaleix P.
Published: April 2010

Abstract. An embedded forecasting system was developed for the North-Western Mediterranean including a regional model and a coastal model of the Gulf of Lion. The system is based on the Symphonie hydrodynamic free surface model and on the variational initialization and forcing platform VIFOP. It was shown that a pre-modeling period of 7 days before beginning the forecast allows the growth of the small scales. The embedded forecasts have been compared to the MFS observing system. It was basically found that in the North-Western Mediterranean, the MFS basin-scale model and thus the regional model forecasts are characterized by large negative biases of salinity in the first 100 m under the surface leading thus to too light subsurface waters. The underestimation of temperature by the regional model just below the surface and its overestimation at 30 m deep can be associated to an overestimation of the turbulent mixing. The regional and coastal models allow to represent a number of processes especially those induced by the wind as coastal upwelling under stratified conditions, dense water formation over the Gulf of Lion shelf, deep mixing in the convection zone or influence on the Northern Current penetration in the Gulf of Lion.
A high resolution free surface model of the Mediterranean Sea
Authors: Tonani M. , Pinardi N. , Dobricic S. , Pujol I. , Fratianni C.
Published: April 2010

Abstract. This study describes a new model implementation for the Mediterranean Sea which has the presently highest vertical resolution over the Mediterranean basin. The resolution is of 1/16°×1/16° in horizontal and 71 unevenly spaced vertical levels. This model has been developed in the frame of the EU-MFSTEP project and it is the operational forecast model presently used at the basin scale. For the first time in the Mediterranean, the model considers an implicit free surface and this characteristics enhances the model capability to simulate the sea surface height variability. In this study we show the calibration/validation experiments done before and after the model has been used for forecasting. The first experiment consist of six years of a simulation forced by a perpetual year forcing and the other experiment is a simulation from January 1997 to December 2004, forcing the model with 6 h atmospheric forcing fields from ECMWF. For the first time the model Sea Level Anomaly is compared with SLA and with ARGO data to provide evidence of the quality of the simulation. The results show that this model is capable to reproduce most of the variability of the general circulation in the Mediterranean Sea even if some basic model inadequacies stand out and should be corrected in the near future.
Variability in the Subtropical-Tropical Cells and its Effect on Near-Surface Temperature of the Equatorial Pacific: a Model Study
Authors: Lübbecke J. F. , Böning C. W. , Biastoch A.
Published: April 2010

Abstract. A set of experiments utilising different implementations of the global ORCA-LIM model with horizontal resolutions of 2°, 0.5° and 0.25° is used to investigate tropical and extra-tropical influences on equatorial Pacific SST variability at interannual to decadal time scales. The model experiments use a bulk forcing methodology building on the global forcing data set for 1958 to 2000 developed by Large and Yeager (2004) that is based on a blend of atmospheric reanalysis data and satellite products. Whereas representation of the mean structure and transports of the (sub-)tropical Pacific current fields is much improved with the enhanced horizontal resolution, there is only little difference in the simulation of the interannual variability in the equatorial regime between the 0.5° and 0.25° model versions, with both solutions capturing the observed SST variability in the Nino3 region. The question of remotely forced oceanic contributions to the equatorial variability, in particular, the role of low-frequency changes in the transports of the Subtropical Cells (STCs), is addressed by a sequence of perturbation experiments using different combinations of fluxes. The solutions show the near-surface temperature variability to be governed by wind-driven changes in the Equatorial Undercurrent. The relative contributions of equatorial and off-equatorial atmospheric forcing differ between interannual and longer, (multi-)decadal timescales: for the latter there is a significant impact of changes in the equatorward transport of subtropical thermocline water associated with the lower branches of the STCs, related to variations in the off-equatorial trade winds. A conspicuous feature of the STC variability is that the equatorward transports in the interior and along the western boundary partially compensate each other at both decadal and interannual time scales, with the strongest transport extrema occurring during El Ni\\~no episodes. The behavior is rationalized in terms of a wobbling in the poleward extents of the tropical gyres, which is manifested also in a meridional shifting of the bifurcation latitudes of the North and South Equatorial Current systems.
Agulhas ring injection into the South Atlantic during glacials and interglacials
Authors: Zharkov V. , Nof D.
Published: April 2010

Abstract. Recent proxies analysis suggest that, at the end of the last glacial, there was a significant increase in the injection of Agulhas rings into the South Atlantic (SA). This brought about a dramatic increase in the salt-influx (from the Indian Ocean) into the SA helping re-start the then-collapsed meridional overturning cell (MOC), leading to the termination of the Younger Dryas (YD). Here, we propose a mechanism through which large variations in ring production take place. Using nonlinear analytical solutions for eddy shedding we show that there are restricted possibilities for ring detachment when the coast is oriented in the north-south direction. We define a critical coastline angle below which there is rings shedding and above which there is almost no shedding. In the case of the Agulhas region, the particular shape of the African continent implies that rings can be produced only when the retroflection occurs beyond a specific latitude where the angle is critical. During glaciation, the wind stress curl (WSC) vanished at a latitude lower than that of the critical angle, which prohibited the retroflection from producing rings. When the latitude at which the WSC vanishes migrated poleward towards its present day position, the corresponding coastline angle decreased below the critical angle and allowed for a vigorous production of rings. Simple process-oriented numerical simulations (using the Bleck and Boudra model) are in very good agreement with our results and enable us to affirm that, during the glacials, the behavior of the Agulhas Current (AC) was similar to that of the modern East Australian Current (EAC), for which the coastline slant is supercritical.
Turbulence closure: turbulence, waves and the wave-turbulence transition – Part 1: Vanishing mean shear
Authors: Baumert H. Z. , Peters H.
Published: April 2010

Abstract. A new two-equation, closure-like turbulence model for stably stratified flows is introduced which uses the turbulent kinetic energy (K) and the turbulent enstrophy (Ω) as primary variables. It accounts for mean shear – and internal wave-driven mixing in the two limits of mean shear and no waves and waves but no mean shear, respectively. The traditional TKE balance is augmented by an explicit energy transfer from internal waves to turbulence. A modification of the Ω-equation accounts for the effect of the waves on the turbulence time and space scales. The latter is based on the assumption of a non-zero constant flux Richardson number in the limit of vanishing mean-flow shear when turbulence is produced exclusively by internal waves. The new model reproduces the wave-turbulence transition analyzed by D'Asaro and Lien (2000). At small energy density E of the internal wave field, the turbulent dissipation rate (ε) scales like ε~E2. This is what is observed in the deep sea. With increasing E, after the wave-turbulence transition has been passed, the scaling changes to ε~E1. This is observed, for example, in the swift tidal flow near a sill in Knight Inlet. The new model further exhibits a turbulent length scale proportional to the Ozmidov scale, as observed in the ocean, and predicts the ratio between the turbulent Thorpe and Ozmidov length scales well within the range observed in the ocean.
Problems with estimating oceanic heat transport – conceptual remarks for the case of Fram Strait in the Arctic Ocean
Authors: Schauer U. , Beszczynska-Möller A.
Published: April 2010

Abstract. While the concept of oceanic heat transport – or rather heat transport divergence – is known since long, it is sometimes applied inaccurately. Often temperature transports are computed across sections with unbalanced volume flow which then depend entirely on the choice of the temperature scale. The consequences of such arbitrariness are demonstrated with a simple calculation exercise for the passages to the Arctic Ocean. To circumvent the arising difficulties for the Fram Strait as an example we propose a stream tube concept to define a net zero volume flow section which can, with coarse assumptions, be used to determine oceanic heat transport by the Atlantic water flow. Weaknesses of this approach and consequences for observational strategies are discussed.