ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are as... more ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are assimilated into a global ocean model has been produced for the period 1958-1998. The assimilation scheme is a univariate, variational optimum interpolation of temperature only in which the first guess is produced by a global ocean general circulation model with a horizontal resolution of 0.5 degree, 31 levels in the vertical and forced by twice daily NCEP reanalysis. The temperature analysis shows encouraging improvement over a corresponding forced ocean simulation and significantly reduces some of the model errors such as the location and amplitude of the surface temperature front along the Gulf Stream and the vertical thermal structure of the region. Decadal evolution of warm and cold anomalies in the Sea Surface Temperature (SST) and upper Ocean Heat Content (OHC) in the subtropical and subpolar gyres of the North Atlantic is detected and investigated. Conventional EOF analysis shows that an important mode of variability of the wintertime upper ocean climate over the North Atlantic during the period of study is characterized by a tripole pattern both in SST and upper OHC. This mode shows a more complicated pattern with respect to a similar mode detected in the analysis of different available SST products. In our ocean analysis the one sign anomalies characteristic of the subtropical gyre extends northward into the Labrador Sea region. The first EOF of the upper OHC winter anomalies shows a similar pattern and exhibits variability on quasi-decadal time scales as in the case of the SST. Finally, in order to identify possible propagating processes of winter SST and OHC anomalies an Extended EOF analysis will be presented.
ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are as... more ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are assimilated into a global ocean model has been produced for the period 1958-1998. The assimilation scheme is a univariate, variational optimum interpolation of temperature only in which the first guess is produced by a global ocean general circulation model with a horizontal resolution of 0.5 degree, 31 levels in the vertical and forced by twice daily NCEP reanalysis. The temperature analysis shows encouraging improvement over a corresponding forced ocean simulation and significantly reduces some of the model errors such as the location and amplitude of the surface temperature front along the Gulf Stream and the vertical thermal structure of the region. Mechanisms of SST and heat content anomalies propagation in the Atlantic Ocean are investigated using both the ocean analysis and a 100-year experiment conducted with a coupled general circulation model (CGCM). The atmospheric component in the CGCM has a T42 horizontal resolution and 19 vertical levels; while the oceanic counterpart has a variable mesh in the latitudinal direction (resolution is 2 degrees in longitude and 0.5 to 2 degrees in latitude) and 31 vertical levels. The ocean component of the CGCM is coupled with a dynamic sea-ice model, that is integrated using the same horizontal grid of the ocean model. The skill with which the formation, propagation and decay of decadal thermodynamic fluctuations in the Atlantic Ocean can be simulated by the CGCM has been examined through the comparison with the ocean analysis. In this study, a particular emphasis is given to the contribution that atmosphere-ocean interaction and ocean advection have on the surface and subsurface temperature anomalies propagation.
The simulated North Atlantic atmosphere–ocean variability is assessed in a subset of models from... more The simulated North Atlantic atmosphere–ocean variability is assessed in a subset of models from HighResMIP that have either low-resolution (LR) or high-resolution (HR) in their atmosphere and ocean model components. In general, the LR models overestimate the low-frequency variability of subpolar sea-surface temperature (SST) anomalies and underestimate their correlation with the NAO compared to ERA5 reanalysis. These biases are substantially reduced in the HR models, and it is shown that the improvements are related to a reduction of intrinsic (non-NAO-driven) variability of the subpolar ocean circulation.To understand the mechanisms behind the overestimated intrinsic subpolar ocean variability in the LR models, a link is demonstrated between the biases in subpolar ocean variability and known biases in the mean state of the Labrador-Irminger seas. Supporting previous studies, the Labrador-Irminger seas are found to be too cold and too fresh in the LR models compared to observation...
Recent studies have established that the typical atmospheric and oceanic resolutions used for the... more Recent studies have established that the typical atmospheric and oceanic resolutions used for the CMIP5 coordinated exercise (Coupled Model Intercomparison Project, phase 5), i.e., around 40km-150km globally, are limiting factors to correctly reproduce the climate mean state and variability. In the context of ocean forecasting like the Copernicus Marine Environment Monitoring Service (CMEMS, https://marine.copernicus.eu/), increasing resolution appears necessary to improve the quality of service and to satisfy the users' needs in the operational application. Resolving scales below 100 kilometers, particularly submesoscale processes (1-50 km), is necessary to better represent the circulation in the open ocean and improve the large-scale representations. Therefore, Ocean General Circulation Models (OGCM) have to evolve to resolve the global ocean flows at kilometric scale by exploiting massively parallel architectures and reducing I/O bottlenecks. In the framework of the ESiWACE p...
<p>Understanding the causes of the variability of the North Atlantic and Me... more <p>Understanding the causes of the variability of the North Atlantic and Mediterranean overturning circulations, and the possible correlation between them is important to disentangle the processes which link the two ocean basins. In this study, we hypothesize that the Gibraltar inflow transport is the main driver of the basin-mean sea surface height variability in the Mediterranean Sea and that they are both anti-correlated to the Atlantic Meridional Overturning Circulation (AMOC) in the North Atlantic.</p><p>We analyze here the AMOC and the Mediterranean mean sea surface height (SSH) in an ensemble of eddy-permitting global ocean reanalyses and the Gibraltar inflow transport using an eddy-resolving Mediterranean Reanalysis over the period 1993-2019. In this contribution, firstly we extend the results obtained in past literature with observations (2004-2017 period) and confirm the anti-correlation between the Mediterranean mean sea level and the upper branch of the AMOC at 26.5°N over the 1993-2019 period. Secondly, for the first time, we examine the correlation of the different components of the AMOC and the Gibraltar inflow transport and find significant anti-correlations at interannual time scales.</p><p>We show that during years of weaker/stronger AMOC and higher/lower SSH in the Mediterranean Sea, a stronger/weaker Azores Current results in stronger/weaker Gibraltar inflow transport. We argue that the anticorrelation between AMOC and the mean sea level of the Mediterranean Sea is explained by the anticorrelation between AMOC and the Gibraltar inflow transport which in turn is changed by the wind driven Azores current strength.</p>
<p>Recent intercomparison studies among ocean/sea-ice Reanalyses (s... more <p>Recent intercomparison studies among ocean/sea-ice Reanalyses (such as ORA-IP) have shown large discrepancies in many sea-ice-related fields, despite a rather general agreement in the sea-ice extension. The low accuracy of sea-ice thickness measurements together with the highly non-gaussian distributions of related uncertainty, made multivariate sea-ice data assimilation (DA) strategies still at an early stage, although nearly twenty years of thickness observations are now available. In a standard multivariate scheme, the break of Gaussianity can generate un-realistic corrections due to the poor linear relationship driven by the B matrix.</p><p>One approach to solve the problem is the implementation of anamorphous transformations that modify the probability density functions of ice anomalies into Gaussian ones (Brankart et al. 2012). In this study, a 3DVar DA scheme (called OceanVar), employed in the routinely production of global/regional ocean reanalysis CGLORS (Storto et al, 2016), has been recently extended to ingest sea-ice concentration (SIC) and thickness (SIT) data. An anamorphous operator, firstly developed and made freely available within the SANGOMA project (http://www.data-assimilation.net/), has been updated and adapted for the bivariate assimilation of SIC/SIT within the OceanVar framework.</p><p>We present the comparison among several sensitivity experiments that were performed assimilating different observation datasets and using different DA configurations at 1/4 degree global resolution. Specifically, we assess the impact of ingesting different SIT products, such as SMOS and CRYOSAT-2 data or the merged product CS2SMOS.</p><p>We show that the sole assimilation of SIC improves the spatial representation of SIT with respect to a free run. The inclusion of thickness correction, determined by empirical relations, appears to improve the sea ice characteristics in the Atlantic sector and degrade them in the Siberian region; therefore a refined tuning could probably be beneficial. The spatial error reduces sharply only once CRYOSAT-2 data are assimilated jointly with SIC data. In the present set up, all the experiments generally tend to overestimate the sea-ice volume in the case SMOS data are not assimilated. However, observational errors associated with SMOS data are generally too small, leading to jumps in the volume time series at the beginning of the accretion period if not calibrated correctly.</p><p>The proposed approach is suitable to be used for covarying ocean/sea-ice variables in future coupled ocean/sea-ice DA.</p><p>Storto, A. and Masina, S. (2016), Earth Syst. Sci. Data, 8, 679, doi: 0.5194/essd-8-679-2016</p><p>Brankart, et al. (2012), Ocean Sci., 8, 121, doi: 10.5194/os-8-121-2012</p><p> </p>
Potential impact of sea-ice initialization on the interannual climate predictability over the Wed... more Potential impact of sea-ice initialization on the interannual climate predictability over the Weddell Sea is investigated using a coupled general circulation model. Climate variability in the Weddell Sea is generally believed to have association with remote forcing such as El Niño-Southern Oscillation and the Southern Annual Mode. However, sea-ice variability in the Weddell Sea has been recently suggested to play additional roles in modulating local atmospheric variability through changes in surface air temperature and near-surface baroclinicity. Reforecast experiments from September 1st, in which the model’s sea-surface temperature (SST) and sea-ice concentration (SIC) are initialized with observations using nudging schemes, show improvements in predicting the observed SIC anomalies in the Weddell Sea up to four months ahead, compared to the other experiments in which only the model’s SST is initialized. During austral spring (Oct–Dec) of lower-than-normal sea-ice years in the Wedd...
In a warming climate, observations indicate that the sea ice extent around Antarctica has increas... more In a warming climate, observations indicate that the sea ice extent around Antarctica has increased over the last decades. One of the suggested explanations is the stabilizing effect of increased mass loss of the Antarctic ice sheet. We investigated the sea ice response to changes in the amount and especially the spatial distribution of freshwater. We performed a sensitivity study by comparing a set of numerical simulations with additional supply of water at the Antarctic ocean surface. Here, we analyse the response of the sea ice cover and the on-shelf water column to variations in the amount and distribution of the prescribed surface freshwater flux. <br><br> Our results confirm that an increase in fresh water input can increase the sea ice extent. However, a very strong increase of freshwater will eventually invert the trend. Our experiments suggest that the spatial distribution of the freshwater is of great influence. It affects sea ice dynamics and can strongly alte...
A set of hindcast simulations with the new NEMO3.6 ocean-ice model in the ORCA1 grid and forced b... more A set of hindcast simulations with the new NEMO3.6 ocean-ice model in the ORCA1 grid and forced by the DFS5.2 atmospheric data was performed from 1958–2012. We focussed on simulations that differ only in their sea-ice component: the old standard version LIM2 and its successor LIM3. Main differences between these sea-ice models are the parameterisations of sub-grid-scale sea-ice thickness distribution, ice deformation, thermodynamic processes, and sea-ice salinity. Our main objective was to diagnose the ocean-ice sensitivity to the updated NEMO-LIM sea-ice physics. Results of such analysis have not been published for this new NEMO version.<br><br> In the polar regions, NEMO-LIM3 compares better with observations, while NEMO-LIM2 deviates more, producing too much ice in the Arctic, for example. Differences between NEMO-LIM2 and NEMO-LIM3 do not change in simulations even when the freshwater adjustments are turned off. In the extra-polar regions, the oceanographic con...
Analysis of a global eddy-resolving simulation using the NEMO (version 3.4) general circulation m... more Analysis of a global eddy-resolving simulation using the NEMO (version 3.4) general circulation model is presented. The model has…
ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are as... more ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are assimilated into a global ocean model has been produced for the period 1958-1998. The assimilation scheme is a univariate, variational optimum interpolation of temperature only in which the first guess is produced by a global ocean general circulation model with a horizontal resolution of 0.5 degree, 31 levels in the vertical and forced by twice daily NCEP reanalysis. The temperature analysis shows encouraging improvement over a corresponding forced ocean simulation and significantly reduces some of the model errors such as the location and amplitude of the surface temperature front along the Gulf Stream and the vertical thermal structure of the region. Decadal evolution of warm and cold anomalies in the Sea Surface Temperature (SST) and upper Ocean Heat Content (OHC) in the subtropical and subpolar gyres of the North Atlantic is detected and investigated. Conventional EOF analysis shows that an important mode of variability of the wintertime upper ocean climate over the North Atlantic during the period of study is characterized by a tripole pattern both in SST and upper OHC. This mode shows a more complicated pattern with respect to a similar mode detected in the analysis of different available SST products. In our ocean analysis the one sign anomalies characteristic of the subtropical gyre extends northward into the Labrador Sea region. The first EOF of the upper OHC winter anomalies shows a similar pattern and exhibits variability on quasi-decadal time scales as in the case of the SST. Finally, in order to identify possible propagating processes of winter SST and OHC anomalies an Extended EOF analysis will be presented.
ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are as... more ABSTRACT An ocean analysis where both surface and subsurface hydrographic temperature data are assimilated into a global ocean model has been produced for the period 1958-1998. The assimilation scheme is a univariate, variational optimum interpolation of temperature only in which the first guess is produced by a global ocean general circulation model with a horizontal resolution of 0.5 degree, 31 levels in the vertical and forced by twice daily NCEP reanalysis. The temperature analysis shows encouraging improvement over a corresponding forced ocean simulation and significantly reduces some of the model errors such as the location and amplitude of the surface temperature front along the Gulf Stream and the vertical thermal structure of the region. Mechanisms of SST and heat content anomalies propagation in the Atlantic Ocean are investigated using both the ocean analysis and a 100-year experiment conducted with a coupled general circulation model (CGCM). The atmospheric component in the CGCM has a T42 horizontal resolution and 19 vertical levels; while the oceanic counterpart has a variable mesh in the latitudinal direction (resolution is 2 degrees in longitude and 0.5 to 2 degrees in latitude) and 31 vertical levels. The ocean component of the CGCM is coupled with a dynamic sea-ice model, that is integrated using the same horizontal grid of the ocean model. The skill with which the formation, propagation and decay of decadal thermodynamic fluctuations in the Atlantic Ocean can be simulated by the CGCM has been examined through the comparison with the ocean analysis. In this study, a particular emphasis is given to the contribution that atmosphere-ocean interaction and ocean advection have on the surface and subsurface temperature anomalies propagation.
The simulated North Atlantic atmosphere–ocean variability is assessed in a subset of models from... more The simulated North Atlantic atmosphere–ocean variability is assessed in a subset of models from HighResMIP that have either low-resolution (LR) or high-resolution (HR) in their atmosphere and ocean model components. In general, the LR models overestimate the low-frequency variability of subpolar sea-surface temperature (SST) anomalies and underestimate their correlation with the NAO compared to ERA5 reanalysis. These biases are substantially reduced in the HR models, and it is shown that the improvements are related to a reduction of intrinsic (non-NAO-driven) variability of the subpolar ocean circulation.To understand the mechanisms behind the overestimated intrinsic subpolar ocean variability in the LR models, a link is demonstrated between the biases in subpolar ocean variability and known biases in the mean state of the Labrador-Irminger seas. Supporting previous studies, the Labrador-Irminger seas are found to be too cold and too fresh in the LR models compared to observation...
Recent studies have established that the typical atmospheric and oceanic resolutions used for the... more Recent studies have established that the typical atmospheric and oceanic resolutions used for the CMIP5 coordinated exercise (Coupled Model Intercomparison Project, phase 5), i.e., around 40km-150km globally, are limiting factors to correctly reproduce the climate mean state and variability. In the context of ocean forecasting like the Copernicus Marine Environment Monitoring Service (CMEMS, https://marine.copernicus.eu/), increasing resolution appears necessary to improve the quality of service and to satisfy the users' needs in the operational application. Resolving scales below 100 kilometers, particularly submesoscale processes (1-50 km), is necessary to better represent the circulation in the open ocean and improve the large-scale representations. Therefore, Ocean General Circulation Models (OGCM) have to evolve to resolve the global ocean flows at kilometric scale by exploiting massively parallel architectures and reducing I/O bottlenecks. In the framework of the ESiWACE p...
&amp;lt;p&amp;gt;Understanding the causes of the variability of the North Atlantic and Me... more &amp;lt;p&amp;gt;Understanding the causes of the variability of the North Atlantic and Mediterranean overturning circulations, and the possible correlation between them is important to disentangle the processes which link the two ocean basins. In this study, we hypothesize that the Gibraltar inflow transport is the main driver of the basin-mean sea surface height variability in the Mediterranean Sea and that they are both anti-correlated to the Atlantic Meridional Overturning Circulation (AMOC) in the North Atlantic.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We analyze here the AMOC and the Mediterranean mean sea surface height (SSH) in an ensemble of eddy-permitting global ocean reanalyses and the Gibraltar inflow transport using an eddy-resolving Mediterranean Reanalysis over the period 1993-2019. In this contribution, firstly we extend the results obtained in past literature with observations (2004-2017 period) and confirm the anti-correlation between the Mediterranean mean sea level and the upper branch of the AMOC at 26.5&amp;amp;#176;N over the 1993-2019 period. Secondly, for the first time, we examine the correlation of the different components of the AMOC and the Gibraltar inflow transport and find significant anti-correlations at interannual time scales.&amp;lt;/p&amp;gt;&amp;lt;p&amp;gt;We show that during years of weaker/stronger AMOC and higher/lower SSH in the Mediterranean Sea, a stronger/weaker Azores Current results in stronger/weaker Gibraltar inflow transport. We argue that the anticorrelation between AMOC and the mean sea level of the Mediterranean Sea is explained by the anticorrelation between AMOC and the Gibraltar inflow transport which in turn is changed by the wind driven Azores current strength.&amp;lt;/p&amp;gt;
&amp;amp;lt;p&amp;amp;gt;Recent intercomparison studies among ocean/sea-ice Reanalyses (s... more &amp;amp;lt;p&amp;amp;gt;Recent intercomparison studies among ocean/sea-ice Reanalyses (such as ORA-IP) have shown large discrepancies in many sea-ice-related fields, despite a rather general agreement in the sea-ice extension. The low accuracy of sea-ice thickness measurements together with the highly non-gaussian distributions of related uncertainty, made multivariate sea-ice data assimilation (DA) strategies still at an early stage, although nearly twenty years of thickness observations are now available. In a standard multivariate scheme, the break of Gaussianity can generate un-realistic corrections due to the poor linear relationship driven by the B matrix.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;One approach to solve the problem is the implementation of anamorphous transformations that modify the probability density functions of ice anomalies into Gaussian ones (Brankart et al. 2012). In this study, a 3DVar DA scheme (called OceanVar), employed in the routinely production of global/regional ocean reanalysis CGLORS (Storto et al, 2016), has been recently extended to ingest sea-ice concentration (SIC) and thickness (SIT) data. An anamorphous operator, firstly developed and made freely available within the SANGOMA project (http://www.data-assimilation.net/), has been updated and adapted for the bivariate assimilation of SIC/SIT within the OceanVar framework.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;We present the comparison among several sensitivity experiments that were performed assimilating different observation datasets and using different DA configurations at 1/4 degree global resolution. Specifically, we assess the impact of ingesting different SIT products, such as SMOS and CRYOSAT-2 data or the merged product CS2SMOS.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;We show that the sole assimilation of SIC improves the spatial representation of SIT with respect to a free run. The inclusion of thickness correction, determined by empirical relations, appears to improve the sea ice characteristics in the Atlantic sector and degrade them in the Siberian region; therefore a refined tuning could probably be beneficial. The spatial error reduces sharply only once CRYOSAT-2 data are assimilated jointly with SIC data. In the present set up, all the experiments generally tend to overestimate the sea-ice volume in the case SMOS data are not assimilated. However, observational errors associated with SMOS data are generally too small, leading to jumps in the volume time series at the beginning of the accretion period if not calibrated correctly.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;The proposed approach is suitable to be used for covarying ocean/sea-ice variables in future coupled ocean/sea-ice DA.&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;Storto, A. and Masina, S. (2016), Earth Syst. Sci. Data, 8, 679, doi: 0.5194/essd-8-679-2016&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;Brankart, et al. (2012), Ocean Sci., 8, 121, doi: 10.5194/os-8-121-2012&amp;amp;lt;/p&amp;amp;gt;&amp;amp;lt;p&amp;amp;gt;&amp;amp;amp;#160;&amp;amp;lt;/p&amp;amp;gt;
Potential impact of sea-ice initialization on the interannual climate predictability over the Wed... more Potential impact of sea-ice initialization on the interannual climate predictability over the Weddell Sea is investigated using a coupled general circulation model. Climate variability in the Weddell Sea is generally believed to have association with remote forcing such as El Niño-Southern Oscillation and the Southern Annual Mode. However, sea-ice variability in the Weddell Sea has been recently suggested to play additional roles in modulating local atmospheric variability through changes in surface air temperature and near-surface baroclinicity. Reforecast experiments from September 1st, in which the model’s sea-surface temperature (SST) and sea-ice concentration (SIC) are initialized with observations using nudging schemes, show improvements in predicting the observed SIC anomalies in the Weddell Sea up to four months ahead, compared to the other experiments in which only the model’s SST is initialized. During austral spring (Oct–Dec) of lower-than-normal sea-ice years in the Wedd...
In a warming climate, observations indicate that the sea ice extent around Antarctica has increas... more In a warming climate, observations indicate that the sea ice extent around Antarctica has increased over the last decades. One of the suggested explanations is the stabilizing effect of increased mass loss of the Antarctic ice sheet. We investigated the sea ice response to changes in the amount and especially the spatial distribution of freshwater. We performed a sensitivity study by comparing a set of numerical simulations with additional supply of water at the Antarctic ocean surface. Here, we analyse the response of the sea ice cover and the on-shelf water column to variations in the amount and distribution of the prescribed surface freshwater flux. <br><br> Our results confirm that an increase in fresh water input can increase the sea ice extent. However, a very strong increase of freshwater will eventually invert the trend. Our experiments suggest that the spatial distribution of the freshwater is of great influence. It affects sea ice dynamics and can strongly alte...
A set of hindcast simulations with the new NEMO3.6 ocean-ice model in the ORCA1 grid and forced b... more A set of hindcast simulations with the new NEMO3.6 ocean-ice model in the ORCA1 grid and forced by the DFS5.2 atmospheric data was performed from 1958–2012. We focussed on simulations that differ only in their sea-ice component: the old standard version LIM2 and its successor LIM3. Main differences between these sea-ice models are the parameterisations of sub-grid-scale sea-ice thickness distribution, ice deformation, thermodynamic processes, and sea-ice salinity. Our main objective was to diagnose the ocean-ice sensitivity to the updated NEMO-LIM sea-ice physics. Results of such analysis have not been published for this new NEMO version.<br><br> In the polar regions, NEMO-LIM3 compares better with observations, while NEMO-LIM2 deviates more, producing too much ice in the Arctic, for example. Differences between NEMO-LIM2 and NEMO-LIM3 do not change in simulations even when the freshwater adjustments are turned off. In the extra-polar regions, the oceanographic con...
Analysis of a global eddy-resolving simulation using the NEMO (version 3.4) general circulation m... more Analysis of a global eddy-resolving simulation using the NEMO (version 3.4) general circulation model is presented. The model has…
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