ALBERT

Beschreibung: The South Atlantic meridional transports are evaluated for four state-of-the-art global Ocean Reanalyses (ORAs) and two Free-Running Models (FRMs) in the period 1997–2010. All products employ the Nucleus for European Modelling of the Oceans model, and the ORAs share very similar configurations. The ORA basin interior transports are consistently modified relative to the FRMs, especially in the Argo period, with an improved representation of the south equatorial currents. The ORAs also exhibit systematically higher meridional transports than the FRMs, in closer agreement with large-scale observational estimates at 35° S and western boundary measurements at 11° S. However, the transport impacts by data assimilation still greatly vary between the ORAs, leading to differences up to ~ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the Meridional Heat Transports (MHTs), respectively. Large inter-product discrepancies arise in the ORA western boundary currents at both upper and deep levels explaining up to ~ 85 % of the inter-product differences in their total MHTs, and meridional velocity differences, rather than temperatures differences, drive ~ 83 % of this spread. Further analysis shows that only very confined temperature differences right against the western boundary geostrophically explain the large boundary current velocity differences. These findings suggest that the current data assimilation schemes, even with Argo data, can consistently constrain the basin interior circulation in the ORAs, but not the overturning transport component dominated by the narrow western boundary currents as in the South Atlantic.

Beschreibung: The meridional heat transport (MHT) of the South Atlantic plays a key role in the global heat budget: it is the only equatorward basin-scale ocean heat transport and it sets the northward direction of the global cross-equatorial transport. Its strength and variability, however, are not well known. The South Atlantic transports are evaluated for four state-of-the-art global ocean reanalyses (ORAs) and two free-running models (FRMs) in the period 1997–2010. All products employ the Nucleus for European Modelling of the Oceans (NEMO) model, and the ORAs share very similar configurations. Very few previous works have looked at ocean circulation patterns in reanalysis products, but here we show that the ORA basin interior transports are consistently improved by the assimilated in situ and satellite observations relative to the FRMs, especially in the Argo period. The ORAs also exhibit systematically higher meridional transports than the FRMs, which is in closer agreement with observational estimates at 35 and 11∘ S. However, the data assimilation impact on the meridional transports still greatly varies among the ORAs, leading to differences up to ∼ 8 Sv and 0.4 PW in the South Atlantic Meridional Overturning Circulation and the MHTs, respectively. We narrow this down to large inter-product discrepancies in the western boundary currents (WBCs) at both upper and deep levels explaining up to ∼ 85 % of the inter-product differences in MHT. We show that meridional velocity differences, rather than temperature differences, in the WBCs drive ∼ 83 % of this MHT spread. These findings show that the present ocean observation network and data assimilation schemes can be used to consistently constrain the South Atlantic interior circulation but not the overturning component, which is dominated by the narrow western boundary currents. This will likely limit the effectiveness of ORA products for climate or decadal prediction studies.

Beschreibung: Author Posting. © American Geophysical Union, 2019. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Journal of Geophysical Research-Oceans 124, (2019): 9141-9170, doi: 10.1029/2019JC015210.

Beschreibung: The observational network around the North Atlantic has improved significantly over the last few decades with subsurface profiling floats and satellite observations and the recent efforts to monitor the Atlantic Meridional Overturning Circulation (AMOC). These have shown decadal time scale changes across the North Atlantic including in heat content, heat transport, and the circulation. However, there are still significant gaps in the observational coverage. Ocean reanalyses integrate the observations with a dynamically consistent ocean model and can be used to understand the observed changes. However, the ability of the reanalyses to represent the dynamics must also be assessed. We use an ensemble of global ocean reanalyses to examine the time mean state and interannual‐decadal variability of the North Atlantic ocean since 1993. We assess how well the reanalyses are able to capture processes and whether any understanding can be gained. In particular, we examine aspects of the circulation including convection, AMOC and gyre strengths, and transports. We find that reanalyses show some consistency, in particular showing a weakening of the subpolar gyre and AMOC at 50°N from the mid‐1990s until at least 2009 (related to decadal variability in previous studies), a strengthening and then weakening of the AMOC at 26.5°N since 2000, and impacts of circulation changes on transports. These results agree with model studies and the AMOC observations at 26.5°N since 2005. We also see less spread across the ensemble in AMOC strength and mixed layer depth, suggesting improvements as the observational coverage has improved.

Beschreibung: This work was initiated through the EU COST‐EOS‐1402 project which supported the development of this paper by funding project meetings, both in person and virtual. We would like to thank Aida Azcarate for organizing the funding for the meetings and would like to thank Martha Buckley, Gokhan Danabasoglu, and Simon Josey for useful discussions. Jackson, Storto and Zuo were partially funded, by the Copernicus Marine Environment Monitoring Service (CMEMS: 23‐GLO‐RAN) and Zuo was partially funded by the Copernicus Climate Change Service. Jackson was also partially funded by the joint UK BEIS/Defra Met Office Hadley Centre Climate Programme (GA01101). Haines and Robson acknowledge funding under the NERC RAPID projects RAMOC and DYNAMOC (NE/M005127/1) respectively, and Robson also acknowledges funding from the ACSIS project. Mignac was supported for PhD scholarship by the CAPES Foundation, Ministry of Education of Brazil (Proc. BEX 1386/15‐8). Forget acknowledges support from the Simons Foundation (549931) and the NASA IDS program (6937342). Work by Piecuch was carried out under the ECCO project, funded by the NASA Physical Oceanography, Cryospheric Science, and Modeling, Analysis and Prediction programs, and supported by the Independent Research and Development Program at Woods Hole Oceanographic Institution. Wilson was funded by the NERC UK‐OSNAP project (NE/K010875.1) as part of the international OSNAP program. NorCPM‐v1 reanalysis was cofunded by the Center for Climate Dynamics at the Bjerknes Center, the Norwegian Research Council under the EPOCASA (229774/E10) and SFE (270733) research projects, the NordForsk under the Nordic Centre of Excellence (ARCPATH, 76654), and the Trond Mohn Foundation under the project BFS2018TMT01. NorCPM‐v1 reanalysis received a grant for computer time from the Norwegian Program for supercomputer (NOTUR2, project NN9039K) and a storage grant (NORSTORE, NS9039K). Data for the figures are available to download (from https://doi.org/10.5281/zenodo.2598509). Data from some reanalysis products are available to download (from http://marine.copernicus.eu/services-portfolio/access-to-products/) under product names GLOBAL_REANALYSIS_PHY_001_025 (GLORYS2v4), GLOBAL_REANALYSIS_PHY_001_026 (C‐GLORSv7, GLORYS2v4, GloSea5 and ORAS5) and GLOBAL_REANALYSIS_PHY_001_030 (GLORYS12V1).

Beschreibung: 2020-05-06