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Improving oceanic overflow representation in climate models : the Gravity Current Entrainment Climate Process Team

2009-05 , Legg, Sonya , Ezer, Tal , Jackson, Laura , Briegleb, Bruce P. , Danabasoglu, Gokhan , Large, William G. , Wu, Wanli , Chang, Yeon , Ozgokmen, Tamay M. , Peters, Hartmut , Xu, Xiaobiao , Chassignet, Eric P. , Gordon, Arnold L. , Griffies, Stephen M. , Hallberg, Robert , Price, James F. , Riemenschneider, Ulrike , Yang, Jiayan

Oceanic overflows are bottom-trapped density currents originating in semienclosed basins, such as the Nordic seas, or on continental shelves, such as the Antarctic shelf. Overflows are the source of most of the abyssal waters, and therefore play an important role in the large-scale ocean circulation, forming a component of the sinking branch of the thermohaline circulation. As they descend the continental slope, overflows mix vigorously with the surrounding oceanic waters, changing their density and transport significantly. These mixing processes occur on spatial scales well below the resolution of ocean climate models, with the result that deep waters and deep western boundary currents are simulated poorly. The Gravity Current Entrainment Climate Process Team was established by the U.S. Climate Variability and Prediction (CLIVAR) Program to accelerate the development and implementation of improved representations of overflows within large-scale climate models, bringing together climate model developers with those conducting observational, numerical, and laboratory process studies of overflows. Here, the organization of the Climate Process Team is described, and a few of the successes and lessons learned during this collaboration are highlighted, with some emphasis on the well-observed Mediterranean overflow. The Climate Process Team has developed several different overflow parameterizations, which are examined in a hierarchy of ocean models, from comparatively well-resolved regional models to the largest-scale global climate models.

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The mean state and variability of the North Atlantic circulation: a perspective from ocean reanalyses

2019-11-06 , Jackson, Laura , Dubois, Clotilde , Forget, Gael , Haines, Keith , Harrison, Matthew , Iovino, Doroteaciro , Toyoda, Takahiro , Kohl, Armin , Mignac, Davi , Masina, Simona , Peterson, K. Andrew , Piecuch, Christopher G. , Roberts, Chris , Robson, Jon , Storto, Andrea , Toyoda, Takahiro , Valdivieso, Maria , Wilson, Christopher G. , Wang, Yiguo , Zuo, Hao

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.