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Assessing the Quality of Southern Ocean Circulation in CMIP5 AOGCM and Earth System Model Simulations

TitleAssessing the Quality of Southern Ocean Circulation in CMIP5 AOGCM and Earth System Model Simulations
Publication TypeJournal Article
Year of Publication2019
AuthorsBeadling R.L, Russell J.L, Stouffer R.J, Goodman P.J, Mazloff M.
Date Published2019/09
Type of ArticleArticle
ISBN Number0894-8755
Accession NumberWOS:000481476900002
Keywordsantarctic circumpolar current; atlantic; climate models; configuration; Coupled models; deep; General circulation; global overturning circulation; historical bias; Meteorology & Atmospheric Sciences; Model comparison; Model evaluation; models; Ocean circulation; pacific; performance; sea-ice; sensitivity; transport

The Southern Ocean (SO) is vital to Earth's climate system due to its dominant role in exchanging carbon and heat between the ocean and atmosphere and transforming water masses. Evaluating the ability of fully coupled climate models to accurately simulate SO circulation and properties is crucial for building confidence in model projections and advancing model fidelity. By analyzing multiple biases collectively across large model ensembles, physical mechanisms governing the diverse mean-state SO circulation found across models can be identified. This analysis 1) assesses the ability of a large ensemble of models contributed to phase 5 of the Coupled Model Intercomparison Project (CMIP5) to simulate observationally based metrics associated with an accurate representation of the Antarctic Circumpolar Current (ACC), and 2) presents a framework by which the quality of the simulation can be categorized and mechanisms governing the resulting circulation can be deduced. Different combinations of biases in critical metrics including the magnitude and position of the zonally averaged westerly wind stress maximum, wind-driven surface divergence, surface buoyancy fluxes, and properties and transport of North Atlantic Deep Water entering the SO produce distinct mean-state ACC transports. Relative to CMIP3, the quality of the CMIP5 SO simulations has improved. Eight of the thirty-one models simulate an ACC within observational uncertainty (2 sigma) for approximately the right reasons; that is, the models achieve accuracy in the surface wind stress forcing and the representation of the difference in the meridional density across the current. Improved observations allow for a better assessment of the SO circulation and its properties.

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