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

Abstract

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 (2σ) 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.

Do Multi‐Model Ensembles Improve Reconstruction Skill in Paleoclimate Data Assimilation?

“…Specifically, for several target models (CSIRO, MPI), there is largest improvement over the mid‐ and high‐latitude Southern Ocean. In these regions there are few nearby observations and models tend to disagree on locations of frontal regions (e.g., Beadling et al., 2019; Holmes et al., 2019). For reconstructions of other target models, the lower error in the MME reconstructions is less focused over the high‐latitude southern hemisphere (Figure 2).…”

The GFDL Earth System Model Version 4.1 (GFDL‐ESM 4.1): Overall Coupled Model Description and Simulation Characteristics

“…Midlatitude SST was further raised to match those in CM4.0 by boosting the submesoscale restratification parameterization relative to the strength used in CM4.0 (Adcroft et al, 2019). Southern Ocean SST and mixed layer biases : Another common bias among CMIP6 models (Beadling et al, 2019; Russell et al, 2006) is warm SST and shallow mixed layer biases in the Southern Ocean, which has important implications for representation of the seasonal cycle in pCO 2 and associated carbon fluxes. For both CM4.0 and ESM4.1, incorporation of the Reichl and Hallberg (2018) surface boundary layer parameterization brought strong improvement to both summer and winter mixed layers.…”

On the Role of the Antarctic Slope Front on the Occurrence of the Weddell Sea Polynya under Climate Change

“…AABW formation in CMIP5 models is primarily controlled by these convection events, as coastal processes leading to the formation and cascading of dense shelf waters down the continental slope are not resolved in these relatively coarse resolution models (⇡ 1 ). CMIP5 models, therefore, often exhibit biases in Southern Ocean water mass properties and stratification (Heywood et al, 2014), as well as in their representation of large scale ocean dynamics such as the Antarctic Circumpolar Current (Beadling et al 2019).…”