Representation of Southern Ocean Properties across Coupled Model Intercomparison Project Generations: CMIP3 to CMIP6

Abstract

The air–sea exchange of heat and carbon in the Southern Ocean (SO) plays an important role in mediating the climate state. The dominant role the SO plays in storing anthropogenic heat and carbon is a direct consequence of the unique and complex ocean circulation that exists there. Previous generations of climate models have struggled to accurately represent key SO properties and processes that influence the large-scale ocean circulation. This has resulted in low confidence ascribed to twenty-first-century projections of the state of the SO from previous generations of models. This analysis provides a detailed assessment of the ability of models contributed to the sixth phase of the Coupled Model Intercomparison Project (CMIP6) to represent important observationally based SO properties. Additionally, a comprehensive overview of CMIP6 performance relative to CMIP3 and CMIP5 is presented. CMIP6 models show improved performance in the surface wind stress forcing, simulating stronger and less equatorward-biased wind fields, translating into an improved representation of the Ekman upwelling over the Drake Passage latitudes. An increased number of models simulate an Antarctic Circumpolar Current (ACC) transport within observational uncertainty relative to previous generations; however, several models exhibit extremely weak transports. Generally, the upper SO remains biased warm and fresh relative to observations, and Antarctic sea ice extent remains poorly represented. While generational improvement is found in many metrics, persistent systematic biases are highlighted that should be a priority during model development. These biases need to be considered when interpreting projected trends or biogeochemical properties in this region.

Resolving and Parameterising the Ocean Mesoscale in Earth System Models

“…Several other studies have also noted multi-decadal temporal variations in ACC transports, of 10 to 30 Sv, linked to the occurrence of unrealistic large open-ocean polynya events, in both the Ross and Weddell Sea, which alter the density structure of the Southern Ocean through intense spurious openocean convection [85]. In 1/4°and 1/2°GFDL models [91,92], super-polynyas in the Ross Sea have been shown to drive large centennial-scale variability in the Southern Hemisphere climate.…”

“…1c). Relative to observations from the cDrake array [74], models tend to fall on the lower end of observational uncertainty or underestimate the net ACC volume transport [85]. Models in the eddyparameterised or eddy-rich regime perform better than those in the eddy-present regime which are particularly sensitive to the choice of uncertain grid-scale closures as seen in forcedocean experiments [15,36,37].…”

“…Different aspects of the regional dynamics in the Southern Ocean are closely dynamically linked including the Antarctic circumpolar current (ACC), the Southern hemisphere upper and lower MOC cells, temperature and salinity structure and associated meridional gradients, and sea ice processes [85]. For example, the Southern Ocean mean state and its response to expected future changes in wind stress forcing, including the upper cell of its overturning circulation which influences heat and carbon uptake and isopycnal slopes that drive the ACC, result from a subtle balance between larger opposing wind-forced and eddy-driven cells [86].…”

“…The few CMIP6 models in the eddy-present regime provide a much more realistic representation of the frontal jets of the ACC [85] in contrast to the majority of models which have lower resolution and completely fail to represent any distinct fronts. These higher-resolution models, however, also show distinct counter flows (presumably linked to stationary eddies in Drake Passage) in multi-decadal means which are not evident in the observational means over similar periods (although they may be evident in sections from individual cruises).…”

Antarctic Bottom Water and North Atlantic Deep Water in CMIP6 models

“…A stronger AMOC is associated with larger biases in temperature and salinity in SPG (correlations of +0.33 and +0.37 respectively), and in particular a saltier NADW SPG (+0.34, as in the paleoclimate simulations of Menviel et al, 2020). The Atlantic SMOC is the only transport that is linked to the strength the Antarctic Circumpolar Current (ACC, values from Beadling et al, 2020): the stronger the ACC, the stronger the https://doi.org/10.5194/os-2020-66 Preprint. Discussion started: 10 July 2020 c Author(s) 2020.…”