Southern Ocean Circulation and Eddy Compensation in CMIP5 Models

Abstract

Thirteen state-of-the-art climate models from phase 5 of the Coupled Model Intercomparison Project (CMIP5) are used to evaluate the response of the Antarctic Circumpolar Current (ACC) transport and Southern Ocean meridional overturning circulation to surface wind stress and buoyancy changes. Understanding how these flows-fundamental players in the global distribution of heat, gases, and nutrientsrespond to climate change is currently a widely debated issue among oceanographers. Here, the authors analyze the circulation responses of these coarse-resolution coupled models to surface fluxes. Under a future CMIP5 climate pathway where the equivalent atmospheric CO 2 reaches 1370 ppm by 2100, the models robustly project reduced Southern Ocean density in the upper 2000 m accompanied by strengthened stratification. Despite an overall increase in overlying wind stress (;20%), the projected ACC transports lie within 615% of their historical state, and no significant relationship with changes in the magnitude or position of the wind stress is identified. The models indicate that a weakening of ACC transport at the end of the twenty-first century is correlated with a strong increase in the surface heat and freshwater fluxes in the ACC region. In contrast, the surface heat gain across the ACC region and the wind-driven surface transports are significantly correlated with an increased upper and decreased lower Eulerian-mean meridional overturning circulation. The change in the eddy-induced overturning in both the depth and density spaces is quantified, and it is found that the CMIP5 models project partial eddy compensation of the upper and lower overturning cells.

OMIP contribution to CMIP6: experimental and diagnostic protocol for the physical component of the Ocean Model Intercomparison Project

“…Gregory, 2000), and the response of the Antarctic Circumpolar Current (ACC) and Southern Ocean meridional overturning circulation (MOC) to changes in surface forcing (e.g. Downes and Hogg, 2013;Downes et al, 2015;Farneti et al, 2015).…”

Response of the Ocean Natural Carbon Storage to Projected Twenty-First-Century Climate Change

“…This suggests that in our simulations the wind-driven ACC acceleration is counterbalanced by a buoyancy-driven slowdown. In an extensive analysis of CMIP5 models, Downes and Hogg (2013) showed that a weakened ACC is a common response to increasing freshwater and heat fluxes in many CMIP5 models, while the ACC response to winds is less certain. Their analysis separated these effects statistically (i.e., via linear regression) rather than via manipulation of the model forcing, as was done here.…”

The Flux-Anomaly-Forced Model Intercomparison Project (FAFMIP) contribution to CMIP6: investigation of sea-level and ocean climate change in response to CO<sub>2</sub> forcing

“…The Southern Ocean feature results both from changes to the surface heat flux and from an intensification and southward shift of the westerly wind stress, which strengthens the Ekman drift and tends to tilt the isopycnals (Mikolajewicz and Voss, 2000;Lowe and Gregory, 2006;Landerer et al, 2007;Frankcombe et al, 2013;Bouttes and Gregory, 2014;Kuhlbrodt et al, 2015;Saenko et al, 2015;Marshall et al, 2015). Eddies tend to oppose the latter effect by removing available potential energy, thus partly compensating for the effect of wind-stress change in ζ , and limiting the sensitivity of the circumpolar circulation to wind-stress change (Hallberg and Gnanadesikan, 2006;Böning et al, 2008;Farneti et al, 2010Farneti et al, , 2015Downes and Hogg, 2013). Most AOGCMs used for multidecadal simulations do not resolve ocean eddies at high latitudes, so their results will depend on their parametrisations of eddy advection on isoneutral surfaces (e.g.…”