The sensitivity of global ocean circulation to Southern Ocean winds and eddies

In coarse-resolution  models of the ocean circulation which have fixed surface temperature and salinity, the lateral transport of heat, mechanical energy budget, uptake of anthropogenic carbon dioxide, and structure of the thermohaline circulation are all sensitive to the magnitude of the wind stress and the parameterization of eddy mixing within the Southern Ocean. Increasing the winds increases the upwelling and transformation of deep waters (and thus the uptake of anthropogenic carbon dioxide, and injection of mechanical energy), while increasing the lateral eddy mixing coefficient has the opposite effect.  The Modeling Eddies in the Southern Ocean (MESO) project was designed to examine how much of the sensitivity to changes in winds could be offset by changes in the energetic eddies. Sensitivity studies to test this picture were performed with a hemispheric isopycnal-coordinate circulation model with realistic geometry and idealized forcing at a range of resolutions from relatively coarse (2º) to eddy-permitting  (1/6º).   A comparison between two such cases is shown in Figure 1.

Figure 1: Ocean surface speed in the model with 1º resolution (left) and 1/6º resolution (right). Note the overall envelope of the current is similar in the two runs. Note also the presence of energetic eddies and jets at many locations in the Southern Ocean.  

In our model, the great bulk of density transformation occurs within the surface layers. As we move from coarse to fine resolution we see effects of eddies on both the mean structure of the overturning and the response of the overturning to changes in surface winds. While the presence of resolved eddies does not greatly alter the qualitative picture of the ocean circulation developed over the years, it does result in some significant differences. In particular, eddies play an important role in the overturning cell involving the transformation of dense bottom waters and light tropical waters to intermediate waters.  In the presence of resolved eddies, the upper branch of this cell extends southwards by hundreds of kilometers and the lower branch draws on comparatively lighter deep waters. The response of this cell to changes in the surface winds is also sensitive to the presence of eddies. In non-eddying simulations changing the Ekman transport produces comparable changes in the overturning, much of it involving transformation of deep waters and resembling the mean circulation. In eddy-permitting simulations a significant fraction of the response is compensated by eddy-induced transport drawing from lighter waters, differing significantly from the mean circulation. This significant difference calls into question the ability of coarse-resolution ocean models to accurately capture the impact of changes in the Southern Ocean on the global ocean circulation.