Towards the impact of eddies on the response of the Southern Ocean to climate change

The sensitivity of the meridional overturning circulation (MOC) of the Southern Ocean (SO) to wind stress changes is discussed. Using an idealised SO model in both non- and eddy-permitting configurations, we assess the effects of both, coarsening the horizontal resolution and implementing different parameterisations for the lateral eddy diffusivity appropriate to the Gent and McWilliams (1990) parameterisation, K. We find that the MOC is characterised by an eddy-driven part ψ∗ which generally opposes the wind-driven part and that the increase of the MOC diminishes with amplifying winds, with the possibility that the MOC in the SO may become completely insensitive to wind stress changes. However, for moderate wind stress, the MOC is still significantly increasing in our configuration.The diagnosed lateral eddy diffusivity K in the eddy-permitting version shows strong spatial variability and is increasing with increasing wind stress. Similar to the MOC (but in contrast to ψ∗) the increase of K diminishes with amplifying winds. It turns out that a small increase in the isopycnal slopes is also relevant in order to capture the correct sensitivity of ψ∗ on wind stress. This relation also holds in model configurations with coarser but still eddy-permitting horizontal resolution: decreasing the horizontal resolution decreases K, but increases the isopycnal slopes such that the strength of the MOC including its sensitivity to wind stress is almost unchanged. The parameterisations are able to reproduce the MOC for certain wind stresses, but all parameterisations underestimate the sensitivity of K and thus overestimate the sensitivity of the MOC on wind stress. Our results show that it is indispensable to incorporate the correct sensitivity of K into climate models in order to reproduce the correct sensitivity of the MOC to wind stress and that up-to-date parameterisations for K are only partially successful.