Sensitivity of the overturning circulation in the Southern Ocean to decadal changes in wind forcing

The sensitivity of the overturning circulation in the Southern Ocean to the recent decadal strengthening of the overlying winds is being discussed intensely, with some works attributing an inferred saturation of the Southern Ocean CO2 sink to an intensification of the overturning circulation, whilst others have argued that this circulation is insensitive to changes in winds. Fundamental to reconciling these diverse views is to understand properly the role of eddies in counteracting the directly wind-forced changes in overturning.

Ocean eddies are a ubiquitous feature of the Southern Ocean, and recent work has shown that the strength of the eddy field increases in proportion to wind stress forcing. It has been proposed that eddies thereby limit the circumpolar transport of water around Antarctica (known as the theory of eddy saturation). It is also possible that eddies help to modulate the Southern Ocean's wind-driven overturning circulation, a process that has been dubbed "eddy compensation". Several recent studies have assumed some equivalence between eddy saturation and eddy compensation; we will argue that these two effects are physically distinct and may even occur in isolation from each other.

We use fine-resolution ocean models and novel theoretical considerations to develop a new scaling for the sensitivity of eddy-induced mixing to changes in winds. We demonstrate that changes in Southern Ocean overturning in response to recent and future changes in wind stress forcing are likely to be substantial, even in the presence of a decadally-varying eddy field. This result has significant implications for the ocean's role in the carbon cycle, and hence global climate.