Multi-decadal trends in Southern Ocean eddy activity simulated by a high-resolution ocean model

We investigate the regional responses of eddy kinetic energy (EKE) to the atmospheric forcing trends between 1948 and 2007 with a global configuration of the NEMO-LIM2 ocean-sea ice model, where a two-way nesting technique is used to enhance the horizontal resolution to 1/12° between 30°S and 68°S. A companion experiment forced by a repeated annual cycle is used to remove model spurious drifts from the hindcast experiment, and to identify the variance of EKE of a stochastic nature. The model results show that the wind strengthening occurred in the past decades led to a multi-decadal increase in EKE of about 5 cm² sec^-2 decade^-1 averaged over the whole Southern Ocean. The changes in EKE are however not spatially uniform, and two response regimes are identified. In the Pacific and Indian sectors, the model reproduces the temporal evolution of the satellite-observed EKE and shows a predictable response of EKE to wind changes. In the Atlantic sector, where the stochastic-to-total ratio of EKE variance is nearly 100%, EKE does not match satellite observations, and wind forcing has a weak influence on EKE variability and its multi-decadal trends. The model thus helps to identify those regions with an optimal signal-to-noise ratio needed to detect possible future changes in eddy activity.