The residual circulation of the Southern Ocean: Which spatio-temporal scales are needed?

The Southern Ocean circulation consists of a complicated mixture of processes and phenomena that arise at different time and spatial scales which need to be parametrized in the state-of-the-art climate models. The temporal and spatial scales that give rise to the present-day residual mean circulation are here investigated by calculating the Meridional Overturning Circulation (MOC) in density coordinates from an eddy-permitting global model. The region sensitive to the temporal decomposition is located between 38°S and 63°S, associated with the eddy-induced transport. The " Bolus" component of the residual circulation corresponds to the eddy-induced transport. It is dominated by timescales between 1. month and 1. year. The temporal behavior of the transient eddies is examined in splitting the " Bolus" component into a " Seasonal" , an " Eddy" and an " Inter-monthly" component, respectively representing the correlation between density and velocity fluctuations due to the average seasonal cycle, due to mesoscale eddies and due to large-scale motion on timescales longer than one month that is not due to the seasonal cycle. The " Seasonal" bolus cell is important at all latitudes near the surface. The " Eddy" bolus cell is dominant in the thermocline between 50°S and 35°S and over the whole ocean depth at the latitude of the Drake Passage. The " Inter-monthly" bolus cell is important in all density classes and is maximal in the Brazil-Malvinas Confluence and the Agulhas Return Current. The spatial decomposition indicates that a large part of the Eulerian mean circulation is recovered for spatial scales larger than 11.25°, implying that small-scale meanders in the Antarctic Circumpolar Current (ACC), near the Subantarctic and Polar Fronts, and near the Subtropical Front are important in the compensation of the Eulerian mean flow. © 2013 Elsevier Ltd.