Ekman Pumping and the Energetics of the Southern Hemisphere Eddy Life Cycle

Abstract

n the framework of the Lorenz energy cycle, the climatological and eddy life cycle characteristics of the generation of eddy available potential energy through Ekman pumping (EEPE) are evaluated using Interim European Centre for Medium-Range Weather Forecasts (ECMWF) Re-Analysis (ERA-Interim) data (1979?2011). EEPE exhibits an annual cycle that is maximized during a given hemisphere?s winter, with maximum values in the midtroposphere of the midlatitudes.Spectral analysis of the Southern Hemisphere storm track reveals that positive EEPE is associated with an anomalously small vertical phase tilt. A composite analysis of the Southern Hemisphere eddy life cycle reveals a maximum in EEPE that occurs after the peak eddy amplitude. Eddy life cycles during winter with large values of EEPE have higher values of eddy available potential energy and eddy kinetic energy than life cycles with small EEPE. However, baroclinic energy conversion remains unenhanced in life cycles with large values of EEPE. The lack of enhancement of baroclinic conversion is related to the small vertical phase tilt associated with positive EEPE. Instead, barotropic energy conversion is muted, and it is this muted barotropic decay that results in an amplification of eddy kinetic energy. There is no evidence of reflecting critical latitudes playing a role in this reduction of barotropic decay, as found in previous modeling studies. Rather, during Southern Hemisphere winter, this reduction coincides with the presence of a turning latitude on the equatorward side of the storm track.