The relationship of Ekman pumping to the vertical phase tilt and energization of the Southern Hemisphere storm track in the context of the Lorenz energy cycle

In the framework of the Lorenz energy cycle, the climatological and storm lifecycle characteristics of the generation of eddy available potential energy through Ekman pumping (EEPE) are evaluated using ERA-Interim reanalysis data (1979-2011). EEPE exhibits an annual cycle that is maximized during a given hemisphere's winter and is minimized during a given hemisphere's summer. Maximum values of EEPE are found in the mid-troposphere of each hemisphere's storm track. Annually, the effects of Ekman pumping are to produce a small net generation of eddy available potential energy (PE) in the Southern Hemisphere and a net generation of eddy kinetic energy (KE) in the Northern Hemisphere. Spectral analysis is employed to calculate the daily phase difference between the upper and lower levels of the Southern Hemisphere storm track. It is found that EEPE is positive if the two levels are anomalously in phase while EEPE is negative if the two levels are anomalously out of phase. A composite analysis of the days when KE is anomalously large in the Southern Hemisphere reveals a maximum in EEPE two days after the peak storm amplitude. Furthermore, storms which have anomalously large values of EEPE have higher values of PE and KE than storms with anomalously small EEPE. It is found that the barotropic energy conversion is weaker in storms with anomalously large values of EEPE. This muted energy conversion from KE to zonal kinetic energy contributes to the higher value of KE.