Interannual modulation of eddy kinetic energy in the southeast Indian Ocean by Southern Annular Mode

Interannual variability in the mesoscale eddy field over the southeast Indian Ocean of 15 degrees S-35 degrees S and 60 degrees E-110 degrees E is investigated on the basis of 16 year satellite altimetry observations. Eddy kinetic energy (EKE) in this region appears stronger in 2000-2004 and weaker in 1993-1996, 1998-2000, and 2007. It is found that this interannual modulation of EKE is mediated by the Southern Annular Mode (SAM), with a positive (negative) SAM corresponding to weak (strong) eddy activity in this region. The interannual modulation of the EKE by the SAM is through modulating the baroclinic instability associated with the surface-intensified South Indian Countercurrent (SICC) and the underlying South Equator Current (SEC) system. In the positive phase of the SAM, the southeastern subtropical Indian Ocean is dominated by an anomalous Ekman upwelling, which slackens the southward tilt of the isotherms and thus reduces the SICC. This shall reduce the vertical velocity shear of the SICC-SEC current system, leading to a weak instability and thus a weak eddy activity.