Interannual to multi-decadal forcing of mesoscale eddy activity in the southern Indian Ocean
Andrew Delman (JPL, Caltech)
A region of elevated mesoscale eddy activity spans the subtropical southern Indian Ocean (SSIO) between Madagascar and Australia. The interannual and decadal changes in eddy activity in the SSIO eddy band, as represented by the variability of eddy kinetic energy (EKE), have implications for the large-scale circulation, mixed-layer budgets, and biological activity. An analysis of nearly two and a half decades of sea level anomaly (SLA) data from merged satellite altimetry shows that, in the southeast Indian Ocean east of 90°E, the variations of EKE and SLA are positively correlated on interannual as well as decadal timescales. Moreover, EKE exhibits a multi-decadal increasing linear trend that corresponds to an increasing trend of SLA in the region. The EKE-SLA co-variability in the southeast Indian Ocean does not appear to be associated with a preference for anticyclonic over cyclonic eddy activity; rather, it can be attributed to the common remote forcing from the tropical Pacific associated with the El Niño-Southern Oscillation and the Pacific Decadal Oscillation. In the west central SSIO, the variability of eddy generation is not well explained by Pacific forcing, or by variations in lateral density gradients (i.e., the vertical shear of horizontal velocity). However, wind stress curl just south of the eddy band forces potential vorticity (PV) anomalies that affect PV gradients and conditions for instability in the west central SSIO. This result implies that a similar analysis of PV anomalies in other regions may yield insights into the variability of eddy generation, while robust EKE-SLA correlations in many areas suggest eddy-related impacts on sea level variations and multi-decadal trends.