Intraseasonal Variability in the South Equatorial Current of the East Indian Ocean

Abstract

Satellite altimeter data reveal that the strongest intraseasonal variability in the southeast Indian Ocean occurs in the South Equatorial Current (SEC) during the second half of the year. The length scale of the variability is 100–150 km, with a westward phase speed of 15–19 cm s⁻¹ and dominant periods between 40 and 80 days. A continuously stratified quasigeostrophic model is solved to analyze the baroclinic stability of the climatological SEC. Large growth rate of the instability waves (with e-folding timescale of less than 50 days) can be found east of the active region of intraseasonal variability during the July–September season, when the SEC and the Pacific to Indian Ocean throughflow are at their strongest. Geostrophic current shear in the upper 200-m ocean is crucial for the growth of the instability. The results suggest that the baroclinic instability draws most of its energy from the available potential energy associated with the throughflow, and, to a lesser degree, by local Ekman pumping. The predicted characteristics of the most unstable mode are consistent with both altimeter and profiling float observations. From all the available evidence, baroclinic instability seems to be the main cause of intraseasonal variability in the SEC. However, more field observations are necessary to properly address the possibility of combined barotropic/baroclinic instability.