Variability in Meridional Transport of the Subtropical Circulation in the South Indian Ocean for the Period From 2006 to 2017

Abstract

AbstractThis study estimates variability in meridional velocity and transport in the interior of the south Indian Ocean using in situ hydrographic observations, satellite altimetry, and two reanalysis products for the period from 2006 to 2017. Previous studies used the zonal difference of satellite sea surface height (SSH) between the western and eastern parts of the basin as an index to variability in basinwide meridional geostrophic transport. This study estimates meridional geostrophic velocity in the upper 1,800 m from in situ observations and examines its vertical structure. Results show that zonal SSH difference represents a surface trapped variability in meridional velocity, the amplitude of which is large in the upper 250 m and decreases to zero at about 1,250 m depth. Zonal SSH difference is significantly correlated with zonally integrated meridional transport relative to 1,000 m depth. It is likely that wind variability both in the south Indian Ocean and tropical Pacific Ocean is responsible for this surface trapped variability, as is suggested by past studies. Results of this study also show meridional velocity variability at subsurface, which peaks in magnitude at about 400 to 800 m depths. This variability is not correlated with zonal SSH difference and represents decadal weakening of northward transport of the subtropical circulation for the analysis period. This subsurface variability can contribute to interannual variability in mode water transport, but its detailed forcing mechanism is not known and warrants a further study.