Interannual to decadal variability in subtropical circulation transport in the south Indian Ocean

This study estimates variability in meridional velocity and transport of the subtropical circulation in 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 1800 m from in-situ observations and compares results with SSH variability. 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 1000 m depth. Zonal SSH difference is significantly correlated with zonally integrated meridional transport relative to 1000 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 depth and is not correlated with zonal SSH difference. Waves radiated from the eastern boundary are possibly responsible for the generation of this subsurface flow, but detailed forcing mechanisms are not known in this study. This subsurface flow can contribute to interannual variability in mode water transport and warrants a further study.