A new mode of decadal variability in the Tropical Indian Ocean subsurface temperature and its association with shallow meridional overturning circulation

Abstract

A north-south subsurface dipole mode (SDM) is found to be the leading mode of interannual variability in the thermocline and subsurface temperature over the tropical Indian Ocean. The present study reports the existence of a decadal SDM with a north-south dominant structure. The mechanisms responsible for the formation of the decadal SDM and its close association with the evolution of local atmospheric and ocean circulation over tropical Indian Ocean are examined. Though the interannual SDM also has a north-south dominant dipole structure, it is imprinted with a weak east-west dipole structure as well. Unlike the interannual SDM, decadal SDM is characterized by a pure north-south pattern with the northern mode covering the entire longitudinal extent of equatorial Indian Ocean. The decadal variability in the surface winds along the equatorial Indian Ocean and the associated wind stress curl and their persistence are found to be the forcing mechanisms responsible for the decadal evolution of the north-south mode. Persistent positive wind stress curl anomalies south of 8 o S intensify the downwelling Rossby waves in the south during the positive phase of the decadal SDM. On the other hand, the northern cooling is driven mostly by the equatorial upwelling Kelvin waves, the Ekman divergence and the equatorially trapped reflected upwelling Rossby waves. It is found that the phase transition in the subsurface mode is determined by the strength of the surface winds and the associated changes in the Ekman transport. Consistent with SDM, upper 50-300 m and 500 m oceanic heat contents reveal conventional north-south dipole structure highlighting the importance of SDM on the tropical Indian Ocean heat redistribution. The meridional overturning circulation associated with SDM modulates the conventional shallow meridional overturning circulation in the Indian Ocean (amplifying it during the positive phase and weakening it during the negative phase) and the associated meridional heat transport. • A new decadal subsurface mode of variability in the tropical Indian Ocean is reported. • The subsurface mode is driven by the equatorial winds and associated wind stress curl. • The subsurface mode modulates the shallow meridional overturning circulation.