Interannual Time-Scale Dynamics of Deep Cross-Equatorial Overturning in the Indian Ocean

Abstract

The Deep Cross-Equatorial Cell (DCEC) is the primary branch of Indian Ocean Meridional Overturning Circulation (MOC) in the tropical Indian Ocean, essential for energy redistribution, water exchange, and diapycnal mixing. However, the mechanisms behind its interannual variability remain limited. This study utilized two reanalysis data sets and a series of ocean model experiments with a Hybrid Coordinate Ocean Model and a Linear Ocean Model to investigate the underlying mechanisms. Model experiments highlight the critical role of direct local wind forcing and eastern boundary waves induced by remote equatorial wind forcing in influencing the DCEC variability. Specifically, through the first mode of baroclinic dynamics, direct wind forcing initiates reverse meridional flow at the DCEC core (around 8°S) in both surface and deep ocean layers, leading to interannual variations of the DCEC. During transitions of climate modes like ENSO and Indian Ocean Dipole from positive to negative phases, both positive and negative DCEC anomalies intensify. In addition to direct local wind forcing, the delayed-time Rossby waves reflected from the eastern boundary excited by the equatorial easterly wind in the previous year make substantial contributions (37.8%). The interplay of faster baroclinic Rossby waves at lower latitudes and slower baroclinic Rossby waves at higher latitudes alters the basin-wide pressure gradient, ultimately amplifying interannual DCEC anomalies in the subsequent year.