732 / 2024-09-19 10:44:01

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

the north-south SSH gradient,direct local wind forcing,eastern boundary waves,DCEC,Interannual variability

The Deep Cross-Equatorial Cell (DCEC) is the primary branch of Indian Ocean Meridional Overturning Circulation (MOC) in the tropical Indian Ocean. It is crucial for energy redistribution, water exchange, and diapycnal mixing. Despite its importance, the mechanisms driving the interannual variability of DCEC in the Indian Ocean are poorly understood. This study utilized two reanalysis datasets and two model experiments from 1992 to 2017 to investigate the underlying mechanisms. The findings indicate that direct local wind forcing and eastern boundary waves induced by remote equatorial wind forcing are critical for influencing the DCEC on an interannual scale. Specifically, direct local wind forcing triggers spatial quasi-equatorial (DCEC core) antisymmetric anomalies in sea surface heights through the induction of Rossby waves and Sverdrup transport. These anomalies generate an anomalous north-south pressure gradient across the core, leading to interannual variations in the DCEC. During periods when climate modes, such as ENSO and Indian Ocean Dipole, undergo a transition from positive to negative phase, the magnitudes of both positive and negative anomalies in DCEC are intensified. 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.