Response of the Indian Ocean Meridional Overturning Circulation to the Subtropical Indian Ocean Dipole

Abstract Using the German contribution to Estimating the Circulation and Climate of the Ocean, version 3 (GECCO3), reanalysis data, this work explores the Indian Ocean meridional overturning circulation (IMOC) variability and mechanism during the mature phase of the subtropical Indian Ocean dipole (SIOD). The IMOC is decomposed into the Ekman component, geostrophic component, external mode, and residue. The IMOC exhibits counterclockwise circulation anomaly in 0°–30°S during the mature phase of the SIOD. While the Ekman component dominates in 10°–30°S, the geostrophic component prevails in 5°–20°S. During the mature phase of the positive SIOD events, while an anticyclonic wind anomaly over the southern Indian Ocean causes a convergence and sinking of the seawater near 30°S, a cyclonic wind field anomaly near 10°S induces a divergence and rising, causing a counterclockwise Ekman component anomaly in 10°–30°S. The geostrophic component anomaly in 5°–20°S is caused by the sea level anomaly (SLA) gradient around 10°S related to a Rossby wave. A linear, 1D baroclinic Rossby wave model shows that the negative SLA in the west is caused by local and remote wind forcing, whereas the positive SLA in the east is generated by radiation from the eastern boundary and is slightly contributed by local wind forcing. Further, Parallel Ocean Program, version 2 (POP2), experiments confirmed that the Ekman component anomaly primarily responds to the wind field of the mature phase of the SIOD and revealed that the geostrophic component anomaly is affected by the wind field of both the developing and mature phases of the SIOD.