Suppressed Upwelling Events in the Seychelles-Chagos Thermocline Ridge of the Southwestern Tropical Indian Ocean

Abstract

A prominent subsurface upwelling over the Seychelles-Chagos Thermocline Ridge (SCTR) in the southwestern tropical Indian Ocean is suppressed when downwelling Rossby waves propagate from the eastern Indian Ocean during the positive phase of the Indian Ocean Dipole (IOD) or El Nino periods. Recent studies have suggested that the upwelling can be further suppressed during the co-occurrence years of positive IOD and El Nino, associated with the strong easterly wind anomalies in the eastern Indian Ocean. This study examines the temporal variations in the SCTR upwelling strength during 1968-2017, identifies strong upwelling suppression events, and analyzes their characteristics by focusing on the role of both remote and local wind forcing, not limited to the remote influence linked to the IOD and the El Nino Southern Oscillation. A total of nine events were identified for the 50-year period, with seven of them occurring 3-6 months after the co-occurring peaks of positive IOD and El Nino in the eastern Indian Ocean. However, only IOD exhibited a positive phase before the 2011-2012 suppression event associated with the downwelling-favorable anticlockwise local wind anomalies in the SCTR. Moreover, the 1978-1979 suppression event was primarily caused by the strong anticlockwise wind anomalies in the SCTR, without any significant influence of the positive IOD or El Nino. The results suggest that the role of local winds, as well as the remote forcing, are significant in the upwelling suppression over the SCTR, which would contribute to understanding the potential predictability of the upwelling variations.