On the Relationship Between Wind, SST, and the Thermocline in the Seychelles–Chagos Thermocline Ridge

Abstract

This letter investigates the relationship between wind, sea surface temperature (SST), and thermocline in the Seychelles–Chagos Thermocline Ridge (SCTR, 5°S–10°S, 50°E–80°E) using a combination of satellite data and a reanalysis version of the HYbrid Coordinate Ocean Model from 1993 to 2012. The asymmetry of this relationship during positive and negative Indian Ocean Dipole (IOD) events and the impacts on the SST—thermocline depth (represented by the 20 °C isotherm depth, D20) relationship—are examined. On interannual timescales, an asymmetric relation between SST and zonal wind stress causes a strengthening of easterlies that enhances anticyclonic wind stress curl and local Ekman downwelling, which in turn deepens the D20 and increases the heat content during positive IOD (PIOD) events. In contrast, during negative IOD (NIOD) events, the winds reverse to be westerlies and cause a three times greater impact on remotely generated upwelling Rossby waves. Subsequently, these asymmetric relations cause an asymmetric D20–SST feedback in the SCTR such that a shoaling D20 is ~2.5 times more effective at lowering SST during NIOD events than a deepening D20 is at raising the SST during PIOD events. The changes to D20 are observed to extend into the year following IOD events, persisting into the end of the year following a PIOD event (due to stronger asymmetric reinforcing effects of warm SST anomalies on zonal wind anomalies) but only to May–June of the year following NIOD events.