Seasonal Differences of Decadal Thermocline Depth Anomalies in the Tropical Indian Ocean

Abstract

Abstract The thermocline depth in the tropical Indian Ocean has experienced dramatic decadal variations in recent decades. Using analysis and reanalysis datasets, we find that the decadal thermocline depth anomalies show large seasonal differences. The seasonal differences are modulated by two major modes. The first mode shows a zonal dipole pattern, with opposite thermocline depth anomalies in the equatorial eastern Indian Ocean and western Indian Ocean, and is prominent in summer and winter. The second mode is characterized by marked thermocline depth anomalies in the southern Indian Ocean and is significant in spring and fall. The amplitude of the seasonal oscillation in these two modes has increased substantially in the twenty-first century. Their phase change is in good agreement with the observed thermocline depth anomalies in each season. The results also show that the seasonality of the decadal thermocline depth anomalies arises directly from surface wind variations within the Indian Ocean. The first mode is mainly caused by equatorial zonal wind anomalies. The second mode is dominated by local wind stress curl anomalies. These wind anomalies are both significantly correlated with the ENSO-like SST anomalies in the Pacific Ocean. The findings improve our understanding of the decadal thermocline anomalies, and will help to better evaluate their impact on seasonal phase-locked oceanic and atmospheric processes.