Attributing interdecadal variations of southern tropical Indian Ocean dipole mode to rhythms of Bjerknes feedback intensity

The changes in the intensity of the southern tropical Indian Ocean dipole mode (STIOD) are investigated using observations and the Community Earth System Model Large Ensemble (CESM-LE) project in this study. The positive STIOD is characterized as cold SST anomalies over the tropical southeastern Indian Ocean (SEIO) and warm SST anomalies over the southcentral Indian Ocean (SCIO), which peak in boreal summer. It is suggested that the intensity of interannual variability in the STIOD experienced prominent interdecadal changes from 1970 to the present. The STIOD was relatively weak before the late 1980s, while it is enhanced significantly during the late 1980s to early-2000s and displayed some decrease in the intensity after the early-2000s. As an important generation mechanism, changes in the strengths of the Bjerknes feedback between the SCIO and SEIO mainly contribute to the variations of the STIOD intensity. The changes in Bjerknes feedback are associated with the variations in climatological mean states over the tropical Indian Ocean. The warmer climatological SST strengthens the efficiency of the SEIO SST in driving wind. On the other hand, the combined effects of the Indonesian Throughflow and surface climatological zonal winds alter the mean thermocline depth over the SEIO, contributing to the variations in the relationship between the thermocline depth and SST anomalies in situ. Two sets of historical and RCP8.5 simulations from CESM1-LE with 35 ensemble members are analyzed to confirm the roles of mean state changes on the intensity of interannual variability of STIOD.