Inter‐Decadal Changes of Indian Ocean Dipole Around the Early 1980s and the Related Atmospheric and Oceanic Processes

This study demonstrates a significant inter‐decadal change of Indian Ocean Dipole (IOD) around the early 1980s and its connection to Pacific Decadal Oscillation (PDO). The changes of IOD include an initiation advancing and an increase in the occurrence of IOD events, particularly negative IOD events. Associated is an inter‐decadal strengthening of the coupling between IOD and sub‐surface IOD (Sub‐IOD) since the boreal spring season of IOD years. All these inter‐decadal changes coincide with the phase transition of PDO around the early 1980s, which has exerted its influence through atmospheric and oceanic pathways. On the one hand, negative/positive PDO forces easterly/westerly anomalies over the tropical Indian Ocean (TIO) by modulating Walker circulation, which is responsible for the increase in the occurrence of positive/negative IOD events. On the other hand, through the Indonesian Through Flow, PDO forced oceanic Rossby waves propagate westward from the Pacific to the Indian Ocean which contributes to deepening/shoaling the thermocline in the southwestern Indian Ocean. The inter‐decadal steepening of the TIO thermocline around the early 1980s, together with the strengthening of the surface westerly anomalies, gives rise to the strengthening of the vertical coupling between IOD and Sub‐IOD since the spring, which is mainly responsible for the inter‐decadal initiation advancing of IOD events. Though the initiation advancing of IOD events in the Coupled Model Inter‐comparison Project phase 6 (CMIP6) simulations is not as obvious as that in the reanalysis data, the linkage between positive PDO and the initiation advancing of negative IOD events also can be reproduced.