The multi-year negative Indian Ocean Dipole of 2021–2022

Abstract. The years 2021 and 2022 witnessed negative Indian Ocean Dipole (nIOD) conditions, with the 2022 event being the strongest on record. The dipole mode index was negative since the summer of 2021 and remained negative until early winter 2022, an unprecedented duration of 19 months. This makes it the first such occurrence of a multi-year nIOD. It co-existed with a triple-dip La Niña event during 2020–2022. In this study, we explore the dynamics behind the occurrence of this multi-year nIOD event. The tropical Indian Ocean (TIO) witnessed predominant westerly wind anomalies starting in the summer of 2021 and lasting till the end of 2022, with a record number and duration of westerly wind bursts (WWBs). The anomalous westerlies were supported by the background La Niña state and anomalous convection over the eastern TIO associated with tropical intra-seasonal oscillations. Occurrences of WWBs outside their preferred climatological months and strong westerly wind anomalies modulated the intensity of the zonal currents and the Wyrtki jets in the TIO. The associated heat and mass transfer caused the depression of the thermocline in the eastern TIO, resulting in the sustenance of nIOD conditions. Anomalous westerly wind activity in the TIO during the spring of 2022 served as a bridge between the two nIOD events and sustained it for a record duration. This multi-year nIOD event thus prevented the Indian summer monsoon rainfall from being in large excess, as the monsoon conducive modulation of the Walker circulation was counteracted by the anomalous subsidence over India by the nIOD-modulated regional Hadley circulation.