The Indian Ocean Dipole in a warming world

The Indian Ocean Dipole (IOD) strongly affects the climate of the Indo-Pacific. Observations suggest a shift towards stronger and earlier positive IOD (pIOD) events alongside an increased amplitude of sea surface temperature (SST) anomalies, but uncertainty remains, impeding assessments of ongoing changes. In this Review, we synthesize the available knowledge of projected changes in the IOD during the twenty-first century under anthropogenic warming. Compared to observations, models struggle to simulate the Bjerknes feedback, asymmetry in the strength of positive and negative IOD anomalies and El Niño–Southern Oscillation or monsoonal forcings. Yet several models do capture important feedbacks reasonably well and offer useful tools with which to assess IOD evolution. A pIOD-like SST warming pattern (an enhanced west-minus-east SST gradient) alongside shifts in feedback process drive corresponding changes to the IOD. Over the course of the twenty-first century, robust changes include: enhanced IOD SST variability (as measured by the first principal component of spring SST variability, not the dipole mode index); an increase in strong rainfall pIOD events; an increase and decrease in the frequency of strong-pIOD and moderate-pIOD, respectively, as defined by SST; and an increase in the frequency of early-pIOD events. Palaeo evidence reveals similar increases in the magnitude and frequency of pIOD events underpinned by a similar pattern of mean state change (Last Glacial Maximum, post-1960), reinforcing IOD projections. Sustained international efforts are needed to improve IOD simulations and reduce projection uncertainties. The Indian Ocean Dipole (IOD) exerts strong control on the Indo-Pacific climate. This Review outlines twenty-first-century changes in the IOD, noting robust increases in eastern pole sea surface temperature variability, more frequent strong and early positive IOD events, and less frequent moderate positive IOD events.