Local feedback and ENSO govern decadal changes in variability and seasonal synchronization of the Indian Ocean Dipole

Abstract

The Indian Ocean Dipole (IOD), characterized by an interannual fluctuation in zonal dipole pattern of sea surface temperature anomalies along the tropical Indian Ocean, has a large socioeconomic impact on neighboring countries. Here, we investigate the decadal modulations of IOD variability over the last 123 years (1900-2022), by analyzing the observational reanalysis data in conjunction with a low-order IOD model that accounts for both stochastic forcing and the remote impact of ENSO. The observed decadal changes in IOD variability are primarily attributed to the local air-sea coupled feedback, and secondarily to ENSO. The local feedback during the late winter has intensified since the late-1970s due to the Indian Ocean warming, the suppressed westerly winter monsoon in the southeastern Indian Ocean, the shallowing of the mean thermocline in the southeastern Indian Ocean, and the decrease in mean upwelling in the western Indian Ocean. Each of these enhances the convective instability, anomalous evaporative cooling, and oceanic vertical thermal advections, respectively. Intensified local feedback increases the likelihood of the early onset of IOD events in the late winter. Additionally, ENSO, which has strengthened since the mid-twentieth century, has extended the peak phase of IOD into the late fall in recent decades. Decadal changes in Indian Ocean Dipole variability are attributable primarily to air-sea coupled feedback changes in the Indian Ocean, and secondarily to changes in ENSO, suggest analyses of long-term observations, reanalysis data and a low-order IOD model.