The Effect of Indian Ocean Temperature on the Pacific Trade Winds and ENSO

A notable shift in the El Nino-Southern Oscillation (ENSO) has been observed in the early 21st century, characterized by an increased prevalence of Central Pacific (CP) events and strengthened Pacific trade winds. This shift may be attributed to the warming tropical Indian Ocean (TIO). To investigate this, we conduct perturbation experiments using the Insitut Pierre Simon Laplace climate model and nudge TIO surface temperatures to induce warming or cooling effects. Our findings reveal that TIO warming (or cooling) leads to amplified (weakened) mean trade winds and surface warming (cooling) in the Pacific region. Surprisingly, ENSO variability increases in both TIO cooling and warming scenarios. This result is linked to stronger positive feedbacks and a less stable Bjerknes index for either TIO forcing. Additionally, we find that TIO warming leads to more frequent CP events, meridional widening of wind anomalies, and broadening of the ENSO power spectrum toward lower frequencies. The dominant mode of interannual climate variability, the El Nino-Southern Oscillation (ENSO), has visibly changed in the past three decades, with maximum sea surface temperature (SST) anomalies during El Nino events occurring in the Central Pacific (CP) rather than in the Eastern Pacific and having weaker magnitudes. Recent studies suggest that the observed stronger Pacific trade winds could have contributed to this shift. One possible mechanism driving such changes is the enhanced warming trends in the tropical Indian Ocean (TIO) relative to the rest of the tropics. Here, we conduct sensitivity experiments to investigate the effect of TIO SST on the Pacific. Our experiments with a climate model indicate that the Pacific trade winds change proportionally to the imposed forcing, with stronger trades corresponding to a warmer TIO. We find that ENSO variability increases strongly in the TIO cooling experiments, driven by weaker trade winds associated with TIO cooling. In the TIO warming experiments we find a shift toward CP-like events; however, we also observe a modest ENSO strengthening, which can be attributed to stronger positive feedbacks resulting from the remotely induced warming of the Pacific. Initial response to Indian Ocean warming (cooling) produces La Nina-like (El Nino-line) conditions in the tropical PacificEquilibrium response to Indian Ocean warming (cooling) shows stronger (weaker) Pacific trade winds but warmer (colder) ocean temperaturesBoth warming and cooling of the Indian Ocean result in a stronger El Nino-Southern Oscillation due to greater positive feedbacks in the Bjerknes stability index