The Impacts of Tropical Indian Ocean Warming on the Pacific Trade Winds and ENSO Dynamics

A salient feature of global warming since the 1950s is a faster warming of the tropical Indian ocean (TIO) than of other tropical basins. Here, we study how this enhanced TIO warming can affect the tropical Pacific ocean using the latest coupled climate model from Insitut Pierre Simon Laplace (IPSL-CM6). Specifically, we examine the remote effects of persistent TIO temperature anomalies on the Pacific mean state and ENSO variability in ensemble experiments nudging SST in the TIO region by -2°C, -1°C, +1°C, and +2°C. Previous studies have shown that TIO warming can affect the Walker circulation in the Indo-Pacific by inducing a stationary Gill-type pattern in the atmosphere, including a Rossby wave to the west of, and a Kelvin wave to the east of the TIO temperature anomaly. In turn, the Kelvin wave can change background conditions in the Pacific. Indeed, we find that TIO warming/cooling modifies the magnitude of the equatorial Pacific mean zonal wind stress proportionally to the imposed forcing, with stronger trades winds induced by a warmer TIO. Surprisingly, ENSO variability generally increases, relative to the preindustrial control simulation, in both TIO cooling and warming experiments. We find that a stronger ENSO for weaker trade winds, associated with TIO cooling, is consistent with changes in the Bjerknes stability index that we compute for the Niño3 region. However, the simulated ENSO strengthening for stronger trade winds, associated with TIO warming, is unexpected. A more detail analysis demonstrates that in the latter case the strengthening of ENSO signal is largely associated with the strengthening of the CP-type events, not captured by the conventional Bjerknes index. Thus, our results confirm that the tropical Indian ocean temperature is an important regulator of the Pacific mean state and ENSO characteristics.