Response of Western North Pacific Anomalous Anticyclones in the Summer of Decaying El Niño to Global Warming: Diverse Projections Based on CMIP6 and CMIP5 Models

Abstract

AbstractThe anomalous anticyclone over the western North Pacific (WNPAC) is a key atmospheric bridge through which El Niño–Southern Oscillation (ENSO) affects East Asian climate. In this study, the response of the anomalous WNPAC to global warming under the high-emission scenario is investigated based on 40 models from CMIP6 and 30 models from CMIP5. Despite low intermodel consensus, the multimodel median (MMM) of CMIP6 models projects an enhanced anomalous WNPAC but the MMM of CMIP5 models projects a weakened anomalous WNPAC, both of which reach about 0.5 standard deviation of the decadal internal variability derived from the preindustrial control experiment. As consistently projected by CMIP6 and CMIP5 models, the same magnitude of sea surface temperature anomaly (SSTA) over the tropical Indian Ocean (TIO) stimulates a weaker anomalous WNPAC under a warmer climate, and this mechanism is responsible for the weakened anomalous WNPAC based on the CMIP5 MMM. However, the above mechanism is overwhelmed by another mechanism related to the changes in tropical SSTA based on the CMIP6 MMM. As a result of the enhanced warm SSTA over the TIO and the eastward shift of the warm SSTA over the equatorial Pacific during the decaying El Niño, the warm Kelvin wave emanating from the TIO is enhanced along with the stronger zonal SSTA gradient based on the CMIP6 MMM, enhancing the anomalous WNPAC. The diverse changes in the zonal SSTA gradient between the TIO and the equatorial western Pacific also explain the intermodel diversity of the changes in anomalous WNPAC.