Strengthening Amplitude and Impact of the Pacific Meridional Mode on ENSO in the Warming Climate Depicted by CMIP6 Models

Abstract

Abstract The Pacific meridional mode (PMM) has been suggested to play an important role in modulating the development of El Niño–Southern Oscillation (ENSO). In this study, we examine the projected changes in the PMM and its impact on ENSO under greenhouse gas forcing using the models of phase 6 of the Coupled Model Intercomparison Project. These models can properly reproduce the characteristics of PMM patterns but reveal discrepant PMM–ENSO relationships owing to different wind–evaporation–sea surface temperature (SST) (WES) feedback efficiency and different magnitude of atmospheric convection response to SST anomalies. We select the models that show good performance in simulating the PMM and its impact on ENSO for investigation of future projections. Results show potential increases in both PMM amplitude and its impact on ENSO under the shared socioeconomic pathway (SSP) 585 (SSP585) warming scenario with great intermodel consensus. Diagnosis of the WES feedback indicates increasing sensitivity of latent heat flux to zonal wind speed in a warming climate, which seems to be the main reason for the projected strengthening PMM and its impact on ENSO. In addition, a slightly intensified response of atmospheric convection to SST anomalies in the subtropical Pacific may partially contribute to a stronger PMM–ENSO relationship. The results from this study highlight the increasing importance of the PMM for ENSO development, which calls for more attention to be paid to the PMM for ENSO prediction. Significance Statement Variability of the sea surface temperature in the equatorial Pacific related to El Niño–Southern Oscillation (ENSO) can exert a great impact on global climate. The development of ENSO is partially modulated by the dominant mode of ocean–atmospheric variation in the subtropical North Pacific, namely, the Pacific meridional mode (PMM). This study is aimed to understand the change in the amplitude of the PMM and its impact on ENSO due to climate change. Multimodel projections suggest that the PMM will likely become stronger and exert a greater impact on ENSO since the future warmer climate is favorable for the growth of the PMM. These results call for more attention to be paid to the PMM for ENSO prediction.