The North Pacific Meridional Mode and Its Impact on ENSO in the Second Version of the Chinese Academy of Sciences Earth System Model

Abstract The North Pacific Meridional Mode (PMM) is the strongest interannual air‐sea coupled system in the subtropical northeastern Pacific, which can significantly impact the development of El Niño and Southern Oscillation (ENSO). This study examines performance of the second version of the Chinese Academy of Sciences Earth System Model (CAS‐ESM2), developed primarily at the Institute of Atmospheric Physics, Chinese Academy of Sciences (IAP/CAS), in simulating the PMM, ENSO, and their relationship. It reveals that CAS‐ESM2 can well reproduce the tropical climate mean states, including sea surface temperature (SST), surface winds, and precipitation. Furthermore, the model shows a good ability in reproducing the seasonal evolutions of the PMM and ENSO. Moreover, CAS‐ESM2 effectively simulates the influence of the PMM on subsequent ENSO and the underlying physical mechanisms, including the wind‐evaporation‐SST feedback process, the trade wind charging mechanism and summer deep convection mechanism. However, some improvements are still needed, particularly in representing the periodicity of the PMM, an overestimation of the ENSO intensity and westward extension of ENSO‐related SST anomalies in the tropical Pacific. The results obtained from the CAS‐ESM2 showcase significant progress in understanding the interaction between air‐sea interaction systems over the tropics and subtropics.