CMIP6 models simulation of the connection between North/South Pacific Meridional Mode and ENSO

The subtropical North and South Pacific Meridional Modes (NPMM and SPMM) are well known precursors of El Nino-Southern Oscillation (ENSO). However, relationship between them is not constant. In the early 1980, the relationship experienced an interdecadal transition. Changes in this connection can be attributed mainly to the phase change of the Pacific decadal oscillation (PDO). During the positive phase of PDO, a shallower thermocline in the central Pacific is responsible for the stronger trade wind charging (TWC) mechanism, which leads to a stronger equatorial subsurface temperature evolution. This dynamic process strengthens the connection between NPMM and ENSO. Associated with the negative phase of PDO, a shallower thermocline over southeastern Pacific allows an enhanced wind-evaporation-SST (WES) feedback, strengthening the connection between SPMM and ENSO. Using 35 Coupled Model Intercomparison Project Phase 6 (CMIP6) models, we examined the NPMM/SPMM performance and its connection with ENSO in the historical runs. The great majority of CMIP6 models can well reproduce the pattern of NPMM and SPMM, but reveal discrepant ENSO and NPMM/SPMM relationship. The intermodal uncertainty for the connection of NPMM-ENSO is due to different TWC mechanism. A stronger TWC mechanism will enhance NPMM forcing. For SPMM, few models can simulate a good relationship with ENSO. The intermodel spread in the relationship of SPMM and ENSO owing to SST bias in the southeastern Pacific, as WES feedback is stronger when the southeastern Pacific is warmer.