Evaluation of the performance of CMIP5 and CMIP6 models in simulating the South Pacific Quadrupole-ENSO relationship

The South Pacific Quadrupole (SPQ) is the extratropical South Pacific's second principal sea surface temperature mode. Previous observational studies have shown that the SPQ promotes the onset of the El Nirio-Southern Oscillation (ENSO). The present study evaluates and compares simulations of the SPQ-ENSO relationship by 20 climate models from CMIP6 and their corresponding 20 previous models from CMIP5. It is found that 16 of the 20 pairs of models are able to consistently reproduce the spatial pattern of the SPQ. In terms of simulating the SPQ-ENSO relationship, 9 of the 16 CMIP6 models show significant improvement over their previous CMIP5 models. The multi-model ensemble (MME) of these 16 CMIP6 models simulates the SPQ-ENSO connection more realistically than the CMIP5 MME. Further analysis shows that the performance of the model simulations in reproducing the SPQ-ENSO relationship is strongly dependent on their ability to simulate the SPQ-related surface air-sea coupling processes over the southwestern and southeastern South Pacific, as well as the response of the SPQ-related equatorial subsurface ocean temperature anomalies. The improvement of the CMIP6 models in simulating these two processes is responsible for the improved performance of the CMIP6 models over their CMIP5 counterparts in simulating the SPQ-ENSO relationship.