Weak persistence of Northwest Pacific anomalous anticyclone during post-El Niño summers in CMIP5 and CMIP6 models

An anomalous anticyclone (AAC) recurs over the Indo-Northwest Pacific during post-El Niño summers, regulating climate anomalies in the densely populated South, Southeast, and East Aisa. However, AACs simulated in phase 5/6 of the Coupled Model Intercomparison Project (CMIP5/6) multi-model ensemble mean fail to persist through post-El Niño summers. The present study suggests that this weak persistence bias likely stems from the weakly persistent sea surface temperature (SST) anomalies’ gradient between the North Indian Ocean and tropical Western Pacific. On one hand, the simulated North Indian Ocean warming decays faster compared with its observational counterpart due to the weakened cross-equatorial antisymmetric wind patterns and wind–evaporation–SST feedback, which may arise from the deep thermocline bias in the Southwest Indian Ocean in CMIP models. On the other hand, the long-lasting Western Pacific warming bias may be led by the intensified upper-ocean zonal advection feedback and wind–downwelling–SST feedback, which can be further traced back to the Pacific cold tongue bias in CMIP models. Compared with CMIP5 models, CMIP6 models simulate a more realistic Southwest Indian Ocean thermocline dome and equatorial Pacific cold tongue. Therefore, the overall simulation skill in the summertime AAC persistence is improved in CMIP6 than in CMIP5 models. The persistence of AAC is further investigated during post-Eastern Pacific (EP) and Central Pacific (CP) El Niño summers in CMIP models. The simulated AAC displays a remarkable stronger persistence during post-EP El Niño summer compared to its post-CP counterpart. The present study highlights that reducing mean-state biases in climate models may elevate the simulation skills of summertime AAC persistence, thereby enhancing seasonal prediction and future projection skills of Asian summer monsoon.