Abstract
Significant anomalies in frequency of summer extreme hot day (SEHD) are broadly observed in the Asian monsoon region (AMR) in the post-ENSO summers. The delayed ENSO impacts are mainly conveyed by provoking the Indo-western Pacific Ocean capacitor (IPOC) effect that maintains the anomalous anticyclone in the western North Pacific. The related diabatic heating anomaly can trigger the westward propagating Rossby wave to the Indian subcontinent, which increases the geopotential heights, reduces the cloud cover, and thus increases the seasonal surface temperature and SEHD frequency in the southern AMR. Besides, the reduced atmospheric moisture in the western North Pacific hinders the northward propagation of intraseasonal oscillation (ISO) and modulates the occurrence frequency of individual ISO phases, contributing to the significantly increased/decreased SEHDs in eastern China/Hokkaido of Japan in the post-El Niño summers.
The 25-model-ensemble mean of CMIP6 historical runs can reproduce well the observed SEHD anomalies in the southern AMR in the post-ENSO summers mainly due to the realistic simulation of ENSO impacts on the seasonal surface temperature, although a large inter-model spread exists due to different strength of IPOC effect in each model owing to model biases in the mean state of eastern tropical Pacific, the ENSO variance and teleconnection to the Indian Ocean. Furthermore, future projections under SSP5-8.5 scenario show that the delayed ENSO impacts on the southern AMR remains stable under global warming via the similar mechanism as in the observations and historical runs.
