Extremely Strong Western Pacific Subtropical High in May 2021 Following a La Niña Event: Role of the Persistent Convective Forcing over the Indian Ocean

The variation of the western Pacific subtropical high (WPSH) significantly influences the weather and climate in East Asia. El Niño-Southern Oscillation (ENSO) is considered as one of the most important factors for the abnormal activity of the WPSH. An El Niño event tends to result in an anticyclonic anomaly over the western Pacific in the following spring and summer, leading to a westward-shifted and stronger WPSH. Opposite features can be observed for a La Niña event. Following the typical La Niña event in the winter of 2020/2021, an abnormal cyclonic circulation routinely appeared over the western Pacific in the beginning of 2021, but it was suddenly replaced by an obviously abnormal anticyclone in May. This unanticipated change induced an extremely strong WPSH and posed a challenge for the regional climate prediction. A careful examination of the tropical Indian Ocean revealed a significant abnormal warming process from April to May in 2021, with a peak of positive sea surface temperature anomaly (SSTA) in early May. Consequently, persistent atmospheric convective activity was stimulated by the positive SSTA, accompanied by remarkable and eastward-moving diabatic heating in the tropical Indian Ocean. The convective heating aroused significant easterly anomalies in the form of a Kelvin wave response of the Gill-type mode over the equatorial region from the western Pacific to the eastern Indian Ocean, which induced an abnormal anticyclone through a further positive circulation–convection feedback over the western Pacific. Additional experiments with the LBM model further verify that the persistent convective forcing over the tropical Indian Ocean is responsible for the extremely strong WPSH in May 2021, although during an antecedent La Niña event.