Contrasting East Asian climate extremes in 2020 and 2022 tied to zonal flow

Abstract

Subtropical East Asia (STEA) experienced a historic flood in the summer of 2020, and historic drought and heatwaves in the summer of 2022. Previous studies emphasized the role of western Pacific subtropical high (WPSH), but there is a paradox that the contrasting climate extremes over STEA in 2020 and 2022 are both associated with anomalously strong WPSH. Given that local vertical motion has a dominant control on precipitation variability, here we investigate the mechanism for the variability of vertical motion in STEA. In most extratropical regions of the Northern Hemisphere, ascent (descent) motion aligns with southerly (northerly) flow in the troposphere due to the northward tilting isentropic surfaces. However, isentropic surfaces tilt eastwards over STEA in the summer due to the existence of a strong warm center over the Tibetan Plateau (TP). Thus, the ascent motion over the STEA is insensitive to the strength of southerly flow related to the intensity of the WPSH but sensitive to the strength of westerly flow related to the meridional shift of subtropical jet. The notably strong WPSH in 2020 and 2022 increased water vapor transport into STEA but had little impact on atmospheric vertical motion. However, the East Asian subtropical jet displaced southwards (northwards) in the summer of 2020 (2022), leading to anomalous westerly (easterly) flows in the mid-upper troposphere from TP to STEA on the jet’s southern flank, prompting anomalous ascent (descent) motion in STEA that contributed to the flood (drought) conditions in 2020 (2022). Our results highlight the essential role of anomalous zonal flow in generating surface climate extremes over STEA in the summer because of its strong control of vertical motion.