Influence of meridional circulation on extreme high temperature and weakened rainfall over the Yangtze River Valley in August 2022

Abstract

The Yangtze River Valley (YRV) experienced record-breaking extreme high temperature and weakened rainfall events in August 2022, which resulted in severe disasters and large socioeconomic losses. The importance of the intensification and westward movement of the western Pacific subtropical high (WPSH) with abnormal subsidence has been emphasized in previous studies. However, the source of the abnormal subsidence remains unknown. This study investigates the source of the abnormal subsidence over the YRV and discusses its possible causes by adopting the three-pattern decomposition of the atmospheric circulation (3P-DGAC). Meridional circulation (MC) was the main contributor to the abnormal vertical velocity (114%), while the contribution of zonal circulation (ZC) was negative (–14%). Additionally, the negative rainfall anomaly over the YRV can be explained mainly by the MC. The anomalous MC was characterized as a ‘negative-positive-negative-positive-negative’ quintuple distribution with sinking motion over the YRV. Anomalous MC is closely related to the sea surface temperature anomaly (SSTA) over the three oceans. The negative phase of the Indian Ocean dipole (IOD) and La Niña SSTA leads to an anomalous rising motion of the ZC over the Maritime Continent, favoring the existence of the rising motion of the MC by the coupling effect. The positive phase of the North Atlantic triple (NAT) SSTA results in an anomalous Rossby wave train, which further leads to a sinking motion over the YRV.