Possible mechanism of the mid-high-latitude synoptic-scale disturbances impact on Meiyu precipitation anomalies

Abstract

Precipitation during the Meiyu period in East Asia demonstrated a tendency towards increased frequency of rainstorms and escalating disaster-causing potential, exhibiting significant inter-annual variations in recent years. The relationship between Meiyu precipitation (MP) anomalies, related circulation anomalies and mid-high-latitude synoptic-scale disturbances was investigated by observation and reanalysis data from 1979 to 2018 and the regional climate model (RegCM4.6). Based on the observation results, the physical mechanism of disturbance affecting MP was proposed and verified by numerical simulation experiments. The results show that, during the Meiyu periods, the variation of synoptic-scale disturbances over Lake Baikal correlates negatively with MP anomalies, where the abnormal zonal winds and the disturbance anomalies show a strong spatially consistent coherence in the upper troposphere. Compared to drought years, the weaker conversion of disturbance energy to mean flow and the convergence of anomalous Eliassen-Palm (E-P) flux induce mid-high-latitude anomalous easterly flow during the Meiyu period in flood years. The occurrence of such conditions leads to the southward shift of the East-Asian subtropical jet (STJ). Then, anomalous stronger ascending flow over the Meiyu region finally results in stronger MP. In addition, warm temperature anomaly and warm advection anomaly exist at lower latitudes in flood years, while mid-high latitudes exhibit cold temperature anomaly and cold advection anomaly, which is conducive to the southward shift of the East-Asian STJ and the increase of MP. Based on the above diagnosis, sensitivity experiments were conducted using RegCM4.6. When the mid-high-latitude synoptic-scale disturbances are amplified during flood years or flood decades, the MP correspondingly decreases, and vice versa. The simulation results also show that the convergence (divergence) of mid-high-latitude E-P flux results in a weakening (strengthening) of the westerly flow, which leads to the southward (northward) shift of the East-Asian STJ. The alteration of the STJ's position in East Asia alters the position of the ascending branch of the vertical meridional circulation corresponding to the jet stream, subsequently impacting MP. Therefore, both diagnosis and numerical simulation reveal the possible mechanism of synoptic disturbances in mid-high latitude affecting MP in Yangtze River Basin, thus enriching the study of MP anomaly mechanism during Meiyu period.