Relationships Between Jianghuai Meiyu Anomaly and the Collaborative Evolution of Wave Trains in the Upper and Lower Troposphere in Mid-July of 2020

Junyu Wang

doi:10.3389/feart.2020.597930

Abstract

Based on the ECMWF reanalysis dataset (ERA5), SST , OLR and the daily precipitation data, the relationship between the Meiyu anomaly in the Yangtze-Huaihe river basin (YHB) and the collaborative evolution of wave trains in the upper and lower troposphere in the mid-July of 2020 is analyzed. The results show that the key circulation background for the long-lasting Meiyu season in the summer of 2020 is the coexistence and collaborative evolution of the Silk-Road (SR) wave train in the upper troposphere and the – + Pacific-Japan (PJ) wave train in the middle-lower troposphere, which is in favor of the movement towards each other of the South Asia high (SAH) and the western Pacific subtropical high (WPSH), resulting in the upper-level divergence over the YHB where is the right side of the jet entrance area. The WPSH strengthens and extends westward, transporting abundant water vapor to the YHB from the Western Pacific and South China Sea. Meanwhile, the meridional circulation develops at the middle and high latitudes, which is in favor of cold-dry air and warm-moist air strongly interact over the YHB, leading to the long-lasting Meiyu season. The coordinative evolution of the SR and PJ wave trains not only affects the persistence and intensity of precipitation in the YHB, but also can be regarded as a precursor signal for the occurrence of persistent heavy rainfall to some extent. The wave-activity flux dispersing eastward along the upper-level jet is beneficial to the development and maintenance of the SR wave train, and the poleward dispersing wave-activity flux is related to the development and maintenance of the PJ wave train. Additionally, along with the weakening El Nino in 2019, the anomalous convection caused by the warm SST in the Western Pacific warm pool is conducive to the formation and maintenance of the PJ wave train. The abnormal convection caused by the warm SST in the Indian Ocean and the east coast of the tropical Atlantic may affect the SR wave train. These are also one of the reasons for the long Meiyu season in 2020.

Highlights

Persistent precipitation events frequently occur during Meiyu season in the Yangtze-Huaihe river basin (YHB) in the context of global warming
The mid-July is still in the Meiyu period when the climate mean state is the summer drought period. In this period do the SR and PJ wave trains have abnormal maintenances? Are there some collaborative evolutions between them? the main purpose of the present study is to examine the relationship between the late ending of the 2020 Meiyu and the collaborative evolutions of the SR and PJ wave trains, and to preliminary investigate the formation mechanisms of the two wave trains, so as to improve understanding of the impact of atmospheric wave trains on extreme precipitation events, as well as provide a reference basis for predicting summer drought and flood in East Asia
The results show that the negative phase of the SR wave train in the middle and high latitudes at 200-hPa was yet unobvious on July 10 (Figure 6A)

Summary

Introduction

Persistent precipitation events frequently occur during Meiyu season in the YHB in the context of global warming. The SR wave train is an atmospheric circulation anomaly that frequently occurs in the upper troposphere of middle latitudes in the Northern Hemisphere in summer (Lu et al, 2002; Enomoto et al, 2003; Ambrizzi et al, 1995; Sato and Takahashi, 2006). Lu and Fu (2009) believed that the SR wave train could regulate the interactions among the members of the Eurasian circulation systems and links the East Asian summer monsoon and the Indian summer monsoon. They believed it could significantly affect the temperature and precipitation in eastern Asia. The daily intense precipitation appearing only at the first half of events dominantly occur in midto-late spring, which are associated with the SR wave train (Shang et al, 2020)

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