Biweekly oscillation of the Meiyu-season precipitation in 2016 and 2020 over the Yangtze-Huaihe river basin：A comparative analysis

The quasi-biweekly oscillation (QBWO) characteristics of Meiyu precipitation in the Yangtze-Huaihe River Basin (YHRB) in 2016 and 2020 are compared by filtering and composite methods using Sea Surface Temperature (SST), atmospheric circulation reanalysis data and daily precipitation data in China. The results show that precipitation exhibited significant QBWO characteristics in both years. However, the QBWO and the low-frequency precipitation in 2016 are stronger than that in 2020. The low-frequency anticyclone moving northwestward in the lower troposphere over the South China Sea (SCS) and the western Pacific and the low-frequency cyclone migrating southeastward under the influence of the westerly jet in the mid-high latitudes are the main reasons for the QBWO of Meiyu precipitation in the YHRB in 2016. In the Meiyu period of 2020, the QBWO precipitation was mainly influenced by the low-frequency cyclones moving southwestward from southern Japan, the low-frequency anticyclones moving northwestward from the Philippine Sea and the low-frequency cyclones moving westward associated with the polar vortex westward rotating in the mid-high latitudes. The continuous SST warming over the SCS, the tropical Western Pacific and the tropical Indian Ocean triggered and maintained the QBWO of convective activities and atmospheric circulations over the SCS and the Western Pacific in 2016 through air-sea interaction. In 2020, positive SST anomalies in the Northwest Pacific and the Kuroshio and its extension region triggered alternative generation and migration of low-frequency cyclones and anticyclones in the region via air-sea interaction, which regulated the advance and retreat of the Western Pacific Subtropical High and thereby affected the QBWO precipitation in the YHRB. In 2016 and 2020, on the quasi-biweekly scale, the anomalous anticyclone in the lower troposphere was favorable for the SST increase, and positive SST anomalies further triggered an anomalous cyclonic in the lower atmosphere, which led to negative SST anomalies and subsequently promoted the development of anomalous anticyclone. Through the air-sea interaction, the QBWO signals in the lower troposphere continuously propagates to the Jianghuai region, affecting the QBWO of rainfall there during the Meiyu period.