Distinction and influences of the quasi-biweekly oscillation among three typical flood years in the lower reaches of the Yangtze river

Based on the ERA5 reanalysis data, the typical flood years of 1999, 2016 and 2020 in the lower reaches of the Yangtze river are analyzed, and the quasi-biweekly oscillation component is extracted by the method of power spectrum analysis and Lanczos filtering. Common characteristics of different typical flood years are summarized, and their key influencing factors are respectively analyzed using in-phase composite study. The results are as follows: (1) 1999, 2016 and 2020 are the most typical flood years in the lower reaches of the Yangtze river, with a significant quasi-biweekly oscillation period of 10—20 d, especially from June to July. (2) In the middle and high latitudes, the low-frequency circulation is dispersed along the wave train disturbance, which is conducive to the maintenance of the large-scale background of "two ridges and one trough" and the enhancement of the disturbance in the lower reaches, causing precipitation anomalies by changing the low-frequency circulation and vertical movement over the lower reaches of the Yangtze river. The quasi-biweekly oscillation over the low-latitude region is another source of low-frequency in the lower reaches of the Yangtze river. The low-frequency circulation in 850 hPa is symmetrically distributed around 10°N, and the low-frequency divergent wind points from the South China Sea to the lower reaches of the Yangtze river. The low-frequency tropical convective activities can affect the precipitation anomaly in the lower reaches of the Yangtze river through vertical circulation. The interaction between circulations at the high and low latitudes, combined with the thermal forcing and the strong convergence of low-frequency water vapor movement, provides a favorable condition for anomalous precipitation in the lower reaches of the Yangtze river. (3) The distribution characteristics and key influencing factors of precipitation in the three typical flood years in the lower reaches of the Yangtze river are obviously different. In 1999, the low-frequency anticyclone circulation over the western North Pacific was located to the west of its normal position, the divergent wind pointed from the South China Sea to the lower reaches of the Yangtze river, and the low-frequency water vapor came from the Bay of Bengal and the South China Sea. In 2016, the coupling zone of the south-north low-frequency circulation system in the lower troposphere presented a northeast-southwest orientation. The low-latitude convective activities were vigorous, and the heating effect of the atmospheric heat source propagating northward was stronger. In 2020, the vertical circulation in the middle and high latitudes was more complete, and the southeasterly water vapor transport in the Northwest Pacific was the main source of water vapor. The results are of great significance to understand the anomalous variations of summer precipitation in the lower reaches of the Yangtze river.