Effects of Low-frequency Oscillation on the Persistent Extreme Precipitation in Sichuan Basin

Based on precipitation data of surface meteorological stations and NCEP/NCAR reanalysis data during 1981-2016, the persistent extreme precipitation is defined for Sichuan Basin, and 15-30-day low-frequency oscillation characteristics of precipitation and atmosphere are analyzed in detail by means of wavelet analysis, synthetic analysis and Butterworth filtering, which provide theoretical basis and reference for extended period forecast.Results show that persistent extreme precipitations in Sichuan Basin are concentrated from June to September, which generally last for 3 days. The precipitation has low-frequency oscillation characteristics of 15-30 days and 30-60 days, and mainly 15-30-day oscillation. During the precipitation, the South Asia high and upper jet form a high-level divergence field, the subtropical high is strong and extends westward, the low pressure in the Lake Baikal and the Sea of Okhotsk extends southward, and the airflow in South China Sea and the airflow in the west side of subtropical high converge and pass northward. During the precipitation, low-frequency systems in each height layer and each latitude cooperate with each other in three-dimension space to form a low-frequency circulation which is favorable for precipitation. The vertical baroclinicity of the low-frequency system is conducive to the accumulation of unstable energy and provides energy conditions for precipitation.In the low-frequency flow field, during the precipitation period, the low-level and middle-level north-south airflows merge into the basin to form a convergence area, and the upper layer appears as a northerly wind. The low-latitude cyclone in the lower layer is generated in the western Pacific Ocean and gradually moves northwestward to the South China Sea to bring warm and humid airflow. The southeast side of the mid-high-latitude Lake Baikal generates a cyclone which then moves eastward to the vicinity of the Okhotsk Sea to enhance the northerly wind transport. The mid-high-latitude Eurasia low-pressure center in the middle layer corresponds to the transverse trough in the polar region of the original field. With the transverse trough turning vertical, the center of the low-pressure moves to the southeast, arriving at Mongolia in precipitation, and splits the small trough to the downstream, strengthening in the Sea of Japan, followed by the high-pressure center to the Ural Mountains. During the precipitation period, the divergence of Sichuan Basin at high-level is positive, which is favorable for the continuous convergence and upward movement at lower and middle layers.