Characteristics of summer tropopause folds over southeastern Central Asia and their influences on precipitation

Abstract

Tropopause folding activities in the summer play a significant role in atmospheric circulations and weather anomalies. Using rain gauge data and the Climate Prediction Center (CPC) precipitation observations as well as the ERA5 reanalysis dataset during July–August (JA) of 1979–2020, three-dimensional circulation structure during 592 frequent tropopause fold days that occurred over southeastern Central Asia and their impacts on precipitation over Central Asia and western China are investigated. During the frequent tropopause fold days, a significant dipole precipitation anomaly pattern is observed with negative precipitation anomalies over Central Asia and positive precipitation anomalies over western China. The contribution of precipitation (extreme precipitation) in the frequent tropopause fold days to the JA climatology over the northwestern slope of the Tibetan Plateau (TP) and the southern Tarim Basin can reach above 40% (50%). Compared with that in no fold days, daily mean precipitation in the frequent tropopause fold days can increase to three times over the northwestern TP and the Tarim Basin. Strong upward motions associated with low level convergence (high level divergence) and high specific humidity are found downstream of the fold region, which is favorable for the occurrence of precipitation, especially extreme precipitation over western China. In contrast, strong downward motions associated with low level divergence (high level convergence) and low specific humidity suppress the occurrence of precipitation over Central Asia. Further analysis reveals that precipitation over western China is closely related to the strength of tropopause fold, with significant positive correlation over the northwestern TP and the Tarim Basin. As the tropopause fold strengthens, the intensity of upward motion and specific humidity in the middle to lower troposphere downstream of the fold region increase, resulting in changes in the frequency and location of precipitation over western China. These findings highlight important impacts of the atmosphere around the tropopause on precipitation over western China.