Eastward-Propagating Tropopause Folds Amplify Winter Rainfall of Yangtze River Valley

This study analyzes 40 years (1981–2020) of ERA5 reanalysis data to investigate the impact of winter tropopause folds (TF) on precipitation in the Yangtze River Valley. While previous studies have focused on local effects of TF over the Qinghai–Tibet Plateau (QTP), our results reveal that TF are systematically propagating weather events. Originating in QTP, TF propagates eastward thousands of kilometers and significantly affecting precipitation downstream. During intense TF years, averaged total winter precipitation in the Yangtze River Valley increases by 30–50 mm. TF induces a zonal dipole pattern of precipitation, with positive anomalies to the downstream and negative anomalies to the upstream, shifting downstream with TF. With the passage of the TF, positive precipitation anomalies of 3-5 mm·d-1 were observed in the Yangtze River Basin. Further analysis indicates that the TF will trigger an intrusion of high vortex air in upper troposphere, and will induce a cold vortex downstream and a warm high-pressure anomaly upstream at around 500 hPa, which will further disrupt the water vapor in lower troposphere. In addition, the strong boundary of the TF impedes the westerly jet and alters the vertical motion of the troposphere, leading to subsidence upstream and convective rise downstream. It is proposed that the systematic eastward propagation of the TF influences local advection and convergence-divergence processes, which anchors the precipitation patterns in the Yangtze River Valley. These findings emphasize the importance of the TF as a moving weather system that exerts a cross-regional influence on Yangtze River Valley precipitation.