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

This study analyzes 40 years (1981–2020) of ERA5 data to investigate the impact of winter tropopause folds (TFs) on precipitation in the Yangtze River valley. While previous studies have focused on local effects of TFs over the Qinghai–Tibet Plateau (QTP), our results reveal that TFs are systematically propagating weather events. Originating in the QTP, TFs propagate eastward by thousands of kilometers and significantly affect precipitation downstream. During intense TF years, the averaged total winter precipitation in the Yangtze River valley increases by 30–50 mm. TFs induce a zonal dipole pattern of precipitation, with positive anomalies to the downstream and negative anomalies to the upstream, propagating downstream with TFs. With the passage of a TF, positive precipitation anomalies of 3–5 mm d⁻¹ are observed in the Yangtze River Basin. Further analysis indicates that TFs will trigger an intrusion of high vortex air in the upper troposphere, and will a cold vortex downstream and a warm high-pressure anomaly upstream at around 500 hPa, which will further disrupt the water vapor in the 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 TFs influences local advection and convergence–divergence processes, which anchors the precipitation patterns in the Yangtze River valley. These findings emphasize the importance of TFs as a moving weather system that exerts a cross-regional influence on Yangtze River valley precipitation.