Impacts of the local temperature anomalies over Mongolian Plateau on heavy rainfall events in north China during July 2023

From July 29th to August 2nd, 2023, an exceptional precipitation event, referred as 237HRE, struck North China, causing widespread flooding in the Haihe River basin. Utilizing reanalysis data and the Weather Research and Forecasting (WRF) model, this study delves into the reasons behind the unusual westward extension and northward shift of the Western Pacific Subtropical High (WPSH), as well as the extreme precipitation during 237HRE. Our findings indicate that during 237HRE, the WPSH underwent a significant anomalous westward extension and northward shift, forming a stable and enduring high-pressure barrier. This barrier caused the typhoon's residual vortex and water vapor transport to stagnate in North China, thereby creating the conditions for this extreme precipitation event.

The diagnostics reveal that the positive temperature anomaly over the Mongolian Plateau intensified local temperature advection, contributing to the enhancement and northwestward extension of the WPSH. This mechanism has been thoroughly validated using the Interactive Global Grand Ensemble (TIGGE) dataset, that better forecasts of 237HRE usually benefited from better forecasts of the WPSH. Additionally, WRF sensitivity experiments further support this mechanism, demonstrating that when the positive temperature anomalies are weakened, the WPSH retreats eastward and weakens considerably, as well as the extreme event. In particular, the typhoon residual vortex moves to the southeast at an increased speed with the influence of steering currents. Under these atmospheric circulation configurations, the moisture transport pathway also shifts eastward, altering its relative relationship with the Taihang Mountains, leading to rainfall patterns expanding eastward and the intensity weakening in North China. These findings highlight the crucial role of local temperature anomalies over Mongolia in modulating the position of WPSH, which is essential for understanding and predicting the extreme precipitation events in North China.