Trend turning of North China summer extreme precipitations around early 2000s and its possible reason

Abstract

Abstract This study focuses on regional extreme precipitation (REP) in North China. We found a trend turning in summer (July and August) REP frequencies and intensities from a decrease trend in 1961–2002 to an increase trend in 2003–2020, accompanied by a southward shift of the extreme rain belt, and an enhanced connection with the Ural blocking and the Western Pacific Subtropical High Pressure (WPSH) in 2003–2020. Rains in North China are accompanied by a west-east low-high dipole (LHD) at upper troposphere. During the REP, the high of the LHD at Northeast China (the NEH) is strongly amplified from a pre-existing stationary ridge at northeast Asia under the influence of eastward propagating Rossby wave energies along the subpolar/subtropical wave guide over Eurasia. For the REP years, an enhanced stationary ridge over the Ural Mountains in the period 2003–2020 replaces the stationary Ural trough in the period 1961–2002, favouring the development of Ural Blockings (UB) and leading to a change of the Rossby wave propagation path from along the subtropical wave guide in 1961–2002 to along the polar wave guide in 2003–2020. Therefore, a connection between the NEH and the UB forms, which may lead to a higher probability of extreme precipitation in North China since blocking is a major source of strong circulation anomalies. The mean summer WPSH expands more westward-northward in 2003–2020 than in 1961–2002, providing the background conditions for further westward-northward expanding of the daily WPSH under the influence of the NEH before the REP day leading to strong moisture transport from north Pacific and increased intensity and probability of extreme precipitation over southern North China.