Summer extreme precipitation patterns and synoptic-scale circulation precursors over the Tibetan Plateau

Abstract

In the context of global warming, the extreme summer precipitation over the Tibetan Plateau (TP) has changed significantly. In this study, the summer (June–August) extreme precipitation on the TP was classified into three spatial types by applying the K-means clustering method to the Third Pole Region long time-series high-resolution (1/30°) precipitation dataset (TPHiPr, 1979–2020). The characteristics of the circulation anomalies and precursors corresponding to the extreme precipitation on the TP in summer during 1979–2020 were investigated. The results showed that the summer extreme precipitation of the TP can be categorized into northwestern (NW), southeastern (SE), and southern Himalayan (HS) types based on extreme precipitation thresholds. The NW and SE types are mainly influenced by anomalous signals in the mid-to-high latitude regions upstream of them, whereas the HS type is controlled by the localized subtropical anomalous circulation. On the 8th day before the onset of the NW type, an anomalous cyclone was observed in the western Atlantic Ocean (60°W, 50°N), which triggered the west-to-east quasi-latitudinal propagation of Rossby waves. On the onset day of the NW type, the upper troposphere showed positive-negative-positive geopotential height anomalies along the latitudinal 40°N from the Caspian Sea, the western part of Xinjiang, to the northeastern TP. Moisture entered from the Arabian Sea along the southeastern edge of the anomalous cyclone on the southwestern TP and converged in the northwestern TP. Compared to the NW type, the precursors of the SE type appeared at higher latitudes and were more intense. On the 8th day before the onset of the SE type, an anomalous cyclone occurred near Greenland (60°W, 70°N) and excited Rossby waves propagating southeastward. On the onset day of the SE type, the upper troposphere showed negative-positive-negative geopotential height anomalies across Eurasia from the Ural Mountains and the Iranian Plateau to the northern TP in the northwest-southeast direction and entered the southeastern TP from the Bay of Bengal along the southeastern edge of the anomalous cyclone in the southern TP. On the 6th day before the onset of the HS type, the anomalously high pressure in the middle and lower layers of the low-latitude region extended westward, and a significant anticyclonic anomalous circulation occurred on the southern TP on the onset day of the HS type, enabling the delivery of moisture from the Bay of Bengal to the southern foothills of the Himalayas. Additionally, an anomalous cyclone perched in the northeastern TP at a geopotential height of 200 hPa strengthened westerly winds in the southern TP and contributed to the maintenance of the anticyclonic system on the southern TP.