Objective Clustering of Spatial Patterns of Summer Extreme Precipitation Frequency over the Huaihe River Basin and Their Formation Mechanisms

Based on the daily precipitation over the Huaihe River basin and reanalysis datasets from 1961 to 2016, the spatial patterns of the summer extreme precipitation frequency over the Huaihe River basin are objectively classified through the K-means cluster and rotated empirical orthogonal function analysis. Their formation mechanisms are investigated via observational diagnosis and numerical simulations. Results show that: (1) The spatial distribution of the summer extreme precipitation frequency over the Huaihe River basin can be objectively divided into three types: S-Type, C-Type, and N-Type, in which the extreme precipitation mainly falls south of 33°N, 32°–36°N, and north of 34°N, respectively. (2) The S-Type is related to the anomalous southwestward shift of the northwestern Pacific subtropical high (NWPSH). Cyclonic anomalies north of NWPSH hinder the water vapor further to the north, resulting in more extreme precipitation in the south of the Huaihe River basin. For the C-Type, the Huaihe River basin is controlled by saddle circulation anomalies pattern. Therefore, extreme precipitation is mainly confined to the central Huaihe River basin. The N-Type is associated with the northwestward shift of the NWPSH. The southerly wind could transport water vapor to the north of the Huaihe River basin, resulting in more extreme precipitation in the region. (3) The formation and the related circulation anomalies of the S-Type and N-Type are closely associated with El Niño-like and La Niña-like sea surface temperature anomalies, while the C-Type-related saddle circulation pattern is linked to two southward-propagated stationary Rossby waves over the Eurasian continent induced by the Arctic Sea ice anomalies over the Barents Sea/Kara Sea.