A statistical and dynamical characterization of large-scale circulation patterns associated with summer extreme precipitation over the middle reaches of Yangtze river

The large-scale circulation associated with extreme precipitation over the middle reaches of Yangtze river (MRYR) in early summer (June and July) are classified into three canonical patterns via hierarchical clustering. The clustering results reveal a clear connection between the MRYR extreme precipitation and anomalous moisture convergence over this region with the eastward expansion of South Asia High and intensified westerly jets providing additional forcing for local rising motion. In all three clusters, the anomalous moisture convergence results from anomalous low-level southwesterlies encountering anomalous northerlies from mid-high latitudes. The southwesterly anomaly is associated with the expansion of the Western Pacific Subtropical High (WPSH). However, the anomalous northerlies weakening the northward advance of the Mei-yu front are mainly driven by different extratropical circulation anomalies in the three clusters. These anomalies range from zonally-elongated barotropic disturbances to developing baroclinic disturbances that are potentially tied to upstream storm tracks. All three clusters are characterized by a meridional dipole in geopotential height anomaly over the tropical-subtropical East Asia. The northern and also more pronounced node of the dipole is located over the MRYR with a cyclonic (anti-cyclonic) height anomaly in the lower (upper) troposphere, suggesting the critical role played by anomalous latent heating of extreme MRYR rainfall in driving the formation of this dipole. This dipole anomaly projects effectively onto the negative phase of the Pacific-Japan teleconnection pattern and acts partly as a positive feedback to the westward expansion of the WPSH. Also discussed are the implications of the identified large-scale circulation patterns for model evaluation and operational forecasting of extreme precipitation events.