Clustering circulation in eastern Asia as a tool for exploring possible mechanisms of extreme events and sources of model error

Every summer in eastern Asia, the combined effects of complex topography, multiple potential moisture sources and a mixture of sub-tropical and mid-latitude dynamics produces an environment, in which subtle differences in spatial patterns of atmospheric circulation can profoundly affect the geographic distribution of rainfall. Understanding and quantifying these patterns is a fundamental requirement if we are to understand, predict or project rainfall events likely to cause impacts to society in the region. To help the scientific community with this task, a method is presented here, in which spatial patterns of summer sea level pressure over eastern Asia are clustered into a set of eight circulation types, with examples given of their potential use. We find that the within-season prevalence of individual circulation types exhibit a strong relationship with the seasonal progression of the summer monsoon whilst providing a spatially coherent, physical interpretation of the monsoon for all parts of eastern Asia. Beyond this, the clustering approach permits the attribution of notable rainfall anomalies in almost any location within the domain used to build the clusters. Examples are given, showing how circulation specific moisture transport anomalies in one part of China can result in anomalously dry days in another part of the country. Two further applications of the clustering approach are demonstrated using climate simulations. The first is of circulation specific model errors which can allow targeted model development. The second provides information about anomalies which are plausible, but have yet to be observed, possibly due to limitations in the observed record.