Summer climate variability in Taiwan and associated large-scale processes

This study investigates major characteristics of summer (June-August) climate variability in Taiwan during the period of 1950-2000 and associated regulating processes imposed by large-scale background variations, including the western North Pacific summer monsoon (WNPSM), the Pacific interdecadal climate change (PICC) related to abrupt climate change in the late 1970s, and the western North Pacific (WNP) tropical cyclone (TC) activity. In Taiwan, summer rainfall and temperature anomalies tend to be opposite in sign: anomalous cold summer is often wet, and vice versa. Based upon characteristics of temperature and rainfall anomalies in Taiwan, low-level circulation pattern overlying Taiwan, and the phase of PICC, we categorize Taiwan's summer climate variability into six major types. The major regulatory mechanism for four of these types is anomalous vertical motion over an east-west elongated low-level circulation anomaly across Taiwan, a feature associated with the WNPSM variability. For the other two types, the primary regulating mechanism is anomalous moisture transport by a southwest-northeast oriented low-level circulation anomaly over the WNP, which is a salient feature of the PICC. Influence of the WNP TC activity on Taiwan's summer climate is examined in terms of TC rainfall (rainfall in Taiwan induced by TC passages) variability. For the types of summer climate variability with a low-level anomalous high over the WNP, the circulation enhances vertical wind shear to suppress TC genesis and the subsequent TC activity over the WNP, leading to decreased TC rainfall in Taiwan. On the other hand, increased TC rainfall does not necessarily occur in Taiwan for summer climate variability types with a low-level anomalous low over the WNP. A favorable condition for TC rainfall in Taiwan is to have more TCs forming in a region southeast of Taiwan, which are then steered by the WNP anomalous low to recurve northward toward Taiwan.