ISO-Modulating Effects on the East Asian Summer Monsoon Circulation Patterns Associated with Southern Taiwan’s Monsoon Rainfall

Abstract

n this study, the circulation patterns of the summer monsoon associated with monsoon rainfall in southern Taiwan were analyzed and two types of monsoon patterns were defined. The first type was characterized by a broad low pressure area extending northeastward from a low pressure center near southern China to southern Japan. Strong southwesterly flows were observed over the southern flank of the monsoon trough. The second type of monsoon pattern was characterized by a strong westward-extending anticyclone in the area north of Taiwan (including central China, South Korea, and Japan), and a weaker east?west-elongated monsoon trough south of Taiwan, driving the easterly flow to southern Taiwan. The modulating effect of intraseasonal oscillations (ISOs) on these monsoon flow patterns indicates that as ISOs propagate northwestward toward southern China, they create favorable conditions for developing and strengthening southwesterly flows and convection. However, because of the latitudinal limit of northwestward-propagating ISOs, only the edge of the strongest convection over the southern flank of the ISO cyclonic circulation reaches southern Taiwan during the westerly phase. Thus, although the westerly ISO pattern appears to be stronger than the easterly pattern, it brings less rainfall to southern Taiwan. Through the tightening of pressure gradients, the ISO typically generates anomalous cyclones (anticyclones) that can affect the southwesterly and northeasterly flows near its southern (northern) and northern (southern) rims. Therefore, fluctuations in the low-frequency background flow can exert a notable effect on the monsoon rainfall and associated circulation systems near Taiwan.