Association between Northward-Moving Tropical Cyclones and Southwesterly Flows Modulated by Intraseasonal Oscillation

Tropical cyclones (TCs) of a particular track type move northward along the open oceans to the east of Taiwan and later pass over or near northern Taiwan. Their northward movement may be associated with intensified monsoon southwesterly flows from the northern South China Sea (SCS) toward Taiwan. Prolonged heavy rainfall then occurs in western Taiwan across the landfall and postlandfall periods, leading to severe floods. Characteristics of this TC-southwesterly flow association and related large-scale regulatory processes of intraseasonal oscillations (ISOs) are studied. For summers from 1958 to 2009, 16 out of 108 TCs affecting Taiwan exhibit the aforementioned northward-moving track. Among them, four TCs (25%) concur with enhanced southwesterly flows. Intensified moisture supplies from the SCS result in strong moisture convergence and prolonged heavy rainfall in western Taiwan. Both 30-60- and 10-24-day ISOs make positive contributions to the TC-southwesterly flow association. Both ISOs exhibit the northward progress of a meridional circulation pair from the tropics toward Taiwan. During landfall and the ensuing few days, Taiwan is surrounded by a cyclonic anomaly to the north and an anticyclonic anomaly to the south of these two ISOs. The appearance of anomalous southwesterly westerly flows acts to prolong heavy rainfall in western Taiwan after the departure of a TC. The TC-southwesterly flow association tends to occur during the minimum phase of the 30-60-day ISO featuring a cyclonic anomaly in the vicinity of Taiwan but in various phases of the 10-24-day ISO. Rainfall in western Taiwan increases when these two ISOs simultaneously exhibit a cyclonic anomaly to the north of Taiwan.