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

Abstract

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.

Heavy Rainfall Induced by Tropical Cyclones across Northern Taiwan and Associated Intraseasonal Oscillation Modulation

“…The topographic lifting effect of steep terrain resulted in torrential rainfall in southern Taiwan. Hong et al's result is consistent with that of Chen and Shih (2012) in that the concurrence of cyclonic anomalies from both 30-60-day and 10-24-day ISOs may act to induce significant rainfall over Taiwan during a TC event. Also implied is that a type-2 TC across northern Taiwan may be associated with intensified southwesterly flows and prolonged heavy rainfall as it is modulated by the ISOs.…”

“…Its rainfall can be attributed to two major sources: the southwesterly monsoon from May to August and tropical cyclone (TC) activity from June to October (e.g., Gray 1975;McBride 1995;Chen and Chen 2003;Ding and Sikka 2006;Chen et al 2007; Wang and Chen 2008;Chen et al 2010b;Chen and Chen 2011;Chou et al 2011). Evolution of the summer monsoon system over Asia and the WNP features two major intraseasonal oscillations (ISOs) of 30-60 and 10-24 days (e.g., Hartmann et al 1992;Chen 1993, 1995;Chen et al 2000;Mao and Chan 2005;Ding 2007).…”

“…Recently, Chen and Shih (2012) found that TC-induced rainfall in Taiwan can be regulated by large-scale ISOs to vary significantly. For summer TCs moving northward along the eastern coast of Taiwan and later passing through or near northern Taiwan, 25% of these TCs are associated with intensified monsoon southwesterly flows from the SCS toward Taiwan.…”

“…It is of interest to ask whether the TC southwesterly flow association and accompanying prolonged heavy rainfall also occur for TCs of a different track type. Chen and Shih (2012) pointed out that a type-6 The topography of Taiwan and 10 major meteorological stations around the coastal plain areas of this island. TC low pressure system over northern Taiwan tends to provide favorable conditions for seasonal southwesterly flows to move northward from the SCS onto Taiwan.…”

Boreal Summer Intraseasonal Oscillation Impact on Western North Pacific Typhoons and Rainfall in Taiwan

“…Typhoons are considered the largest weather-related disaster in Taiwan (e.g., Wu and Kuo 1999). Chen and Shih (2012) and Chen et al (2013) studied the interaction between the ISO and some specific types of typhoon cases affecting Taiwan. There is still a gap in understanding the ISO influence on WNP typhoons and the summer rainfall in Taiwan.…”