Decreasing Trend in Rainfall over Indochina during the Late Summer Monsoon: Impact of Tropical Cyclones

Abstract

We examined the decreasing trend in rainfall during the late summer monsoon season (September) in Thailand from 1951 to 2000 and associated changes in tropical cyclone (TC) activity. Thailand receives significant rainfall from May to October and experiences two rainy peaks in late May to early June and in September. A previous study reported a decreasing trend in September rainfall in Thailand and, based on a regional climate model, suggested that the trend was associated with local deforestation. However, the long-term trend may also be affected by changes in large-scale circulation. Thus, the purpose of this study was to investigate changes in large-scale circulation associated with the decreasing rainfall trend.Westward-propagating TCs from the South China Sea and the western North Pacific brought most of the rainfall over Thailand in September. TCs include tropical depressions, tropical storms, severe tropical storms, typhoons, and residual lows. 70% of the rainfall amount in September was estimated to be associated with TCs.The 50-year time-series of September rainfall over Thailand showed a significant decreasing trend. TC activity defined by 700-hPa relative vorticity, showed a weakening trend over the Indochina Peninsula. TC tracks also suggested the weakening of TC activity over this area. The long-term trend in rainfall during the late summer monsoon season was closely associated with changes in TC activity over the Indochina Peninsula; these changes were likely caused by changes in the major course of TCs. Concurrent with the changes in TC tracks, there was a change in the TC steering current around the Philippines archipelago and Taiwan. This led to the TC activity over the Indochina Peninsula being inactive, probably resulting in the long-term decrease in rainfall over Thailand.

Climatological analysis of passage-type tropical cyclones from the Western North Pacific into the South China Sea

“…Seasonal variations in probability for a WNP TC to move into the SCS are likely related to changes in major TC tracks among different months. TC tracks are generally considered to be highly controlled by steering flows, which are normally represented by 500-hPa circulations (e.g., Wu and Wang 2004;Camargo et al 2007;Takahashi and Yasunari 2008). The seasonal features of steering flows are delineated by 1950 -2010 means of 500-hPa winds and height.…”

Long-term changes in rainfall and tropical cyclone activity over South and Southeast Asia

“…However, weaker TCs, including tropical depressions and residual lows, can also bring rainfall. Takahashi and Yasunari (2008) showed that weaker TCs also contribute to the total rainfall. To estimate TC activity including that of weaker TCs, the perturbation kinetic energy (PKE) of the low level winds was defined using high frequency components of u and v, i.e.,…”

“…For example, Singh et al (2000) showed that the frequency of TCs over the north Indian Ocean has increased in May and November and weakened in September over the last 100 years, which was likely to affect long-term changes in rainfall. Takahashi and Yasunari (2008) and Takahashi et al (2009) analyzed observational data and proposed that the long-term decrease in September rainfall over Indochina in the second half of the twentieth century, which was found by Kanae et al (2001), could be explained by the weakening of TC activity over the South China Sea and Philippine Sea regions in September. In addition, Takahashi et al (2009) found that the long-term decrease in rainfall over the Indochina Peninsula was most significant along the eastern coast of Vietnam, where rainfall is strongly affected by TC activity.…”

“…In addition, Takahashi et al (2009) found that the long-term decrease in rainfall over the Indochina Peninsula was most significant along the eastern coast of Vietnam, where rainfall is strongly affected by TC activity. Takahashi and Yasunari (2008) also estimated the contribution of TCs including typhoons, tropical storms, tropical depressions, westward-moving intraseasonal disturbances, and residual lows to the total rainfall amount, finding that 70% of the rainfall was associated with TC activity.…”

Monthly adjustment of Global Satellite Mapping of Precipitation (GSMaP) data over the VuGia^|^ndash;ThuBon River Basin in Central Vietnam using an artificial neural network

“…While most Southeast Asian regions experience a simple summertime rainfall maximum during the Asian summer monsoon period, the maximum rainfall period in central Vietnam is from November to December (Matsumoto, 1997;Yokoi et al, 2007). Although some studies have been conducted to understand the mechanisms and characteristics of rainfall over Vietnam (e.g., Yokoi and Matsumoto, 2008;Takahashi and Yasunari, 2008;Chen et al, 2012;Nguyen-Thi et al, 2012;Takahashi, 2012), large knowledge gaps remain. One of the reasons is that the rain-gauge network in Vietnam is still not dense enough to meet the demands of practitioners and researchers.…”