Changes in summer monsoon rainfall in the Upper Chao Phraya River Basin, Thailand

Abstract

We determined the effects of climate change on pre-monsoon (May-June-July: MJJ) and monsoon (August-September-October: ASO) season rainfall in the Upper Chao Phraya River Basin, Thailand, by downscaling surface rainfall from large-scale atmospheric variables, i.e. surface air temperature (SAT), sea level pressure (SLP), and zonal and meridional wind (u and v, respectively). The data were obtained from the Geophysical Fluid Dynamics Laboratory (GFDL) model and used as predictors in a modified k-nearest neighbor (k-nn) model. Under climate change scenarios A2 and B2, during 2011-2100, the increasing trends of annual SAT over northern Thailand and the South China Sea vary from 1.65 to 3.47°C century -1. By the end of the 21st century, the annual SAT anomalies range from + 2 to +10°C. The increasing trends of annual SLP over the Gulf of Thailand and northern Thailand range from 0.40 to 0.83 mb century -1. Depending upon the regions and scenarios, increasing and decreasing trends of annual u and v were observed. From the modified k-nn model, the effects of climate change on MJJ and ASO rainfall indicate decreasing trends during 2011-2100 with a maximum decrease by 6.16 mm yr -1 , corresponding to the ASO rainfall under Scenario B2. In terms of effects on the frequency of extreme events, dry (wet) conditions during 2011-2100 showed a greater (lesser) chance of occurrence than the climatology, with the exception of ASO rainfall under the Scenario A2, which showed a greater chance of being both dry and wet. With a probability > 70%, dry MJJ and ASO conditions will be observed more often than wet, especially the dry ASO under Scenario B2, which was predicted for the 55 yr from 2046 -2100. KEY WORDS: Climate change · Thailand rainfall · Upper Chao Phraya River Basin · Ping River BasinResale or republication not permitted without written consent of the publisher Clim Res 49: 155-168, 2011 terms of intensity and frequency of extreme events. Increasing greenhouse gas concentrations, in particular CO 2 , greatly influence global warming (Mitchell 1989, Maslin 2007, NIC 2009, NASA 2010. Consequently, climate change is expected to occur due to increasing global surface temperatures (UNEP 2003, Trenberth 2008. Since rainfall in Thailand shows significant links to large-scale atmospheric variables (LSAVs), e.g. temperature and pressure (Chen & Yoon 2000, Singhrattna et al. 2005b, the changing climate affects rainfall via atmospheric-oceanic circulations. The objective of this study was to determine the effects of climate change on summer monsoon season rainfall in the Upper Chao Phraya River Basin (Thailand). Rainfall simulation by a stochastic statistical model is a tool for long-term planning in water resources and an adaptation strategy to deal with future climate change. METHODSBased on the significant relationships between local hydroclimates (e.g. precipitation) and LSAVs (e.g. sea level pressure), LSAVs have been used as predictors in multiple regressions to forecast hydroclimates (Hamlet et al. 2002, McCa...

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