Change in rainfall seasonality in Thailand during 1955-2018

Abstract

Spatiotemporal changes in Thailand were analyzed in rainfall seasonality for long-term data (1955–2018). The results showed that the dimensionless rainfall seasonality index (DSI) and relative entropy (RE) in Thailand ranged from rather seasonal with a short drier season to markedly seasonal with a long drier season. In addition, climatology of the DSI and RE captured the dominant feature of Thailand’s rainfall pattern which is under the influence of phase reversals between the Southwest and Northeast monsoons and topographical effects. From a long-term perspective, the DSI in Thailand showed a significant decreasing trend, indicating a tendency for the annual rainfall to be more spread out over the months. Furthermore, the decrease in the RE was more attributable to the observed decreasing trend in the DSI than normalized mean annual rainfall. Similar to the DSI, the timing of peak rainfall significantly decreased, suggesting a delay in the peak rainfall of 1.2 d per decade. In contrast, the rainfall duration significantly increased at the rate of 0.61 d per decade, highlighting a lengthening of the rainfall season. Therefore, these observations provide an indication of an earlier onset or a delayed withdrawal or both, as well as a shift in its associated rainfall peak for the summer monsoon in Thailand. These changes in rainfall seasonality will have profound socioeconomic implications for Thailand, especially with the on-going warmer background. To better understand short-term variations in Thailand’s rainfall seasonality, their relationship with regional and global natural climate modes needs to be further elaborated.