20–60-day intraseasonal variation of summer rainfall in Thailand and its associated large-scale atmospheric moisture circulation

Variations of rainfall at sub-seasonal time scale can cause both wet and dry conditions that are commonly associated with anomalies of large-scale atmospheric moisture circulation. This study examines the characteristics of the sub-seasonal variations of summer rainfall in Thailand and their association with large-scale atmospheric moisture circulation. We analyzed long-term daily rainfall data from the Thai Meteorological Department (TMD) spanning from 1979 to 2019, along with data on atmospheric water vapor flux and transport obtained from the ERA-5 reanalysis. Here, we used a 20–60-day bandpass filter for both the TMD and ERA-5 datasets. The results showed that summer rainfall in Thailand is characterized by high intensity and variability of 20–60-day rainfall, particularly in the northeastern, eastern, and western coastal areas of upper Thailand, as well as in the western coastal area of lower Thailand. The characteristics of the 20–60-day atmospheric water vapor anomalies associated with wet and dry rainfall events varied both spatially and temporally, depending on the sub-region and month. Indeed, the wet conditions in upper Thailand during the summer season are strongly associated with southwesterlies (easterlies) of moisture convergence from the eastern Indian Ocean (South China Sea), which induced a total moisture budget of about 24 × 10 8 kg/s. In contrast, dry conditions are controlled by westerly anomalies of moisture divergence, with a moisture budget of −21.4 × 10 8 kg/s. For lower Thailand, wet conditions of the 20–60-day rainfall variation are associated with southwesterly anomalies and a cyclonic-like pattern of moisture convergence over the Andaman Sea, while an anti-cyclonic atmospheric moisture divergence is dominant, leading to below-normal rainfall (with a water budget of 2.7 × 10 8 kg/s and −2.4 × 10 8 kg/s for wet and dry events, respectively). The results presented here will benefit our understanding of regional intraseasonal rainfall variation, improving sub-seasonal to seasonal forecasting in Thailand which will help improve our water resource management and disaster risk reduction.