| Summary: | Temporal variations of droughts/wet spells in Thailand for the period 1951-2005 were examined on the basis of the gridded Palmer Drought Severity Index (PDSI) data. PDSI is the most dominant index for drought monitoring, climatology and variability across different climates. The PDSI variations in Thailand were correlated well with the annual streamflow records, indicating that PDSI is a good proxy for monitoring and assessing droughts/wet spells and it can be further used as an index of annual-mean streamflow variations. An empirical orthogonal function (EOF) analysis of PDSI revealed a linear trend and an El Niño-Southern Oscillation (ENSO)-induced mode of multi-year variations as the leading pattern. The ENSO cycle and its shift toward more warm phases after about 1976 appeared to be largely responsible for interannual variations and the recent progressive dying trend in Thailand. From 1951 to 2005, there were also large interannual/decadal variations in the occurrence frequencies in severe/extreme droughts (PDSI< -3) and very/extremely wet spells (PDSI> 3) with the coherent jump occurred in the mid 1970s. Similar to the leading PDSI EOF1 mode, these annual occurrence frequencies were closely related to ENSO events which extreme events tended to happen more frequently during ENSO years. Patterns of EOF-derived PDSI variations were consistent with the observed surface temperature warming in Thailand. These results provide evidence that Thailand will experience the increasing risks of severe and extreme droughts/floods in the near future as a result of the combined effects of a more vigorous hydrological cycle and enhanced surface drying due to anthropogenic global warming and the anomalous oscillations of ENSO.
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