A Study of the Time Lags of the Indian Ocean Dipole and Rainfall Over Thailand by Using the Cross Wavelet Analysis

The aim of this research was to study the time lags of the Indian Ocean Dipole (IOD) and monthly mean rainfall over Thailand by using the Cross Wavelet Analysis. Historical data of monthly mean rainfall during the period 1979-2008 in a total of 80 locations spread throughout Thailand are used in this study. This data was obtained from the archives of the Meteorological Department. The regions of Thailand have been divided into five parts: the Northern, Northeastern, Central, Eastern, and Southern regions of Thailand. The seasons for monthly mean rainfall can be divided into two seasons, dry and wet. This study used the Dipole Mode Index (DMI) covering a 30-year period from January 1979 to December 2008. The DMI is an indicator of the east-west temperature gradient across the tropical Indian Ocean, linked to the IOD or Zonal Mode. The analysis of the cross wavelet transform of the DMI and monthly mean rainfall over Thailand represents significant peaks in the years of strong IOD at a scale around 11 months. This implies that the IOD leads rainfall around two months. Although there may be other factors that affect rainfall over Thailand, it can be concluded that rainfall over Thailand is influenced by the IOD.