The historical data of monthly mean rainfall during the period 1979–2008 from 80 locations spread throughout Thailand, obtained from the archives of the Meteorological Department were used (Fig. 1). Besides, 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 Indian Ocean Dipole or Zonal Mode (Fig. 2).