El Nino Watch
El Nino moves the Pacific rain clouds east, so air sinks over Southeast Asia. In Thailand, El Nino years are usually drier and hotter than normal, most clearly in the hot season and early rainy season.
El Nino basics
The Thai Meteorological Department (TMD) studied rain and temperature in El Nino years over 50 years (1951-2000). Its findings, in short:
Why: the rising air and rain clouds move east, towards the central Pacific (see Walker circulation). Over Southeast Asia, air tends to sink, which suppresses rain clouds.
How this meets the monsoons: Thailand's rain comes mostly from two monsoons. El Nino does not switch them off. It shifts the odds towards a weaker or shorter rainy period and a longer, hotter dry season. Koh Samui, on the Gulf side, gets its main rain from the northeast monsoon (October to January), not from the southwest monsoon. See Koh Samui's seasons.
For Samui the worry is the chain: less rain -> reservoirs not full -> a long dry season (February to April 2027) with more heat -> water shortage. This is exactly what happened in 2026: the PWA started rotating water supply on Samui from 3 August 2026 after a long dry spell (see Samui's water supply).
El Nino does NOT prevent floods. The northeast monsoon can still bring heavy rain and floods in November-December, even in an El Nino year.