An El Nino usually starts in March-June, peaks in the northern late autumn and winter (about November to February) and fades in the following spring (May-July).
El Nino basics
NOAA's description of a typical event:
Develops during March to June.
Peaks during the northern late autumn and winter. The CPC says December to April; NOAA Climate.gov says November to February. The warm anomaly is usually largest around November-January.
Weakens during May to July of the following year.
Lasts typically 9 to 12 months. Some events have lasted 2 years, or even 3 to 4.
Why the winter peak: the equatorial Pacific is at its warmest then, so a small extra warming moves the tropical rain a lot, which in turn feeds back on the winds.
The effects on land often lag the ocean peak. For Thailand, the dry season right after the peak (February to April) is when El Nino drought and heat usually hurt most.
Why it matters for Koh Samui
The 2026 El Nino is expected to peak around late 2026. The dry, hot season that follows (February-April 2027) is the danger window for Samui's water.