El Nino

El Nino basics

During El Nino, the surface of the central and east-central equatorial Pacific (roughly from the date line to the coast of South America) warms up. The easterly trade winds weaken, or even reverse in the far west.

What typically happens:

• The pool of very warm water that normally sits near Indonesia spreads east.
• The big rain clouds follow the warm water east, towards the central Pacific.
• Over Indonesia, northern Australia and much of Southeast Asia, air sinks instead of rising, so it rains less.
• Air pressure rises at Darwin and falls at Tahiti, so the SOI turns negative.

The name ("the boy" in Spanish, a reference to the Christ child) comes from Peruvian fishermen who noticed warm water off their coast around Christmas.

How to read it

NOAA declares El Nino conditions when the one-month relative Nino 3.4 anomaly is +0.5 °C or more, the 3-month RONI is expected to stay above +0.5 °C, and the atmosphere responds (weaker trade winds, more rain in the central Pacific).

Why it matters for Koh Samui

For Thailand, El Nino years usually bring less rain and higher temperatures, most clearly in the hot season and early rainy season. On Koh Samui that means a higher risk of a hard dry season (water, heat, haze). See Why El Nino affects Thailand.

Sources

• NOAA CPC -- ENSO frequently asked questions
• NOAA CPC -- ENSO Alert System definitions
• NOAA Climate.gov -- What is ENSO, in a nutshell

Related

• ENSO (El Nino-Southern Oscillation)
• La Nina
• Why El Nino affects Thailand
• Weak, moderate, strong and "super" El Nino
• Typical El Nino lifecycle
• The 2026 El Nino

ENSO status now

ONI (NOAA CPC)

+1.80 °C, JJA 2026 (previous +1.39 °C)

RONI (NOAA CPC)

+1.36 °C, JJA 2026

Nino 3.4 weekly (NOAA CPC / OISST)

+3.0 °C anomaly, week of 16 Sep 2026 (SST 29.6 °C)