El Nino Watch

Thermocline

The boundary in the ocean between the warm surface layer and the cold deep water. Its depth controls how much cold water can reach the surface.

El Nino basics

The ocean is layered. On top sits a warm, well-mixed layer. Below it, the temperature drops quickly. That zone of fast cooling is the thermocline.

Along the equatorial Pacific, the thermocline is normally tilted:

  • Deep in the west (around 150 m or more), because the trade winds pile warm water up there.
  • Shallow in the east, near South America, so cold deep water easily comes up to the surface (this is called "upwelling").

During El Nino the tilt flattens: the thermocline gets deeper in the east (NOAA gives 150 to 175 m as typical during El Nino in the east-central Pacific). The cold water can no longer reach the surface, so the surface warms.

How to read it

NOAA's subsurface charts show temperature anomalies along the equator from the surface down to about 300 m. Big warm anomalies near the thermocline (the CPC reported more than +10 °C at depth in September 2026) mean more warm water is on its way to the surface.

Sources

Related

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)