The average temperature anomaly of the top 300 metres of the equatorial Pacific. The same stored heat as the warm water volume, expressed as a temperature instead of a volume.The upper 300 m temperature is a way of expressing the same heat content as the warm water volume, but in units of temperature rather than volume. Instead of asking how much water is warmer than 20 °C, it asks what the average temperature anomaly is across the top 300 meters of the equatorial Pacific. A positive anomaly means the entire layer is warmer than its long-term average; a negative anomaly means it is cooler. This metric is useful because temperature anomalies are easier to compare across different regions and depths than raw volumes.
This metric was developed alongside the warm water volume index as part of the same monitoring effort, with NOAA PMEL providing regular updates. It reflects a broader practice in oceanography of converting heat content into temperature anomalies for easier analysis.
The calculation takes temperature measurements from the same moored buoys and floats, averages them over the top 300 meters, and then subtracts the climatological average for that time of year. The result is a single number that tracks the buildup or release of ocean heat. Because the warm water volume and this temperature anomaly both depend on the same underlying data, they move together, but the temperature form is often more intuitive for interpreting the state of the ocean.
Researchers use it to study the evolution of ENSO and to validate climate models that simulate ocean–atmosphere interactions.
The top 300 meters of the equatorial Pacific hold more heat than the entire atmosphere above it, making this layer a critical buffer for global climate.
