Marine heatwave Bulletin
Forecasts for 20 December 2025​

Key takeaways

  • Mediterranean Sea – The event in the eastern side of the basin is intensifying.
  • North Atlantic – Marine heatwaves in the North Sea and Baltic Sea are intensifying.
  • Southern Ocean – Two events are increasing in intensity at the basin moving to strong and extreme categories.
  • Tropical Atlantic – The marine heatwave, from the Gulf of Mexico to the center of the basin, remains stable.
Waves

Marine heatwave categories

European Zone

Marin heatwave categories forecast for 20 December, 2025. European zone. Credit: Mercator Ocean International

Mediterranean Sea

The marine heatwave in the eastern basin is intensifying, with the area covered by the strongest categories increasing.

North Atlantic

Marine heatwaves in the Bay of Biscay and between Iceland and the British Isles remain stable, with mostly moderate categories and locally strong categories. Marine heatwaves in the North Sea and Baltic Sea are intensifying, with the area covered by strong categories increasing.

Global Ocean

Marin heatwave categories forecast for 20 December, 2025. Global ocean. Credit: Mercator Ocean International

Tropical Atlantic

The marine heatwave, from the Gulf of Mexico to the center of the basin, remain stable, with moderate and strong categories still in effect.

South Atlantic

The marine heatwave around 30°S is decreasing in intensity and extent, with only a few scattered areas remaining in the moderate category.

South Pacific

The marine heatwave around 30°S is decreasing in intensity and extent, with only a few scattered areas remaining in the moderate category.

Indian Ocean 

The marine heatwaves present in the Indian Ocean are decreasing in size and intensity, with only a few scattered areas of moderate intensity remaining.

Southern Ocean

The marine heatwave between 30°E and 120°E is intensifying, with several areas moving into the extreme category. The marine heatwave around 170°W is intensifying, with the appearance of strong categories.

Weekly temperature anomalies

 

 

 

 

 

Mean surface temperature anomaly forecasts for the week of 13 to 20 December 2025. European zone and global ocean. Source: Mercator Ocean International
Mediterranean Sea 0°C to 3°C
North Atlantic <-3 °C to > 3°C
South Atlantic -3°C to 3°C
South Pacific -1°C to 3°C
Indian Ocean -1°C to 2°C
Southern Ocean -0.5°C to 1°C

 

Forecasts for 20 December 2025

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NOTES

Datasets and products: Global Ocean Physics Analysis and Forecast (sea surface temperature) E.U. Copernicus Marine Service/ Mercator Ocean International. https://doi.org/10.48670/moi-00016