The marine heatwave bulletin provides forecasts and analysis of marine heatwave events across the globe and throughout the year. Used datasets include observations (satellite sea surface temperature maps) and numerical model analyses (assimilating satellite and in situ observations) to derive marine heatwave forecasts for a 10-day period. This week’s forecasts were produced using as a comparison the marine heatwave situation on 25/03/2025.
Forecasts for 5 April
Tropical Pacific Ocean
The marine heatwaves present between 120°W and 180°W decreases, with small area in moderate category which still there. Around Papua New Guinea and Indonesia, the marine heatwave increases with moderate to strong categories.
North Atlantic Ocean
The marine heatwave presents of the European coast increases in intensity with strong and severe categories and locally extreme.
Tropical Atlantic Ocean
The global extent of the marine heatwaves in the Tropical Atlantic is decreasing. The marine heatwave around the West Indies disappears. In the Mexico Gulf the extent of the marine heatwave increases with moderate category.
Indian Ocean
The global extent and intensity of the marine heatwave decreases. The marine heatwave in the Arabian Sea and the Bay of Bengal decreases in both intensity and extent. The extent of the marine heatwave to the north-west of Australia remains stable.
European Zone
Mediterranean Sea
The marine heatwave present to the Eastern Mediterranean basin decreases in intensity and in extent. The strong category disappears and the moderate category spatial extent decreases. In the Western Mediterranean basin, the marine heatwave increases with strong category between Corsica and French coast and the Strait of Gibraltar.
Weekly Temperature Anomalies
30/03/2025-05/04/2025
Mean sea surface temperature anomalies in each global ocean region for the week 30 March to 5 April, 2025.
| Mediterranean Sea | 0°C to 1.5°C |
| Tropical Pacific Ocean | -1°C to 3°C |
| North Atlantic Ocean | 0°C à 1.5°C |
| Tropical Atlantic Ocean | -2°C to 2°C |
| Indian Ocean | -1°C to 3°C |
Access the Daily Global Physical Bulletin for a 9-day forecast here.
SURVEY
Help us improve our content! Answer a quick survey about this bulletin here.
What are marine heatwaves?
Marine heatwaves (MHW) are extreme rises in ocean temperature for an extended period of time. They can occur at different locations in the ocean, and their magnitude and frequency have increased over the last couple of decades, with harmful impacts on ecosystems, and human activities. According to the latest report released by the Intergovernmental Panel on Climate Change (IPCC AR6 SYR), it is found with high confidence that in the near-term at 1.5°C global warming, the increasing frequency of marine heatwaves will increase risks of biodiversity loss in the oceans, including from mass mortality events.[2]
How are marine heatwaves calculated?
A marine heatwave is an episode during which the ocean temperature is abnormally warm for at least 5 consecutive days.
Adapted from Hobday et al. (2018)
For any location in the ocean, the normal temperatures are defined for every day of the year using a climatological period (here 1993–2016). A heatwave is identified when the measured daily temperature is within the top 10% of the highest recorded values for that day (i.e., above the 90th percentile, see diagram), and with this condition persisting for at least five consecutive days.
The intensity of the heatwave on any given day is measured as the number of degrees above the climatological average (represented by the bold black line) indicated by the blue arrow. We can either calculate the cumulative intensity over the entire heatwave or record the maximum intensity.
Heatwaves are classified based on their intensity level. To do this, the intensity is compared to the difference between the climatological value and the 90th percentile value. A mhw intensity between 1 and 2 times this difference corresponds to a heatwave of moderate category; between 2 and 3 times, to a strong category; between 3 and 4 times, to a severe category; and a difference greater than 4 times corresponds to an extreme category.
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
[2] IPCC AR6 SYR chapter 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf