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 22/04/2025.
Forecasts for 3 May
North Pacific Ocean
The marine heatwave in the tropical Atlantic Ocean dispersed and only a few scattered areas remain in moderate category. At 60°W the marine heatwave increases in intensity with strong category. Towards the equator and off the coast of Africa, the moderate to locally strong marine heatwave is intensifying with strong category.
Tropical Pacific
The marine heatwave present from the east of the Philippines to the north of Papua New Guinea is decreasing in extent and intensity and is now overall in the moderate category. The marine heatwave in the north-central part of the basin, around 20°N and 160°W, remains stable overall, with moderate and strong categories. The marine heatwave southeast of Papua New Guinea remains stable overall, with moderate and strong categories. The marine heatwave to the southeast of the basin at around 30°S and 110°W remains stable overall, with moderate and strong categories.
South Pacific Ocean
The marine heatwave around New Zealand and in the center of the basin remains stable overall, with moderate to strong categories.
Indian Ocean
The marine heatwave in the Bay of Bengal is diminishing in intensity and surface area, with only a few scattered areas remaining in the moderate category. The marine heatwave in the Arabian Sea remains stable overall, with moderate categories. To the east of Madagascar and north-west of Australia, the marine heatwave is decreasing in intensity and is now generally in the moderate category. To the south of the Indian Ocean, the marine heatwave around 60°S remains stable overall, with moderate, strong and locally severe categories.
European Zone
North Atlantic Ocean
The marine heatwave off Europe’s coasts is intensifying between northern Portugal and the southern British Isles, with an increase in the surface area of strong categories and the local appearance of severe and extreme categories.
Weekly Temperature Anomalies
27 April – 3 May 2025
Mean sea surface temperature anomalies in each global ocean region for the week 27 April to 3 May , 2025.
| North Atlantic Ocean | -3°C to 2°C |
| North Pacific Ocean | -3°C to 3°C |
| Tropical Pacific | -2°C à 2°C |
| South Pacific | -0.5°C to 3°C |
| Indian Ocean | -3°C to 3°C |
Access the Daily Global Physical Bulletin for a 9-day forecast here.
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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.
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
IPCC AR6 SYR chapter 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf