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 08/04/2025.
Forecasts for 19 April
Tropical Atlantic Ocean
The marine heatwave in the Gulf of Mexico and the Caribbean Sea is decreasing in extent, and only a few scattered areas remain in moderate category. Towards the equator and off the coast of Africa, the moderate to locally strong marine heatwave is intensifying and is now predominantly strong. Off the coast of Brazil, the marine heatwave of moderate category is diminishing in extent, and only a few scattered areas remain.
Tropical Pacific Ocean
The marine heatwave extending from the north of Indonesia to the west of Papua New Guinea is now mainly in moderate category and is decreasing in area. Only an area in strong category remains to the south-east of Papua New Guinea. The marine heatwave in the centre of the basin at around 20°N remains in moderate and strong categories and retains its surface area. The area around the equator to the east of the basin is almost no longer affected by a marine heatwave, with the exception of a local patch off Costa Rica in strong to severe categories. The marine heatwave located at around 30°S and 110°W is stable.
Indian Ocean
Marine heatwaves off Madagascar, to the west and south of Australia and in the Arabian Gulf remain stable overall, with moderate and strong categories. In the Bay of Bengal, the moderate marine heatwave is diminishing in extent, with only a few scattered areas remaining.
European Zone
North Atlantic Ocean
The marine heatwave off Europe’s Atlantic coast is diminishing in intensity and extent, with most of the area affected now by moderate categories. The moderate marine heatwave in the North Sea is intensifying greatly, with strong to severe categories developing over almost the entire area. Off the coast of Morocco, the marine heatwave remains stable.
Mediterranean Sea
The marine heatwave of moderate and strong categories in the western Mediterranean basin is intensifying, with the development of strong categories in the centre, extending from north to south. In the eastern Mediterranean basin, the extent of the moderate marine heatwave is decreasing, mainly in the east. The rest of the area remains affected by moderate and strong categories.
Weekly Temperature Anomalies
13 – 19 April 2025
Mean sea surface temperature anomalies in each global ocean region for the week 13 to 19 April, 2025.
| Mediterranean Sea | 0°C to 2°C |
| Tropical Pacific Ocean | -1°C to 3°C |
| North Atlantic Ocean | -3°C à 3°C |
| Tropical Atlantic Ocean | -1°C to 2°C |
| Indian Ocean | 0°C to 3°C |
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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