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 18/03/2025.

Forecasts for 29 March

Tropical Pacific Ocean

The extent of the marine heatwaves remains stable overall, but an intensification is observed around Papua New Guinea and Indonesia, with the presence of strong categories in addition to moderate categories. At 180°W, strong categories are developing.

South Pacific Ocean

The marine heatwave in the central part of the basin, between 180°W and 120°W, is decreasing in extent. Its intensity remains stable, with strong and moderate categories.moderate category.

Tropical Atlantic Ocean

The global extent of the marine heatwaves in the Tropical Atlantic is decreasing. In the Caribbean Sea, the intensity of the marine heatwave is falling and only an area to the south of the Dominican Republic and Puerto Rico remains in the strong category, while most of the rest is now in moderate category. 

Indian Ocean

The marine heatwave east of Madagascar remains stable, with moderate and strong categories. The marine heatwave in the Arabian Sea is decreasing significantly in both intensity and extent. Only the eastern part of the basin remains affected by moderate categories. In the Bay of Bengal, the marine heatwave is also diminishing. Its extent decreased, as is its intensity, with the transition from strong to mostly moderate categories. The extent of the marine heatwave to the north-west of Australia has greatly diminished, levaing only a small area near the coasts in moderate category.

European Zone

Mediterranean Sea

The extent of the marine heatwave present throughout the Mediterranean basin decreases, as well as its intensity, with a decrease of the area in strong categories which are now in moderate categories. 

Weekly Temperature Anomalies

23/03/2025-29/03/2025

Mean sea surface temperature anomalies in each global ocean region for the week 23 to 29 March, 2025.

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.[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