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. [1] This week’s forecasts were produced using as a comparison the marine heatwave situation on 18/02/2025.

Forecasts for 1 March

Indian Ocean

The extent of the marine heatwave to the east of Madagascar is strongly decreasing, with a patch remaining in the moderate and strong categories. In the Arabian Sea and in the Gulf of Bengal, the area of the marine heatwave decreases too. The remaining areas affected by a marine heatwave are mainly in moderate to locally strong categories.

Tropical Pacific Ocean

The marine heatwave present in the west of the basin and affecting Indonesia, the Philippines and Papua New Guinea has seen its extent and intensity greatly reduced, with moderate categories in the majority, with the exception of a few areas in strong categories along the coasts of the south of Indonesia and the Solomon Islands. To the east, the marine heatwave along the coasts of Chile and Peru, which is extending into the basin, has seen its intensity increasing from moderate to strong.

South Pacific Ocean

At the center of the basin between 150°W and 120°W, the marine heatwave is intensifying, going from moderate to strong categories. The extent of the marine heatwave off the Chilean coast is increasing as well as its intensity, with the development of strong categories beside moderate categories.

Tropical Atlantic Ocean

The marine heatwaves in the Caribbean Sea, the Gulf of Guinea as well as in the center of the basin at 30°N intensify, going from moderate to strong categories. In the Gulf of Mexico, the marine heatwave is decreasing in intensity and extent, and is now mostly in moderate category. 

Southern Ocean

The marine heatwave between 0° and 110°E stays stable in area, and intensifies between 50°E and 90°E as the surface area of the strong categories increases. 

European Zone

Weekly Temperature Anomalies

23/02 – 01/03/2025

Mean sea surface temperature anomalies in each global ocean region for the week 23 February to 1 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

[1] Analysis of datasets: SST OSTIA (Copernicus Marine Service), OISST (NOAA), GLO12 (Copernicus Marine Service / Mercator Ocean International), PSY4 (Copernicus Marine Service / Mercator Ocean International), and GLO12 et PSY4 forecasts.

[2] IPCC AR6 SYR chapter 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf