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 11/02/2025.

Forecasts for 22 February

Indian Ocean

The marine heatwave to the east of Madagascar is decreasing in intensity and increases in surface area but remains in the moderate and strong categories. The marine heatwave in the south of Australia is decreasing and has almost disappeared 

Tropical Pacific Ocean

The marine heatwave in the Caribbean Sea is slightly decreasing in intensity but still stabilized in the moderate category. In the Gulf of Mexico, the marine heatwave is decreasing and is now mostly in the moderated category. The marine heatwave in the Gulf of Guinea is intensifying, with a more significant appearance of strong categories. 

South Pacific Ocean

Off New Zealand at 220°W, the marine heatwave towards the center of the basin is decreasing in intensity and in surface area. The marine heatwave off the Chilean coast is decreasing in surface area and intensity and is now mostly in the moderate category.

Tropical Atlantic Ocean

The marine heatwave in the Caribbean Sea is decreasing in intensity, and is now mostly in the moderate category. In the Gulf of Mexico, the marine heatwave is intensifying, and is now mostly in the strong category, with severe categories in places. The marine heatwave in the Gulf of Guinea is intensifying, with the appearance of strong categories. 

Southern Ocean

The marine heatwave between 0° and 60°E intensifies as the surface area of the strong categories increases. Between 90°E and 120°E, the heatwave stays mostly into the moderate and strong categories accompanied by a decrease in its extent. 

European Zone

Mediterranean Sea

The marine heatwave in the Mediterranean Sea is mostly stable with an intensity around the strong category in the eastern basin and moderated for the rest of the basin

Weekly Temperature Anomalies

09/02 – 15/02/2025

Mean sea surface temperature anomalies in each global ocean region for the week 16 to 22 February, 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.

[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