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 24/12/2024.

Forecasts for 4 January

Southern Ocean

The marine heatwave in this region increases in intensity, between 0° 90°E, with strong and severe categories and locally extreme.  

Tropical Pacific Ocean

The marine heatwave in the Tropical Pacific remains stable.  

South Pacific

Off the coast of New Zealand at 150°W, the extent and intensity of the marine heatwave increases with strong to severe categories and locally extreme. A small extent of marine heatwave of moderate and strong categories appears to 30°S. 

European Zone

Mediterranean Sea

Marine heatwaves remain stable.

Tropical Atlantic Ocean

Around the equator, the extent of the marine heatwaves decreases. In the east the marine heatwave intensity decreases in the Gulf of Guinea, with moderate to strong categories.

Weekly Temperature Anomalies

29/12/2024 – 04/01/2025

Access the Daily Global Physical Bulletin for a 9-day forecast here.

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