Mercator Ocean International (MOi) oceanographers examine marine heatwaves across the global ocean throughout the year. They analyse a variety of datasets from observations (satellite sea surface temperature maps) to numerical model analyses (assimilating satellite and in situ observations) and derive marine heatwave forecasts for a 10 day period.[1]

Forecasts for November 9

Europe Zone

Mercator Ocean International (MOi) forecasts are made in comparison with the marine heatwave situation on the 29 October.

For the 9 November, Mercator Ocean International forecasts a marine heatwave onset in the Bay of Biscay with moderate category. In the Baltic Sea, the marine heatwave decreases in intensity and extent, with moderate and strong categories locally. In the Mediterranean Sea (western basin), the marine heatwave increases with moderate and strong category locally. In the Eastern basin, the marine heatwave decreases in spatial extent and intensity into a moderate category.  

Global Ocean

The Mercator Ocean International (MOi) forecasts are in comparison with the marine heatwave situation on the 29 October 2024.

Atlantic Ocean

North Atlantic Ocean – MOi forecasts the onset of a marine heatwave from Iceland to the Marocan coast with moderate category.   

North Tropical Atlantic Ocean – In the North Tropical Atlantic, the marine heatwave is decreasing in extent and intensity with moderate to strong categories.

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave along the coast of Brazil will decrease in extent, while along the coasts of Uruguay and Argentina the extent will increase. The marine heatwave along the coast of Africa will increase in intensity, developing into moderate and strong categories.

Southern Ocean

The marine heatwave in the Southern Ocean, off the South African coast (between 5°W and 90°E) will decrease in spatial extent and intensity, with moderate and strong categories. 

Pacific Ocean

Tropical Pacific Ocean – In the Tropical Pacific, in the North of Papua New Guinea, the marine heatwave increases with moderate and strong categories. 

North Pacific Ocean – In the North Pacific, the marine heatwave in the center of the basin, around 180°W, decreases in intensity and spatial extent with moderate and strong categories. The extent of the moderate and strong categories marine heatwave extending from northern Papua New Guinea to the Californian coast are also decreasing, with mostly moderate category. The spatial extent of the marine heatwave in the Sea of Japan, the East China Sea and the Yellow Sea will decrease. The categories observed are mainly moderate, and locally strong and severe. 

South-East Asian Seas – In the Southeast Asian seas, the marine heatwave remains stable with mostly moderate, locally strong categories. 

South Pacific– In the South Pacific, south of the basin at around 150°W, a marine heatwave can be found with strong category.

Indian Ocean

For the Indian Ocean, MOi forecasts a decrease in the the marine heatwave extent. The spatial extent increases in the Arabian Sea, with strong categories. In the Bay of Bengal intensity decreases and going from strong to mainly moderate categories. In the South of Madagascar, around 50°S a marine heatwave comes out with strong and severe categories. 

Weekly temperature anomalies

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.

Figure 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