Mercator Ocean International (MOi) oceanographers examine marine heatwaves across the global ocean. 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 7 day period.[1]
Forecasts for September 28th
Europe Zone
For the 28th of September, Mercator Ocean International forecasts a slight decrease of the intensity and the extent of the marine heatwave in the Mediterranean Sea. Only mainly moderate categories are present in the eastern basin, with several areas in strong categories along the coasts of Egypt and Libya. In the western basin, the marine heatwave north of the Balearic Sea goes from strong to moderate categories. The marine heatwave in the Baltic Sea is stable. In the Norwegian Sea, the extent of the marine heatwave decreases, as well as its intensity, with a large area of strong to severe categories now mainly in moderate categories.
Global Ocean
Atlantic Ocean
North Atlantic Ocean – MOI forecasts that the marine heatwave extent in the mid North Atlantic is decreasing. The area is in moderate categories overall, with local presence of strong categories.
North Tropical Atlantic Ocean – In the tropical North Atlantic, the marine heatwave is decreasing in area but increasing in intensity. Areas in strong categories are concentrated in the center of the basin as well as in the Caribbean Sea and around the Antilles archipelago. Locally, severe to extreme categories are present in the center of the basin as well as around Haiti, the Dominican Republic and Puerto Rico.
South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the extent of the marine heatwave in moderate categories decreases.
Southern Ocean
The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) remains stable.
Pacific Ocean
Tropical Pacific Ocean – In the Tropical Pacific, a marine heatwave in moderate to strong categories is developing in the west, between 150°E and 180°E. Off the coast of Mexico, the marine heatwave disappears.
North Pacific Ocean – In the North Pacific, the intensity of the marine heatwave in the center of the basin at around 180°W is decreasing, with less area in strong and severe categories. Off the North American coast and around 150°W, the marine heatwave is increasing in intensity, with moderate categories now in strong and locally severe categories. The marine heatwave in the Sea of Japan and the East China Sea is still present, with strong and severe categories, and locally extreme categories in the Yellow Sea.
South-East Asian Seas – Marine heatwaves of mainly moderate and locally strong intensity are developing in the Southeast Asian seas.
South Pacific, to the east of New-Zealand – The marine heatwave is increasing in intensity, with more severe categories and, locally, more severe and extreme categories.
Indian Ocean
In the Indian Ocean, Mercator Ocean forecasts that the intensity and extent of the marine heatwave continues to decrease. There are mostly moderate categories and a few strong categories in the center of the basin, as well as north west of Australia.
Weekly temperature anomalies
| Atlantic Ocean | North 1.5°C to 3°C | North Tropical 0°C to 1.5°C | South Tropical 0°C to 2°C | |
| Southern Ocean | 0°C to 2°C | |||
| Pacific Ocean | North 1°C to 3°C | Tropical 0°C to 1.5°C | South 0.5°C to 1.5°C | South-East Asian Seas 0°C to 1.5°C |
| Indian Ocean | 0.5°C to 2°C |
Consult our 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