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 October 5th
Europe Zone
For the 5th of October, Mercator Ocean International forecasts few marine heat waves of moderate categories in the Mediterranean Sea, only moderate categories are present in some parts of the eastern basin. Int the Baltic Sea, a marine heat wave of moderate and strong categories is observed.
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, a marine heat wave with moderate and strong, locally severe categories are still present in the middle of the basin with similar extend. In the Gulf of Mexico, a marine heat wave of mainly moderate categories is observed.
South Tropical Atlantic Ocean – Dans l’Atlantique Tropical Sud, la canicule marine déjà présente gagne en intensité avec davantage de catégories fortes.
Southern Ocean
The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) remains stable, moderate and strong categories are observed.
Pacific Ocean
Tropical Pacific Ocean – In the Tropical Pacific, a marine heatwave mostly in moderate categories is still present in the west, between 150°E and 180°E.
North Pacific Ocean – In the North Pacific, the intensity of the marine heatwave at the center of the basin at around 180°W is in strong and severe categories. Off the North American coast and around 150°W, the marine heatwave is stable with moderate and strong categories. In the Sea of Japan, in the China sea and in the Yellow Sea the marine heat waves are mostly of moderate categories.
South-East Asian Seas – In the Southeast Asian seas, the marine heat waves are mostly of moderate categories, locally strong.
South Pacific, to the east of New-Zealand – In the South Pacific, to the East of New-Zealand, the marine heatwave stays stable.
Indian Ocean
In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave stays stable with moderate and strong categories. West of Sumatra, a marine heat wave of strong and locally severe categories develops.
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 1.5°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 0.5°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