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 June 25th
Europe Zone
North Sea
For the 25th of June, Mercator Ocean International forecasts the marine heatwave development in the North Sea with moderate and strong category.
Mediterranean Sea
On the Eastern part of Mediterranean Sea, the marine heatwave increases in intensity withsevere and extreme categories.
Global Ocean
Atlantic Ocean
North Atlantic – MOi forecasts that the surface of the marine heatwave in the mid North Atlantic remains stable in extend, but decreases in intensity with moderate category.
North Tropical Atlantic – In the Tropical North Atlantic, the intensity of the marine heatwave decreases to strong and moderate categories. In the Gulf of Mexico, the marine heatwave disappears.
South Tropical Atlantic – In the Southern Tropical Atlantic, the marine heatwave dissipates.
Southern Ocean
The extend of the marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) stays stable.
Pacific Ocean
Tropical Pacific – In the Tropical Pacific, the marine heatwave remains stable.
North Pacific – In the North Pacific, the marine heatwave around 45°N and 170°W increases in surface area with moderate category and locally strong.
South-East Asian Seas – The marine heatwave in the Southeast Asian seas decreases in intensity, with moderate and strong categories.
South Pacific, to the east of New-Zealand– The marine heatwave remains stable.
Indian Ocean
In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave decreases in surface area with moderate categories.
Weekly temperature anomalies
| Atlantic Ocean | North 1.5°C to 3°C | North Tropical 0.5°C to 1.5°C | South Tropical 0°C to 1°C | |
| Southern Ocean | 0°C to 2°C | |||
| Pacific Ocean | North 1°C to 3°C | Tropical 0.5°C to 1.5°C | South 1.5°C to 2°C | South-East Asian Seas 0.5°C to 1°C |
| Indian Ocean | 0.5°C to 2.5°C |
Consult our Daily Global Physical Bulletin for a 9-day forecast ici.
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 a heat episode during which the temperature is significantly higher than a certain threshold for at least 5 consecutive days.
Figure adapted from Hobday et al. (2018)
The seasonally-varying threshold is defined on a daily basis according to a sufficiently long climatic period (in this case 1993-2016). So, for a given place and a given day, knowing all the surface temperatures observed over the last 30 years, a heatwave situation is defined as one where the temperature measured is within 10% of the maximum values observed (i.e. above the 90th quantile, see diagram), for at least 5 consecutive days.
The main characteristics of heatwaves are their duration and intensity. The intensity for a given day corresponds to the value in degrees above the 90th quantile (blue arrow), which can either be calculated as the cumulative intensity throughout the heatwave event, or the maximum intensity.
Heatwaves are categorised on the basis of their deviation from the mean temperature or anomaly (green arrow): a deviation of more than 2 times the difference between the 90th quantile and the mean corresponds to a heatwave in the strong category; a deviation of more than 3 times corresponds to a heatwave in the severe category; and a deviation of more than 4 times corresponds to a heatwave in the 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