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 August 10th
Europe Zone
For the 10th of august, Mercator Ocean International (MOi) forecasts an increase in the intensity of marine heatwaves in the western Mediterranean basin, going from moderate to severe categories, as well as in the western part of the eastern Mediterranean basin, which is now mostly in strong categories. In the eastern part of the eastern Mediterranean basin, the marine heatwave intensity decreases and is now in moderate categories. In the Norwegian Sea, marine heatwaves are intensifying, going from moderate to strong and severe categories, even extreme locally. A moderate category marine heatwave is present in the bay of Biscay.
Global Ocean
Atlantic Ocean
North Atlantic Ocean – MOi forecasts that marine heatwave in the mid North Atlantic decreases in intensity, going from strong to moderate categories.
North Tropical Atlantic Ocean – In the Tropical North Atlantic, the marine heatwave slightly intensifies with the development of strong and locally severe categories. In the Gulf of Mexico and Caribbean Sea, the marine heatwave intensity is strongly increasing, going from moderate to strong, severe and extreme categories.
South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave area is diminishing, with moderate categories and only a few strong categories persisting along the Brazilian coast.
Southern Ocean
The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) continues to decrease in spatial extend.
Pacific Ocean
Tropical Pacific Ocean – In the Tropical Pacific, the marine heatwave decreases in intensity the western part, with moderate and strong categories which remain. On the eastern side, almost all of the marine heat wave vanishes.
North Pacific Ocean – In the North Pacific, between the coast of Japan and around 180°W, the marine heatwave is decreasing in intensity. A marine heatwave in strong category is developing south of Japan and in the East China Sea. Between the coasts of Canada and towards 180°W, the extent of the marine heatwave is increasing, and it is intensifying with strong categories.
South-East Asian Seas – The marine heatwaves in the South-East Asian seas are stable overall, but moderate categories are developing in the South China Sea.
South Pacific, to the east of New-Zealand – The marine heatwave remains stable.
Indian Ocean
In the Indian Ocean, Mercator Ocean forecasts that the intensity of the marine heatwave diminishes in the center of the basin, with fewer strong categories.
Weekly temperature anomalies
| Atlantic Ocean | North 2°C to 3°C | North Tropical 1°C to 2°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°C | South 1°C to 2.5°C | South-East Asian Seas 0.5°C to 1°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 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