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 forecasts for a 7 day period.[1]
Assessment for April 16th
Marine heatwave categories
Zone Europe
Mediterranean Sea
Almost the entire western basin is affected by a marine heatwave of moderate to severe intensity. In the eastern basin, the marine heatwave intensity is increasing with mostly strong and severe categories, as well as extreme categories locally.
Global Ocean
Atlantic Ocean
North Atlantic Ocean – On the North African Atlantic coast, from the Moroccan coast to the Strait of Gibraltar, the marine heatwave extent and intensity is increasing, with categories reaching moderate to strong and locally severe.
North Tropical Atlantic Ocean – In the North Tropical Atlantic and Caribbean Sea the marine heatwave is stable.
South Tropical Atlantic Ocean – In the North Tropical Atlantic and Caribbean Sea the marine heatwave intensity is decreasing, with the majority of the area exposed to events of moderate and strong categories. Except near the African coast, where the intensity of the marine heatwave is increasing.
Southern Ocean
The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) increases in intensity to become predominantly strong and locally severe.
Pacific Ocean
Tropical Pacific Ocean – The marine heatwave due to El Niño condition decreases slightly in the center of the basin. To the west of the basin, south of Papua New Guinea, the marine heatwave decreases in intensity to a moderate category. North of Papua New Guinea, the marine heatwave increases in extent but remains in moderate and strong categories.
South Pacific, east of New Zealand – The marine heatwave remains stable overall.
Indian Ocean
In the Arabian Sea, the marine heatwave remained stable overall. In the Bay of Bengal, the marine heatwave is increasing in intensity, with strong categories covering the entire northern part of the Bay.
Weekly temperature anomalies
Mediterranean Sea – 0.5°C and 2.5°C.
North Atlantic Ocean– 0.5°C and 2°C.
North Tropical Atlantic Ocean – 0.5°C and 1.5°C.
South Tropical Atlantic Ocean – 1°C and 2°C.
Southern Ocean – 1°C and 3°C.
Tropical Pacific Ocean – 0.5°C and 1°C.
South-East Asian Seas – 0.5°C and 1°C.
South Pacific Ocean – 2°C and 3°C.
Indian Ocean – 0.5°C and 2°C.
Forecast for April 23th
Zone Europe
Mediterranean Sea
For the 23rd of April, Mercator Ocean International forecasts the complete disappearance of the marine heatwave in the western basin. In the eastern basin, the marine heatwave will decrease significantly in both intensity and extent. Categories change from strong and severe to moderate with the presence of strong categories locally.
Global Ocean
Atlantic Ocean
North Atlantic Ocean – MOI forecasts that the marine heatwave on the North African Atlantic coast off Morocco will decrease slightly in intensity but increase in extent.
North Tropical Atlantic Ocean – MOI forecasts that the marine heatwave in the North Tropical Atlantic and Caribbean Sea will remain stable. At the equator, the extent of the strong categories increases, but the extreme categories disappear.
South Tropical Atlantic Ocean – The marine heatwave will remain stable.
Southern Ocean
The marine heatwave in the Southern Ocean, off South Africa (between 30°W and 30°E) remains stable.
Océan Pacifique
Tropical Pacific Ocean – In the tropical Pacific Ocean, the marine heatwave due to El Niño condition increases in intensity west of the basin until Papua New Guinea.
South Pacific, east of New Zealand – The marine heatwave will remain stable.
South-East Asian Seas
In the South-East Asian seas, the marine heatwave is increasing in intensity, with most of the area in severe category.
Indian Ocean
In the Indian Ocean, the marine heatwave in the Arabian Sea remains stable. In the Bay of Bengal, the marine heatwave is decreasing in intensity.
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