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 17th

Europe Zone

For the 17^th of august, Mercator Ocean International (MOi) forecasts an increase in the intensity of marine heatwaves in the western Mediterranean basin, with strong category. In the eastern Mediterranean basin, the marine heatwave intensity decreases with mostly moderate category. In the Norwegian Sea, marine heatwaves decrease in area and increases in intensity with strong and severe categories. In the Bay of Biscay the marine heatwave increases in intensity with strong and severe categories and locally extreme.  

Global Ocean

Atlantic Ocean

North Atlantic Ocean – MOi forecasts that marine heatwave in the mid North Atlantic decreases in intensity, with strong to moderate categories. 

North Tropical Atlantic Ocean – In the Tropical North Atlantic, the marine heatwave remains stable. 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 decreases in extent, with moderate categories the Brazilian coast. 

Southern Ocean

The marine heatwave 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, the marine heatwave decreases in extent the western part, with moderate and strong categories which remain. On the eastern side, almost all of the marine heatwave vanishes.  

North Pacific Ocean – In the North Pacific, between the coast of Japan and around 180°W, the marine heatwave is decreasing in extent. The marine heat wave in the Japan Sea and China Sea decreases in extent with mostly strong category. Between the coasts of Canada and towards 180°W, the marine heatwave remains stable. 

South-East Asian Seas – The marine heatwaves in the South-East Asian seas are stable overall, but strong 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. In the Bay of Bengal a marine heat wave grows with moderate and strong categories.  

Weekly temperature anomalies

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