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 31st

Europe Zone

For the 31^st of august, Mercator Ocean International (MOi) forecasts that marine heatwaves in almost whole the Mediterranean basin stays mostly stable with moderate and strong categories, locally severe. Between Greece and Libya, the marine heat wave reach severe and locally extreme categories. In the Norwegian Sea, marine heatwaves intensifies slightly above 70° N, with more moderate categories. A marine heat wave also develops East of the Strait of Gibraltar, with severe and locally extreme categories. Offshore of the Bay of Biscay, the marine heat wave stays mostly stable in intensity, moderate categories remain. 

Global Ocean

Atlantic Ocean

North Atlantic Ocean – MOi forecasts that the marine heatwave in the mid North Atlantic increases in intensity with the development of strong and locally severe categories. 
Along the Canadian coasts, the marine heat wave around Nova Scotia stays stable with moderate and strong categories. 

North Tropical Atlantic Ocean – In the Gulf of Mexico and in the Caribbean Sea, the marine heatwave intensifies with the development of strong and severe categories, locally extreme. In the center of the basin, the marine heat wave intensifies, with the development of moderate, locally strong, categories. 

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave decreases in intensity, only moderate categories. 

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 has almost disappeared, with the exception of localized sport offshore the Mexican coast.

North Pacific Ocean – In the North Pacific, between the coast of Japan and around 180°W, the marine heatwave stays stable, moderate and locally strong categories are observed. The marine heat wave in the Japan Sea and in the China Sea increases both in extent and intensity, with moderate and strong categories, locally extreme. Between the coasts of Canada and towards 180°W, the marine heatwave stays stable with moderate and strong categories. 

South-East Asian Seas – The marine heatwaves in the South-East Asian decrease in intensity, moderate and strong categories are still present 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, moderate and strong categories are observed at the center of the basin. 

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