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 July 27th

Europe Zone

For the 27^th of July, Mercator Ocean International (MOi) forecasts a decrease in spatial extent of the marine heatwave in the Mediterranean Sea with moderate and strong categories, locally severe. In the Norway Sea the marine heatwave become stronger in intensity with strong and severe category, locally extreme.

Global Ocean

Atlantic Ocean

North Atlantic Ocean – MOi forecasts that the surface of the marine heatwave in the mid North Atlantic increases in intensity with the development of strong and locally severe categories. 

North Tropical Atlantic Ocean – In the Tropical North Atlantic, the marine heatwave increases in spatial extent with strong categories gain in extent. In the Gulf of Mexico, the marine heatwave decreases in intensity and spatial extent.

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave stays stable. 

Southern Ocean

The marine heatwave in the Southern Ocean, off the South African coast (between 30°W and 30°E) decreases in spatial extent.

Pacific Ocean 

Tropical Pacific Ocean – In the Tropical Pacific, the marine heatwave decreases in spatial extent in the Western part, moderate and strong categories remain. On the Eastern side, the marine heat waves intensities and is now mostly of strong categories, locally severe and extreme. 

North Pacific Ocean – In the North Pacific, between the coast of Japan and around 180°W, the marine heatwave decreases in spatial extent with mainly moderate and strong categories. Between the coasts of Canada and around 170°W, the marine heatwave almost disappears.  

South-East Asian Seas – The marine heatwave present in the seas of Southeast Asia disappears. 

South Pacific, to the east of New-Zealand – The marine heatwave decreases in spatial extent.  

Indian Ocean

In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave decreases in spatial extent, moderate and locally strong categories remain mainly in 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