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 September 7^th

Europe Zone

For the 7^th of September, Mercator Ocean International forecasts a strengthening of marine heatwaves in the western Mediterranean basin, with moderate to severe categories. In the eastern Mediterranean basin, marine heatwaves are decreasing in intensity, with moderate to severe categories.
In the Baltic Sea, marine heatwaves are intensifying, with strong and locally severe categories covering coasts from Poland to Estonia. In the Norwegian Sea, the marine heatwave is becoming more widespread, with moderate to severe categories. The marine heatwave in the Bay of Biscay disappeared entirely. 

Global Ocean

Atlantic Ocean

North Atlantic Ocean – MOi forecasts that the marine heatwave in the mid North Atlantic is decreasing both in extent and intensity, though still with moderate to severe categories. Along Canada’s coasts, the marine heatwave decreased until it almost disappeared, moderate categories remain only. 

North Tropical Atlantic Ocean – The marine heatwave in the Gulf of Mexico is easing, with only moderate and strong categories remaining. In the center of the basin, the marine heatwave remains stable overall, with moderate and strong categories still observed. 

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave decreases in extent, we’re still observing mainly 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 to the north of Papua New Guinea decreased in intensity, with only moderate categories observed overall. Off the coast of Mexico, the marine heatwave is increasing in intensity, with mostly strong categories observed. 

North Pacific Ocean – In the North Pacific, the marine heatwave in the center of the basin, towards 180°W, is increasing in intensity, with more severe categories. Between the North American coasts and towards 180°W, the marine heatwave is also increasing in intensity, with moderate and strong categories. The marine heatwave in the Sea of Japan and East China Sea continues to increase in intensity, with moderate and strong categories, locally extreme. 

South-East Asian Seas – Marine heatwaves in the Southeast Asian seas continue to decrease in intensity and have almost disappeared, with only a few moderate marine heatwaves left.

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 and extent of the marine heatwave continues to decrease, 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 an episode during which the ocean temperature is abnormally warm for at least 5 consecutive days.

Figure adapted from Hobday et al. (2018)

For any location in the ocean, the normal temperatures are defined for every day of the year using a climatological period (here 1993–2016). A heatwave is identified when the measured daily temperature is within the top 10% of the highest recorded values for that day  (i.e., above the 90th percentile, see diagram), and with this condition persisting for at least five consecutive days.

The intensity of the heatwave on any given day is measured as the number of degrees above the climatological average (represented by the bold black line) indicated by the blue arrow. We can either calculate the cumulative intensity over the entire heatwave or record the maximum intensity.

Heatwaves are classified based on their intensity level. To do this, the intensity is compared to the difference between the climatological value and the 90th percentile value. A mhw intensity between 1 and 2 times this difference corresponds to a heatwave of moderate category; between 2 and 3 times, to a strong category; between 3 and 4 times, to a severe category; and a difference greater than 4 times corresponds to an 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