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 October 26th

Europe Zone

– For the 26^th October, Mercator Ocean International forecasts only a few spots of marine heat waves in moderate categories in the Mediterranean Sea.  
In the Baltic Sea some moderate and locally severe marine heatwaves are observed.  

Global Ocean

Mercator Ocean International predictions are generated in comparison with the marine heatwaves of 15 October.

Atlantic Ocean

North Atlantic Ocean – MOi forecasts that marine heatwaves will persist with moderate and strong categories. 

North Tropical Atlantic Ocean – In the tropical North Atlantic, the marine heatwave remained stable in extent, but decreases in intensity, with less surface area in extreme and severe categories. In the Gulf of Mexico, the marine heatwave disappears completely. 

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave along the coast of Brazil extends towards the East and reinforces with the development of moderate and severe categories.  
Strong categories develop along the Argentinian coast.  
The marine heatwave along the coast of Africa extends, still with strong and locally severe categories. 

Southern Ocean

The marine heat wave in the Southern Ocean, off the South African coast (between 5°W and 90°E) remains stable, with moderate and strong categories but very locally severe and extreme.  

Pacific Ocean

Tropical Pacific Ocean – In the Tropical Pacific, a marine heatwave of moderate intensity forms off Central America. The marine heatwave to the west of the basin to the north of Papua New Guinea decreases in both extend and intensity with mainly moderate categories.  

North Pacific Ocean – In the North Pacific, the marine heatwave at the center of the basin, around 180°W, remains stable in both extends and intensity with moderate and strong categories.  
A marine heatwave in moderate, locally strong categories, extends from North of  Papua New Guinea towards the Californian coast.  The marine heatwaves in the Sea of Japan, China Sea and Yellow Sea decreases in intensity with mostly moderate, locally strong categories.  

South-East Asian Seas – In the Southeast Asian seas, the marine heat wave decreases in intensity with mostly moderate, locally strong categories.  

South Pacific, to the east of New-Zealand – In the South Pacific, south of the basin at around 150°W, the marine heat wave diminishes in area until it almost disappears.  

Indian Ocean

– In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave remains overall stable with moderate categories spread in the basin. Strong categories are observed in the center and in the East near Indonesia.

Weekly temperature anomalies

Access the 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