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 12th

Europe Zone

For the 12^th of October, Mercator Ocean International forecasts few marine heat waves of moderate categories in the Mediterranean Sea, only moderate categories are present in some parts of the eastern basin. Int the Baltic Sea, a marine heat wave of moderate and strong categories is forecasted. 

Global Ocean

Atlantic Ocean

North Atlantic Ocean – MOI forecasts that the marine heatwave extent in the mid North Atlantic is decreasing. The area is in moderate categories overall, with local presence of strong categories. 

North Tropical Atlantic Ocean – In the tropical North Atlantic, the marine heat wave is still present in the middle of the basin and increases in intensity with strong and severe categories. In the Gulf of Mexico, the marine heat wave decreases in extent with moderate and strong categories. 

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave already presents reinforces with more strong categories. Along the African coast, the marine heat wave increases en extent with moderate and strong categories.  

Southern Ocean

The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) remains stable, moderate and strong categories are observed. 

Pacific Ocean

Tropical Pacific Ocean – The marine heatwave present in the west, between 150°E and 180°E increases in intensity with strong category.  

North Pacific Ocean – In the North Pacific, the intensity of the marine heatwave at the center of the basin at around 180°W increases with strong and locally severe categories. Off the North American coast and around 150°W, the marine heatwave increases in intensity with moderate and strong categories. In the Sea of Japan, in the China sea and in the Yellow Sea the marine heat waves decreases in intensity with mostly moderate and locally strong categories. 

South-East Asian Seas – In the Southeast Asian seas, the marine heat waves decreases in spatial extent with moderate and locally strong categories. 

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

Indian Ocean

In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave decreases in extent with moderate and strong categories. West of Sumatra, the marine heat wave increases with 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 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