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 November 2

Europe Zone

Mercator Ocean International (MOi) forecasts are made in comparison with the marine heatwave situation on the 22th October. 

For the 2^nd of November, Mercator Ocean International forecasts a slight intensification of the situation in the Mediterranean Sea, with the development of areas of moderate category marine heatwave in the western basin, as well as the local development of strong categories.  A moderate category marine heatwave is developing in the North Sea and the English Channel. In the Baltic Sea, the marine heatwave of generally moderate category remains stable.

Global Ocean

Mercator Ocean International (MOI) forecasts are made in comparison with the marine heatwave situation on the 22th October. 

Atlantic Ocean

North Atlantic Ocean – MOi forecasts the decrease of the extent of the marine heatwaves for all intensities.  

North Tropical Atlantic Ocean – In the North Tropical Atlantic, the marine heatwave is decreasing in extent and intensity in the west, with fewer areas in severe and strong categories. Near the equator and near the coast of Africa, the intensity of the marine heatwave is increasing from moderate to strong and severe categories. 

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, marine heatwaves along the coasts of Brazil and Argentina, as well as along the coast of Africa are stable. 

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, the situation remains broadly stable in the west, with moderate and locally strong category conditions, while in the east the moderate category marine heatwave near Central America has disappeared. 

North Pacific Ocean – In the North Pacific, the marine heatwave in the center of the basin, around 180°W, remains stable in intensity with moderate and strong categories, but its extent is decreasing slightly.  The intensity and extent of the moderate and strong categories marine heatwave extending from northern Papua New Guinea to the Californian coast are also decreasing, with less strong categories. The intensity of the marine heatwave in the Sea of Japan, the East China Sea and the Yellow Sea is increasing slightly. The categories observed are mainly moderate, and locally strong and severe. 

South-East Asian Seas – In the Southeast Asian seas, the marine heat wave decreases in intensity with mostly moderate, locally strong categories. Its extent declined strongly, as the South China Sea is now free from marine heatwave. 

South Pacific– In the South Pacific, south of the basin at around 150°W, the marine heat wave area diminishes and is now practically absent.

Indian Ocean

In the Indian Ocean, MOi forecasts that the extent of the marine heatwave will remain stable overall. Intensity is increasing in the Arabian Sea, with a development of strong categories, as well as near Indonesia, while in the Bay of Bengal intensity is decreasing and going from strong to mainly moderate categories. 

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