Mercator Ocean Bulletin: Marine heatwave forecasts – May 21st 2024

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 May 21st

Marine heatwave category map
Figure 1: Map of marine heatwave categories for May 21st 2024 forecast for the world ocean as a whole. GLO12 analysis. Source : Mercator Ocean International 

Europe Zone

North Sea

For the 21st of May, Mercator Ocean International forecasts that a marine heatwave of categories strong and severe will appear in the eastern part of the basin. 

Global Ocean

Atlantic Ocean

North Atlantic – MOi forecasts that the marine heatwave in the North Atlantic continues to decrease, with a complete vanishing offshore the Moroccan coast. In the central part, the marine heatwave decreases in intensity, and is mostly of moderate category. In the western part, it reinforces with a development of strong categories.

Tropical North Atlantic – In the Tropical North Atlantic the marine heatwave north of the equator remains stable with mainly strong categories to locally severe.
Gulf of Mexico – The marine heatwave in the Gulf of Mexico intensifies with mostly strong categories; locally categories higher than extreme are forecasted. 

South Tropical Atlantic – In the South Tropical Atlantic and at the equator, the marine heatwave remains stable, with categories ranging from moderate to severe.

Southern Ocean

The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) is stable with categories mostly strong and locally extreme.

Pacific Ocean 

Tropical Pacific – In the Tropical Pacific the marine heat wave remains stable with mostly moderate and strong categories.

North Pacific – In the North Pacific, the marine heatwave located around 45°N and 170°W increases in intensity with strong categories and even locally severe. 

South-East Asian Seas – The marine heatwave in the South China Sea remains stable, with mostly moderate and strong categories.

South Pacific, to the east of New-Zealand – The marine heatwave remains stable.  

Indian Ocean

Arabian Sea – The marine heatwave decreases, with most areas in moderate categories, strong categories remain locally.

Bay of Bengal – The marine heatwave stays stable with a large area of strong category.

South of the Indian Ocean, between 30°S and 60°S – The predominantly moderate to severe marine heatwave remains stable.

Weekly temperature anomalies

Weekly temperature anomaly maps for marine heatwaves
Figure 2 : Weekly surface temperature anomaly map for the week of May 14 to May 21st 2024. GLO12 forecast. Source : Mercator Ocean International
North Sea
1.5°C to 3°C
Atlantic OceanNorth
0.5°C to 1.5°C
North Tropical
0.5°C to 2.5°C
South Tropical
0.5°C to 1°C
Southern Ocean
0.5°C to 3°C
Pacific OceanNorth
2°C to 3°C
0.5°C to 1°C
1°C to 3°C
South-East Asian Seas
0.5°C to 1.5°C
Indian Ocean
0.5°C to 2.5°C

Consult our Daily Global Physical Bulletin for a 9-day forecast ici.

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