Mercator Ocean Bulletin: Marine heatwave forecasts – June 4th 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 June 4th

Marine heatwave category map
Figure 1: Map of marine heatwave categories for June 4th 2024 forecast for the world ocean as a whole. GLO12 analysis. Source : Mercator Ocean International 

Europe Zone

North Sea

For the 4th of June, Mercator Ocean International (MOi) forecasts the North Sea marine heatwave now only covers the northern area, where it decreases in intensity, with moderate to locally strong categories. Moderate categories are developing in the Celtic Sea.  

Mediterranean Sea

MOi forecasts the development of a marine heatwave in moderate to locally strong categories in the eastern basin. 

Global Ocean

Atlantic Ocean

North Atlantic – MOi forecasts that the marine heatwave in the North Atlantic is developing in the east in moderate categories, off the Moroccan coast, while in the west it fades away in the Sargasso Sea. 

Tropical North Atlantic – In the Tropical North Atlantic the marine heatwave remains stable, except in the Caribbean Sea and in the Gulf of Mexico, where it decreases in strength with moderate to strong categories.

South Tropical Atlantic – In the South Tropical Atlantic, the marine heatwave strength slightly decreases, in particular off the African coasts, with a change from strong to moderate categories.   

Southern Ocean

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

Pacific Ocean 

Tropical Pacific – In the Tropical Pacific the marine heat wave remains globally stable with mostly moderate and strong categories. The development of a strong categories marine heat wave is forecasted north of Papua New Guinea. 

North Pacific – In the North Pacific, the marine heatwave located around 45°N and 170°W decreases in intensity, with a change from mainly strong and severe categories to moderate and strong categories. 

South-East Asian Seas – The marine heatwave intensity in the South China Sea keeps decreasing in strength with a slight decline in the strong categories surface in favor of moderate ones. 

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

Indian Ocean

The overall situation in the Indian Ocean is stable. However, Mercator Ocean forecasts moderate to locally strong marine heatwave off the Indian coasts as well as the coast off Bangladesh and Myanmar.

Weekly temperature anomalies

Weekly temperature anomaly maps for marine heat waves
Figure 2 : Weekly surface temperature anomaly map for the week of May 28th to June 4th 2024. GLO12 forecast. Source : Mercator Ocean International
North Sea
0.5°C to 3°C
Atlantic OceanNorth
1°C to 2°C
North Tropical
1°C to 1.5°C
South Tropical
0.5°C to 2.5°C
Southern Ocean
0°C to 2°C
Pacific OceanNorth
2°C to 3°C
0.5°C to 1°C
1°C to 2°C
South-East Asian Seas
1°C to 2°C
Indian Ocean
0.5°C to 1.5°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