Mercator Ocean Bulletin: Marine heatwave forecasts – June 11th 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 11th

Marine heatwave category map — *Figure 1: Map of marine heatwave categories for June 11th 2024 forecast for the world ocean as a whole. GLO12 analysis. Source : Mercator Ocean International*

Europe Zone

North Sea

For the 11th of June, Mercator Ocean International (MOi) forecasts that the marine heatwave in the North Sea between Great Britain and Norway will diminish until it disappears altogether.

Mediterranean Sea

MOi forecasts that the marine heatwave in the eastern basin of the Mediterranean Sea will intensify, with moderate to strong categories. In the western basin, a moderate category of marine heatwave is forecast locally.

Global Ocean

Atlantic Ocean

North Atlantic – MOi forecasts that the surface of the marine heatwave in the mid North Atlantic will increase, but the intensity of the marine heatwave will remain stable, with moderate and locally strong categories.

North Tropical Atlantic – In the Tropical North Atlantic, the surface and intensity of the marine heatwave diminished off the Spanish and Moroccan coasts. In the central North Atlantic and Caribbean Sea, the marine heatwave remains stable, with moderate to locally extreme categories. In the Gulf of Mexico, the marine heatwave is increasing in intensity, with categories going from strong to extreme.

South Tropical Atlantic – In the Southern Tropical Atlantic, the marine heatwave remains stable, with moderate and strong categories.

Southern Ocean

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

Pacific Ocean

Tropical Pacific – In the Tropical Pacific, the marine heatwave remained stable, except off the coast of Mexico, where it diminished in intensity, becoming mostly moderate.

North Pacific – In the North Pacific, the marine heatwave around 45°N and 170°W is decreasing in intensity, with moderate and strong categories still present. But the surface of the marine heatwave in the strong category is decreasing.

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

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

Indian Ocean

In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave between the north of Madagascar and the island of Sumatra will increase in intensity, with more surface in the strong category.

Weekly temperature anomalies

Weekly temperature anomaly maps for marine heatwaves — *Figure 2 : Weekly surface temperature anomaly map for the week of June 4th to June 11th 2024. GLO12 forecast. Source : Mercator Ocean International*

North Sea	0.5°C to 1°C
Atlantic Ocean	North 0°C to 0.5°C	North Tropical 1°C to 3°C	South Tropical 0.5°C to 2°C
Southern Ocean	0°C to 2°C
Pacific Ocean	North 2°C to 3°C	Tropical 0.5°C to 1.5°C	South 1°C to 2°C	South-East Asian Seas 0.5°C to 1°C
Indian Ocean	0.5°C to 2.5°C

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 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 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf