Mercator Ocean International (MOi) oceanographers examine marine heatwaves across the global ocean. They analyse a variety of datasets from observation analyses (satellite sea surface temperature maps) to model analyses (assimilating satellite and in situ observations) and model forecasts.^¹

Assessment for December 5

Marine heatwave categories

• Atlantic – the marine heatwave persists to the south-west of the Iberian peninsula and the Moroccan coast. It continues at moderate to locally high levels.
• Mediterranean – the situation remains stable, with mainly moderate to strong marine heatwaves in the eastern part.
• North Tropical Atlantic – the marine heatwave that has been present from the Iberian Peninsula to the Caribbean archipelago for several months is remaining stable. In the Caribbean, the heatwave is continuing at a high level. The marine heatwave in the South Atlantic, covering a zonal strip from South Africa to Brazil, is intensifying, with an increase in the surface area in the strong category.
• South Atlantic – the marine heatwave in the southern South Atlantic remains stable.
• Tropical Pacific – the marine heatwave linked to the formation of El Niño remains stable at overall moderate levels, and locally strong in the east.
• Indian Ocean – the marine heatwave in the western Indian Ocean remains stable. The section of marine heatwaves at 30° south latitude remains stable but is decreasing in size.
• Southeast Asian seas – the marine heatwave that has been present in the South-East Asian seas for over a month, around Malaysia, remains stable.

Weekly temperature anomalies

• Atlantic Europe’s seafront– 1.5°C to 2°C
• Mediterranean – 2°C
• North Tropical Atlantic – 1°C to 1.5° C
• South Tropical Atlantic – 2.5°C (South Africa and Brazil)
• South Atlantic – 1.5°C to 2°C
• Tropical Pacific – 1.5°C to 3°C
• Indian Ocean (west) – 1°C to 2°C (between Madagascar and Australia: 1.5°C and 2.5°C)
• Indian Ocean (South East Asian Seas) – 0.5°C to 1°C

Forecasts for December 12

Europe zone

• For December 12, the Mercator Ocean International (MOi) forecasting system is predicting a slight intensification of the marine heatwave off the Iberian peninsula and the Moroccan coast, with an increase in the surface area of the severe category of marine heatwave.
• The situation remains stable throughout the Mediterranean basin.

Global Ocean

• For December 12, MOi is predicts an increase in the intensity of the marine heatwave in the North Tropical Atlantic, with a transition from strong to severe category off the coast of Africa. In the Caribbean, the situation remains stable with moderate to strong categories.
• In the South Equatorial and South Tropical Atlantic, the marine heatwave is still present, but the extent of the strong intensities is diminishing.
• In the Tropical Pacific, the situation remains stable with mostly moderate categories and locally strong in the east.
• The marine heatwave in the Indian Ocean is intensifying to the south of India, rising from moderate to strong.
• In the northern part of the South-East Asian seas, the moderate category of marine heatwave is increasing.

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.

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.

¹Analysis of datasets: OSTIA sea surface temperature observations analysis (Copernicus Marine Service), OISST sea surface temperature observations analysis (NOAA), GLO12 model (Copernicus Marine Service, Mercator Ocean International)

² IPCC AR6 SYR 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf