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 November 28

Marine heatwave categories

• Atlantic – a marine heatwave is persisting to the south-west of the Iberian peninsula at moderate to locally high levels.
• Mediterranean – the whole basin is no longer in a marine heatwave condition, apart from a few residual signs near the coasts.
• North Tropical Atlantic – the marine heatwave that has been present from the Iberian Peninsula to the Caribbean archipelago for several months remains stable over most of its length. In the Caribbean, the heatwave is continuing at a high level.
• South Tropical Atlantic – the marine heatwave in the southern tropical Atlantic covering a zonal strip from South Africa to Brazil is worsening, with a larger surface area in strong categories. However, there has been a local reduction in the extreme categories off Angola and Namibia.
• Southern Tropical Atlantic – the marine heatwave remains stable, reaching severe categories.
• Tropical Pacific – the marine heatwave linked to the formation of El Niño remains stable at generally moderate to strong intensity levels.
• Indian Ocean – the marine heatwave in the western Indian Ocean is diminishing in intensity, with a large part of it shifting from strong to moderate, particularly off Madagascar.
• Southeast Asian seas – the marine heatwave which has been present for more than a month over Malaysia has eased in intensity, lowering from strong to moderate intensity.

Weekly temperature anomalies

• Atlantic Europe – 1.5 to 2°C
• Mediterranean Sea – 3°C
• North Tropical Atlantic – 1 to 1.5°C
• South Tropical Atlantic – 2.5°C
• Tropical Pacific – 1.5 to 3°C
• Indien Ocean (Madagascar to Australia) – 1 à 2°C
• Indien Ocean (Southeast Asian seas ) – 0.5 à 1.5°C

Forecasts for December 5

Europe zone

• For December 5th, the Mercator Ocean International (MOi) forecasting system predicts a continuation of the marine heatwave off the Iberian Peninsula and the Moroccan coast, with moderate to severe categories.
• The marine heatwave in the western Mediterranean continues to diminish, remaining locally and at a moderate level.

Global Ocean

• The marine heatwave in the North Tropical Atlantic will remain stable with moderate to strong categories. In the Caribbean, the situation will equally remain stable with moderate to strong categories.
• In the Southern Tropical Atlantic, the marine heatwave remains stable with strong to locally severe levels.
• In the Tropical Pacific, the situation remains stable with mostly moderate and strong categories.
• The marine heatwave in the western Indian Ocean remains stable in intensity, with strong to moderate levels.
• In the southern Indian Ocean, off the west coast of Australia at around 30°S, a strong category of marine heatwave is developing.

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