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 January 9

Marine heatwave categories

• North Atlantic (Europe zone) – marine heatwaves are continuing along the entire North Atlantic seafront, with moderate to high intensity locally.
• Mediterranean – The situation remains stable throughout the basin. The extent of the marine heatwave is stable, with moderate to strong and locally extreme levels close to the coast, mainly in the eastern Mediterranean.
• North tropical Atlantic – the marine heatwave remains stable with moderate to strong categories. In the Caribbean, the heatwave remains strong.
• South Equatorial and South Tropical Atlantic – the marine heatwave is still present with mainly strong categories.
• Tropical Pacific – the situation remains stable with mainly moderate categories and locally strong in the west due to el Niño.
• Indian Ocean (west) – the marine heatwave remains stable with strong intensities to the north of Madagascar.
• South East Asian Seas – the marine heatwave remains stable with moderate categories.

Weekly temperature anomalies

• North Atlantic (Europe zone) – 1.5 to 2.5°C
• Mediterranean – 1.5 to 3°C
• North Tropical Atlantic – 1° to 1.5° C
• South Tropical Atlantic – 1 to 1.5° C
• Tropical Pacific– 1.5 to 2.5°C and 3°C (to the East of the basin)
• Indian Ocean (western) – 2 to 3°C (to the North of Madagascar)
• South East Asian Seas – 1 to 2°C

Forecasts for January 16

Europe zone

• For January 16, the Mercator Ocean International (MOi) forecasting system predicts that marine heatwaves will remain present along the entire North Atlantic coast of France, Spain and Portugal, with generally moderate categories which could reach varied levels locally.
• The situation is slightly easing up over the eastern Mediterranean basin. The extent of the marine heatwave is stable, with categories ranging from moderate to severe.

Global Ocean

• For January 16, MOi forecasts that the marine heatwave in the North Tropical Atlantic will remain with moderate to severe categories. In the South Equatorial and South Tropical Atlantic, the marine heatwave is still present despite the drop in intensity.
• In the Tropical Pacific, the situation is still present with mostly moderate categories and locally strong to severe in the west. The situation is intensifying to the east of the basin, with locally strong categories.
• The marine heatwave in the Indian Ocean is still present, with a decrease in intensity to the north of Madagascar and increase in the middle of the Indian Basin towards 30°S.
• In the northern part of the South East Asian seas, the marine heatwave remains stable with moderate to strong categories.

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