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 October 31

Marine heatwave categories

• For the Atlantic Ocean in Europe, the marine heatwave that has been raging along the Atlantic coast, from the Iberian Peninsula to the English Channel, continues to diminish, no longer affecting the Bay of Biscay and falling to moderate levels over almost all the regions affected.
• In the Mediterranean, the marine heatwave is present over almost the entire basin, with moderate to locally strong levels around Corsica.
• In the tropical North Atlantic, the geographical extent of the marine heatwave from the Iberian peninsula to the equator remained stable.
• The marine heatwave in the Caribbean Sea is continuing at high levels of intensity, ranging from severe to extreme.
• In the Tropical Pacific, the marine heatwave linked to the formation of El Niño remains stable, with moderate to strong levels of intensity overall.
• The marine heatwave in the North Pacific remains at moderate to locally strong levels.
• In the south-west Indian Ocean, to the east of Madagascar, the marine heatwave continued at a high category. Further north, the entire equatorial strip is affected by a moderate to locally severe marine heatwave.
• A marine heatwave is developing in Indonesian waters and in the south of the South China Sea, with strong to locally extreme levels.

Weekly temperature anomalies

• Atlantic Ocean (Europe zone) – 2.5 °C
• Mediterranean – 2°C to 3°C
• North Tropical Atlantic – 1°C to 2°C
• Tropical Pacific – 1.5°C to 3°C
• North Pacific – 1°C to 2°C
• Indien Ocean (to the East of Madagascar) – 2 °C to 3 °C
• Indian Ocean (to the North of Madagascar) – 1°C to 2°C

Forecasts for November 7

Europe zone

• For November 7, the Mercator Ocean International (MOi) forecasting system predicts that the situation on the Atlantic coast and around the Iberian Peninsula will be largely free of marine heatwaves, with only a few areas still in the moderate category.
• The marine heatwave in the Mediterranean is easing, affecting only the eastern half of the basin, which is now in the moderate category overall. Temperatures will remain high in this region, with weekly anomalies of over 3°C locally.

Global Ocean

• For November 7, MOi forecasts that the marine heatwave in the Tropical Atlantic will decrease slightly in intensity, with moderate to severe categories. The marine heatwave in the Caribbean remains at a lower level, going from generally severe to extreme to generally strong to severe.
• In the Tropical Pacific, the situation remains stable.
• The marine heatwave in the North Pacific remains stable, generally in the moderate to locally severe category.
• The marine heatwave in the south-west Indian Ocean remains stable. In the northern part, the marine heatwave seems to be intensifying, reaching a strong level over a large part of the area.
• In Indonesian waters, the marine heatwave remains strong to extreme.

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