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 7

Marine heatwave categories

• In Atlantic Europe, the marine heatwave that swept along the Atlantic seafront from the Iberian Peninsula to the North Sea has essentially disappeared, with only a few areas in moderate categories.
• The marine heatwave in the Mediterranean continues to ease in the western part of the basin, hardly affecting the area west of Corsica. However, in the easternmost part of the basin, the marine heatwave is increasing in intensity from moderate to moderate to severe.

• In the Tropical Atlantic, the geographical extent of the marine heatwave present from the Iberian Peninsula to the equator has remained stable, but the intensity of the marine heatwave is decreasing slightly in the northern part, with moderate to severe categories.
• The marine heatwave in the Caribbean Sea continues to be severe, but is decreasing in intensity, with most categories being strong and locally severe.

• In the Tropical Pacific, the marine heatwave linked to the formation of El Niño remains stable, with overall moderate to strong levels of intensity.
• The marine heatwave in the North Pacific is continuing at moderate to strong and locally severe levels.
• The marine heatwave in the south-west Indian Ocean remains at moderate to strong levels and is now developing from the north of Madagascar to the south of the eastern Indian Ocean. Further north, the whole of the Arabian Sea and the Bay of Bengal are affected by a marine heatwave that is becoming increasingly intense, with moderate to strong levels.
• The marine heatwave in the South East Asian Seas and the South China Sea is increasing in intensity, with mostly strong and locally extreme categories.

Weekly temperature anomalies

• Atlantic Europe – 2°C
• Mediterranean – 3°C
• North Tropical Atlantic – 1.5°C
• Tropical Pacific – 1.5°C to 3°C
• North Pacific – 1°C to 3°C
• Indian Ocean – 1°C Arabian Seas and Bay of Bengal
• Indian Ocean – 2.5°C North to Madagascar
• Seas of Southeast Asia – 1.5 °C

Forecasts for November 14

Europe zone

• For November 14, the Mercator Ocean International (MOi) forecasting system predicts a slight re-intensification of the marine heatwave around the Iberian Peninsula and the Moroccan coast with moderate categories.

• The marine heatwave in the western Mediterranean is easing. The eastern part of the basin remains stable with moderate to strong marine heatwaves.

Global Ocean

• For November 14, MOi forecasts that the marine heatwave in the Tropical Atlantic will remain stable with moderate to strong categories. The marine heatwave in the Caribbean Sea is decreasing in intensity from strong to severe to moderate to strong. In the Southern Tropical Atlantic, a marine heatwave is appearing with moderate to strong levels.

• In the Tropical Pacific, the situation remains stable. In the western tropical Pacific, the marine heatwave is becoming more intense, with locally extreme levels.
• The marine heatwave in the North Pacific is diminishing in extent but remains at moderate to locally strong levels overall.
• The marine heatwave in the south-west Indian Ocean is decreasing in extent in its eastern part. In the western part of the Indian Ocean, the marine heatwave is intensifying, reaching high levels over most of the area.
• In the South-East Asian seas, the marine heatwave remains stable at strong to extreme levels.

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