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 14

Marine heatwave categories

• In Atlantic Europe, few areas in moderate categories persist on the Iberian peninsula to the west of Gibraltar.
• In the western Mediterranean, only a small area to the south of the Tyrrhenian Sea is still affected by a marine heatwave. In the eastern part, on the other hand, we are still seeing moderate to strong categories.

• In the Tropical Atlantic, the geographical extent of the marine heatwave from the Iberian Peninsula to the equator, which has been raging for several months, remains stable over most its majority.
• The marine heatwave in the Caribbean Sea has diminished in intensity, with the categories now essentially strong.
• A marine heatwave is also developing in the Southern Tropical Atlantic, stretching along the Agulhas current.
• In the Tropical Pacific, the marine heatwave linked to the formation of El Niño remains stable overall, with moderate to strong levels of intensity. A decrease in intensity in the eastern part of the region is observed.
• The marine heatwave in the North Pacific is diminishing in extent, with mostly moderate categories remaining locally.
• South-west Indian Ocean is intensifying in its western part, with strong to severe levels. It is still developing to the north of Madagascar, reaching the southern centre of the Indian Ocean.
• Arabian Sea and the Bay of Bengal, the marine heatwave is also becoming more intense, with strong to severe levels.

Weekly temperature anomalies

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

Forecasts for November 21

Europe zone

• For November 21, Mercator Ocean International’s (MOi) forecasting system is predicting an intensification of the marine heatwave around the Iberian Peninsula and the Moroccan coast, with moderate to strong categories.
• The marine heatwave in the western Mediterranean is easing. The eastern part of the basin will essentially fall into the moderate category.

Global Ocean

• For November 21st, MOi forecasts that the marine heatwave in the Tropical Atlantic will remain stable with moderate to severe categories. The marine heatwave in the Caribbean Sea remains stable with mainly strong categories.
• In the South Atlantic, the marine heatwave is increasing in extent but also in intensity, with severe to extreme categories.
• In the Tropical Pacific, the situation remains stable with mostly moderate categories.
• The marine heatwave in the North Pacific continues to diminish in extent, with locally moderate levels remaining.
• The marine heatwave present in the south-west Indian Ocean is extending to reach the eastern part of the southern Indian Ocean. In the western Indian Ocean, the marine heatwave is diminishing in intensity.
• In the South-East Asian Seas, the marine heatwave is decreasing in extent and intensity to strong to severe 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