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 5 September

• Europe zone :
  • The marine heat wave in the Mediterranean is now present in the easternmost part of the basin, within a moderate category, with weekly temperature anomalies of around 1°C.
  • A moderate heatwave is beginning to develop in the North Sea and around the UK and Ireland, with weekly temperature anomalies of between 1°C and 2°C.
  • The geographical extent of the heatwave in the north-east Atlantic off the Iberian Peninsula has diminished. It remains in the moderate category, with weekly temperature anomalies of around 1°C.

• Global zone :
  • In the Tropical Atlantic, the heat wave continues to spread geographically westwards and southwards along the South American coast. It continues to intensify, with strong to severe categories in the centre of the basin. Weekly temperature anomalies in this area are between 1.5°C and 2.5°C.
  • A moderate to severe heatwave is also developing in the Gulf of Mexico, with weekly temperature anomalies of around 1°C.
  • In the Tropical Pacific, the heatwave linked to the formation of El Nino is continuing. The weekly temperature anomaly in the eastern part of the basin is around 3°C.
  • The heatwave in the north-east Pacific is intensifying, moving from moderate to strong in places. In this area, weekly temperature anomalies are reaching 3°C.
  • The situation remains stable in the western Indian Ocean, where the heatwave in the Arabian Sea remains in the moderate to strong category, with weekly temperature anomalies of around 1°C.

Forecasts up to 12 September

• Europe zone :
  • For 12 September, MOi predicts an intensification of the heatwave in the North-East Atlantic, with moderate to strong categories off the Iberian Peninsula, in the North Sea, in the English Channel and around the UK. This corresponds to the atmospheric heatwave currently affecting Europe.

• Global Zone:
  • For September 12, MOi forecasts a strengthening of the heat waves in the North Atlantic, in the centre of the basin, with moderate to strong categories, and weekly temperature anomalies in the order of 1.5°C to 2°C.
  • The North Tropical Atlantic heat wave remains stable.
  • The heatwave in the Gulf of Mexico is intensifying, moving up to the strong category over almost the entire area.
  • The situation is stable in the Pacific and the western Indian Ocean.

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