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 19 September

• Europe zone :
  • Over the whole Europe Zone, there are no longer any strong categories, but only moderate categories which extend over the whole zone. The Europe zone is showing weekly temperature anomalies of between 1°C and 2°C.

• Global zone :
  • In the Tropical Atlantic, the heatwave is easing in the centre of the basin as it spreads westwards, with weekly temperature anomalies of around 1.5°C.
  • The moderate to strong heatwave in the Gulf of Mexico is still present, with weekly temperature anomalies of around 1.5°C.
  • In the Tropical Pacific, the heatwave linked to the formation of El Niño persists. The weekly temperature anomaly in the eastern part of the basin is around 3°C.
  • The heatwave in the north-east Pacific is still present, with weekly temperature anomalies of around 2°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.
  • A moderate to strong heatwave has developed to the west of the Pacific Ocean, in the China Sea. In this area, weekly temperature anomalies are around 0.5°C in the South China Sea, and up to 3°C in the North China Sea.

Forecasts up to 26 September

• Europe zone :
  • For September 26, forecasts show a reduction in the extent of heat waves in the North-East Atlantic, around the UK and Ireland, as well as in the North Sea and off the Iberian Peninsula.
  • The heatwave in the western Mediterranean disappears.
• Global zone:
  • For September 26, forecasts show that the heatwave in the Tropical Atlantic will continue.
  • In the Tropical North-East Pacific, the heatwave linked to the formation of El Niño is extending to reach the Solomon Islands with moderate to strong categories.
  • The heatwave in the western Indian Ocean should intensify, reaching extreme levels.
  • The heatwave in the North Pacific is spreading to reach the coast of southern Japan, with moderate to severe 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