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 moderate Mediterranean marine heatwave in the easternmost part of the basin continues, with weekly temperature anomalies of around 1.5°C.
  • A moderate to strong heatwave has developed in the North Sea and around the UK and Ireland, with weekly temperature anomalies of between 1°C and 2.5°C.
  • The heatwave in the north-east Atlantic off the Iberian Peninsula has spread, reaching the strong category in the Bay of Biscay, with weekly temperature anomalies of around 2°C.

• Global zone :
  • In the Tropical Atlantic, the heat wave is intensifying in the centre of the basin, with strong, severe and locally extreme categories. Weekly temperature anomalies in this area are between 1°C and 2°C.
  • The moderate to severe heatwave in the Gulf of Mexico has intensified, 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 east of the basin is around 3°C.
  • The heat wave in the north-east Pacific is easing. In this area, weekly temperature anomalies are reaching 3°C locally.
  • 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 19 September

• Europe zone :
  • For September 19, forecasts show a reduction in the extent of heat waves in the North-East Atlantic, around the UK and Ireland, and in the North Sea. Their intensity is also decreasing, and they are being upgraded to moderate category, with weekly temperature anomalies of around 2°C.
  • A moderate heatwave is developing in the north-western Mediterranean, with weekly temperature anomalies ranging from 1°C to 2.5°C.
• Global zone:
  • For September 19, it is predicted an easing of the heat wave in the centre of the North Atlantic basin.
  • The heat wave in the Tropical Atlantic is intensifying in the western part of the basin with severe categories.
  • The heatwave in the Gulf of Mexico continues to extend into strong to locally severe categories.
  • In the northeast tropical Pacific, the heat wave linked to the formation of El Nino is intensifying.
  • The situation is stable in the north-east Pacific and in 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