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 October 10
Europe zone
- In Europe, current marine heatwaves have intensified.
- The marine heatwave along the Atlantic seaboard has intensified, rising from moderate to strong, and even locally severe, covering a large area from the Iberian Peninsula to the English Channel, as well as the Irish Sea.
- The marine heatwave in the western Mediterranean is also intensifying, rising from moderate to severe and even reaching severe levels, particularly in the Adriatic Sea. In the eastern part of the basin, moderate category heatwaves are returning to seasonal normality.
- Weekly temperature anomalies range from 1.5°C to over 3°C along the Atlantic coast and in the western Mediterranean, respectively .
Global zone
- In the Tropical Atlantic, marine heatwave stretching from the Iberian Peninsula to the band in the centre of the basin is evolving, moving from moderate to strong and locally severe category. The extent of the marine heatwave still covers the majority of the Tropical Atlantic zone, and has done so for several months. The moderate category areas in the centre of the basin are moving up to the strong to severe category. The associated temperature anomalies are of the order of 1°C to 2°C.
- The marine heatwave zone covering the Gulf of Mexico and the Caribbean Sea remains at a high level, in the strong to severe category, with extreme categories locally. Weekly temperature anomalies in these areas are around 1.5°C.
- Generally speaking, since March 2023 the surface temperature in the North Atlantic has been higher than in any previous year since 1982, with conditions never seen before for an October in this area [Figure 2].
- In the Tropical Pacific, the marine heatwave linked to the formation of El Niño is still persisting at a moderate intensity, but is diminishing off Mexico, with areas in the strong category moving into the moderate category. The weekly temperature anomaly in the eastern basin is in the region of 1.5°C to 3°C.
- The marine heatwave in the North Pacific remains at a moderate level, with the centre of the basin moving from moderate to strong category. Weekly temperature anomalies are around 2°C in the central part, but can reach up to 3°C in the western part.
- In the western Indian Ocean, the marine heatwave remains stable off Madagascar and is intensifying in the Arabian Sea, where a change to strong to severe categories has been observed. Weekly temperature anomalies are in the region of 1°C to 2°C.
Forecasts for October 17
Europe zone
- For October 17, the Mercator Ocean International (MOI) forecasting system predicts the situation will remain stable in the Mediterranean.
- The marine heatwave is weakening in the Bay of Biscay as far as the English Channel, with a return to the moderate category for the most part.
Global zone
- For October 17, MOI is forecasts an evolution of the marine heatwave in the Tropical Atlantic, which is increasing in intensity, with moderate to strong categories rising to strong to severe levels, from the Iberian Peninsula to the centre of the basin. The marine heatwave in the Caribbean Sea and the Gulf of Mexico remains at a severe to extreme level locally.
- In the Tropical Pacific, the situation remains stable.
- The marine heatwave in the North Pacific is changing from moderate to non-heatwave conditions in the centre of the basin.
- The marine heatwave in the western Indian Ocean is intensifying and moving from moderate to strong, locally severe, with strong categories being more prevalent.
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