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 21
Marine heatwave categories
- Atlantic – a marine heatwave persists to the south-west of the Iberian peninsula, continuing at a moderate to locally strong level.
- Mediterranean (eastern region) – the marine heatwave is reducing its spatial extent and decreasing in intensity to a generally moderate level. In the Adriatic, it remains at a high level.
- Tropical Atlantic – the marine heatwave that has been present from the Iberian Peninsula to the Caribbean archipelago for several months remains stable over most of its length. In the Caribbean, the heatwave continues at a high level.
- Southern Tropical Atlantic The marine heatwave covering a zonal strip from South Africa to Brazil, which has been present for around ten days, has developed rapidly. It is now expanding, particularly in the eastern part of the basin in the Benguela upwelling off Angola and Namibia, and is intensifying to reach levels that are generally strong and locally severe to extreme.
- South Atlantic – a marine heatwave is appearing in the southern region, reaching severe categories.
- Tropical Pacific – the marine heatwave linked to the El Niño formation remains stable, with moderate to strong levels of intensity overall.
- Indian Ocean (western region) – the marine heatwave which has been present for more than a month, is maintaining its intensity at a strong to locally severe level. In its southern part, a strong branch extends from Madagascar to the coast of Australia.
- Southeast Asian seas – The marine heatwave that has been present for over a month has subsided and is now only present locally off Malaysia.
Weekly temperature anomalies
- Atlantic Europe – 1.5 to 2°C
- Mediterranean Sea – 1.5 to 3°C
- Tropical Atlantic – 2°C
- Tropical Pacific – 1.5°C to 3°C
- Indien Ocean (Madagascar to Australia) – 1°C à 2.5°C
- Indien Ocean (Southeast Asian seas ) – 1°C à 1.5°C
Forecasts for November 28
Europe zone
- For November 28, Mercator Ocean International (MOi) forecasting system predicts a continuation of the marine heatwave off the Iberian Peninsula and the Moroccan coast with moderate to strong categories.
- The marine heatwave in the western Mediterranean continues to diminish, being present only locally and at a moderate level.
Global Ocean
- For November 28, MOi predictions show that the marine heatwave in the North Tropical Atlantic will remain stable with moderate to strong categories. In the Caribbean, the situation remains stable with mainly strong categories.
- In the Southern Tropical Atlantic, the marine heatwave is developing and extending over the entire eastern part of the basin, reaching strong to locally severe levels; to the west of the basin, the situation is more mixed, with moderate to locally strong levels.
- o In the Tropical Pacific, the situation remains stable with moderate and strong categories in the eastern part of the basin.
- o The marine heatwave in the western Indian Ocean is decreasing in intensity, with a large part of the area moving from strong to moderate, particularly off Madagascar.
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