Mercator Ocean International (MOi) oceanographers examine marine heatwaves across the global ocean. They analyse a variety of datasets from observations (satellite sea surface temperature maps) to numerical model analyses (assimilating satellite and in situ observations) and derive forecasts for a 7 day period.¹
Assessment for 6 February
Marine heatwave categories
North Atlantic Ocean (Europe zone)
The surface area of the strong categories of marine heatwave present for several months remains stable along the coasts of Morocco and to the south-west of the Iberian Peninsula.
Bay of Biscay – The marine heatwave has intensified slightly, rising locally from moderate to strong category.
Mediterranean Sea
The situation remains stable over most of the basin, as well as in the eastern part, with marine heatwaves in the moderate to locally strong categories.
West of the basin – a slight intensification in the western part, mainly in the Tyrrhenian Sea, rising locally from moderate to strong category.
North Tropical Atlantic Ocean
The situation remains stable over most of the basin, as well as in the eastern part, with marine heatwaves in the moderate to locally strong categories.
Mer des Caraïbes – A slight intensification in the western part, mainly in the Tyrrhenian Sea, rising locally from moderate to strong category.
South Tropical Atlantic Ocean
Brésil et la Namibie – the marine heatwave remains stable.
Southern Ocean off South Africa (30°W and 30°E) – A marine heatwave, mainly in the moderate to strong category, is present.30°W and 30°E – marine heat wave with categories ranging from moderate to strong.
Tropical Pacific Ocean
In the East – The marine heatwave linked to the formation of El Niño remains largely stable, with moderate to strong categories.
150 et 180°W – intensification of the marine heatwave from moderate to severe.
Coral Sea and Eastern Australia – The heatwave is diminishing in intensity with a reduction in the extent of the strong categories.
Pacifique Sud, à l’Est de Nouvelle Zélande – The current marine heatwave is intensifying slightly, with a greater spread of moderate and strong categories.
Indian Ocean
Arabian Sea -The heatwaves off the Horn of Africa (Somali Peninsula), to the east of Madagascar and off the Bay of Bengal, are stable overall, with moderate and strong categories.
Weekly temperature anomalies
- North Atlantic (Europe zone) Ocean – 1°C to 3°C
- Mediterranean Sea – 1 °C to 2°C
- North Tropical Atlantic Ocean – 1° C to 2.5° C
- South Tropical Atlantic Ocean– 1 °C to 2°C
- South Atlantic South – 1°C à 3°C.
- Tropical Pacific Ocean– 1.5°C to 3°C
- South Pacific Ocean – 1.5 °C 3°C
- Mer d’Arabie – 1°C to 3°C
Forecasts for 13 February
Europe zone
North Atlantic Ocean – The Mercator Ocean International (MOi) forecasting system predicts that the situation will remain stable off the coasts of Morocco, the Iberian Peninsula and the Bay of Biscay.
Mediterranean Sea – the forecast indicates slight easing in the western basin, with the majority of the area being downgraded to moderate and the situation stable in the eastern part.
Global Ocean
North Tropical Atlantic – The marine heatwave in the North Tropical Atlantic is intensifying slightly in the centre of the basin. The marine heatwave in the Caribbean Sea is decreasing in intensity, going from strong to moderate overall.
Southern Tropical Atlantic – The marine heatwave between Namibia and the tip of Brazil is decreasing in intensity in the southern part of the zone (15°S to 30°S), going from strong to moderate.
South Atlantic -The marine heatwave in the Southern Ocean to the south-west of South Africa (between 30°W and 30°E) is intensifying slightly.
Eastern Tropical Pacific – Stable with mostly moderate and locally strong categories in the easternmost part of the basin.
Western Tropical Pacific – The marine heatwave to the east of New Zealand is easing off, with a transition from strong to moderate categories for the most part.
Indian Ocean – The marine heatwaves in the Arabian Sea, off the Horn of Africa (Somali Peninsula) and to the east of Madagascar are easing. The marine heatwave off the Bay of Bengal is intensifying to the west of Indonesia, with generally strong 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