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 March 12th
Marine heatwave categories
North Atlantic Ocean
The marine heatwave that has been present for several months along the North-East Atlantic seaboard is continuing to diminish in intensity and extent, with most of the categories being moderate.
Mediterranean Sea
Across the Mediterranean basin, the marine heatwave is continuing to gradually diminish in intensity and extent. The vast majority of marine heatwaves are now in the moderate and locally severe categories.
North Tropical Atlantic Ocean
North Tropical Atlantic and Caribbean Sea – The extent remains stable, but the intensity of the marine heatwave is increasing, with strong to locally severe categories.
Off the Equator
The marine heatwave is decreasing slightly in intensity, with strong to severe categories in the centre and west of the basin.
South Tropical Atlantic Ocean
In the South Tropical Atlantic, to the east of the basin, the marine heatwave is increasing in intensity, with most of it in the strong category.
Southern Ocean
Off South Africa (between 30°W and 30°E) – The marine heatwave is diminishing in intensity, with strong to locally severe categories.
Pacific Ocean
Tropical Pacific Ocean – The marine heatwave linked to El Niño conditions remains stable, with mainly moderate categories and locally strong categories.
South Pacific, east of New Zealand – The marine heatwave is increasing in extent but decreasing in intensity, with mainly moderate and locally strong categories.
Indian Ocean
In the Indian Ocean, the marine heatwaves are disappearing in the eastern part. However, they are increasing in intensity in the western part to the north of Madagascar and off the Horn of Africa in moderate and strong categories.
Weekly temperature anomalies
North-east Atlantic Ocean – 0.5°C to 1.5°C
Mediterranean Sea – 0.5°C to 1.5°C
North Tropical Atlantic Ocean – 1°C to 2°C
Off the Equator – 1°C to 2°C
South Tropical Atlantic Ocean – 1°C to 2°C
Southern Ocean – 1°C to 3°C
Tropical Pacific Ocean – 0°C to 2°C
South Pacific Ocean – 1 °C to 2°C
Indian Ocean – 0°C to 2°C
Forecast for March 19th
Europe Zone
North Atlantic Ocean – For March 19th, the Mercator Ocean International (MOi) forecasting system predicts that the marine heatwave will increase in intensity and surface area along the North-East Atlantic coast, with strong to locally severe categories.
Mediterranean Sea – The marine heatwaves are increasing in extent and intensity in the west of the basin, with the majority of the basin moving into strong categories.
Global Ocean
North Tropical Atlantic – For March 19th, MOi predicts that the marine heatwave in the North Tropical Atlantic will diminish in intensity, with the majority of categories being strong and locally severe in this region.
Caribbean Sea – Marine heatwave intensifies with strong categories.
Equator – The marine heatwave is increasing in intensity, with most of it falling into the strong and severe categories.
South Tropical Atlantic – The surface area of the marine heatwave is increasing in intensity, with mainly strong and locally severe categories.
Southern Ocean – To the south-west of South Africa (between 30°W and 30°E), the marine heatwave is increasing in intensity, with most categories being strong to locally severe.
Tropical Pacific – The marine heatwave linked to El Niño conditions is increasing in intensity, with most categories being strong.
South Tropical Pacific – The marine heatwave to the east of New Zealand in the South Pacific is increasing in intensity, with strong categories.
Indian Ocean – The marine heatwaves are decreasing in intensity in the western part to the north of Madagascar, with most of the categories being moderate.
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.
Figure adapted from Hobday et al. (2018)
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.
[1] Analysis of datasets: SST OSTIA (Copernicus Marine Service), OISST (NOAA), GLO12 (Copernicus Marine Service / Mercator Ocean International), PSY4 (Copernicus Marine Service / Mercator Ocean International), and GLO12 et PSY4 forecasts.
[2] IPCC AR6 SYR chapter 4.3 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf