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 April 2nd
Marine heatwave categories
Atlantic Coast
The marine heatwave that has been present for several months along the Moroccan coast as far as the Bay of Biscay is continuing to ease. The areas around Brittany and in the Bay of Biscay are mainly in moderate and locally strong categories.
Mediterranean Sea
Western Mediterranean Basin – The intensity of the marine heatwave is decreasing: the strong categories are disappearing in favour of the moderate ones over almost the entire basin.
Eastern Mediterranean Basin – The marine heatwave is intensifying, with most of the strong and severe categories developing.
North Tropical Atlantic Ocean
The extent of the marine heatwave remains stable. Its intensity is increasing slightly in its south-western part, with an increase in the extent of the strong categories.
Caribbean Sea
The marine heatwave is diminishing in intensity, with the strong and severe categories being replaced by moderate ones.
Equator and South Tropical Atlantic Ocean
The extent of the marine heatwave remains stable overall. We are seeing a strengthening of the strong and severe categories along the northern coasts of Brazil.
Southern Ocean
Off South Africa (between 30°W and 30°E) – The marine heatwave remains stable.
Pacific Ocean
Tropical Pacific Ocean – The marine heatwave linked to El Niño conditions is strengthening slightly in the centre of the basin, with an increase in the extent of the strong and severe categories. In the west of the basin, near Papua New Guinea, the marine heatwave is decreasing in intensity, with a shift to mostly moderate categories.
South Pacific, east of New Zealand – The marine heatwave remains stable overall.
Indian Ocean
In the Indian Ocean, the strengthening that began last week between north-western Indonesia and the Maldives is being confirmed, with an increase in the surface area of moderate and strong categories.
Weekly temperature anomalies
Atlantic Coast – 0.5°C to 2°C
Mediterranean Sea – 0.5°C to 1.5°C
North Tropical Atlantic Ocean – 1.5°C
Caribbean Sea – 1°C to 1.5°C
Off the Equator and South Tropical Atlantic Ocean – 1°C to 1.5°C
Southern Ocean – 1°C to 3°C
Tropical Pacific Ocean – 0.5°C to 1°C (up to 3°C east of the basin)
South Pacific Ocean – 1 °C to 2°C
Indian Ocean – 0.5°C to 1.5°C
Forecast for April 9th
Europe Zone
North Atlantic Ocean – For 9 April, the Mercator Ocean International (MOi) forecast system predicts that the marine heatwave will almost disappear along the north-east Atlantic seaboard, with only a few moderate and very locally severe zones remaining.
Mediterranean Sea – The marine heatwave will become much more intense throughout the basin, with a large increase in the strong and severe categories.
Global Ocean
North Tropical Atlantic and Caribbean Sea – MOi expects the marine heatwave to remain stable.
South Tropical Atlantic – The marine heatwave will remain stable overall.
Southern Ocean – South-west of South Africa (between 30°W and 30°E) will remain stable overall.
Tropical Pacific Ocean – The marine heatwave linked to El Niño conditions will remain stable.
Southeast Asian Seas – The marine heatwave around Papua New Guinea will increase slightly in intensity with the strong categories developing.
South Tropical Pacific – The marine heatwave to the east of New Zealand in the South Pacific will decrease in intensity, with a reduction in the surface area of the strong categories.
Indian Ocean – The extent of the marine heatwave in the north of the basin will increase in intensity in the south of the Arabian Sea, with the development of moderate and 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.
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