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 19th

Europe zone

• On the Atlantic seaboard, the marine heatwave that has been present for several months along the Moroccan coast up to the Bay of Biscay is intensifying, with an increase in the area covered by strong categories.  The associated weekly temperature anomalies are between 0.5°C and 1.5°C.
• In the western Mediterranean basin, the marine heatwave is becoming more intense, with an increase in the surface area of the strong categories. In the eastern Mediterranean basin, the marine heatwave remains stable, with mostly moderate and severe categories. The associated weekly temperature anomalies are between 0.5°C and 1.5°C.

Global Ocean

• In the North Tropical Atlantic and the Caribbean Sea, the extent of the marine heatwave remains stable but there is a slight increase in the surface area of the strong categories off the Moroccan coast, as well as in the Caribbean Sea. In addition, around 50°W and 15°N, there is a slight decrease in the area covered by strong categories. The associated weekly temperature anomalies are between 1°C and 1.5°C. 
  
  
• At the equator and in the Southern Tropical Atlantic, the extent of the marine heatwave remains stable. There is an increase in the surface area of the strong categories towards the east of the basin. In the centre of the basin, the extreme intensity categories disappear, but there is an increase in the surface area of the severe categories. The associated weekly temperature anomalies are between 1°C and 1.5°C. 
  
  
• The marine heatwave in the Southern Ocean off South Africa (between 30°W and 30°E) is increasing slightly in intensity, with more severe categories, and slightly in extent, with moderate categories appearing. The associated weekly temperature anomalies are between 1°C and 3°C. 
  
  
• In the Tropical Pacific, the marine heatwave linked to El Niño conditions is intensifying, with an increase in the area covered by strong categories. Around 165°W but also around 120°W. The associated weekly temperature anomalies are around 1°C. 
  
  
• In the South Pacific, to the east of New Zealand, the marine heatwave is intensifying with an increase in the area covered by strong categories. The associated weekly temperature anomalies are between 1°C and 2°C. 
  
  
• In the Indian Ocean, the marine heatwave is mostly in the moderate category and the extent of the marine heatwave remains stable. The associated weekly temperature anomalies are between 0.5°C and 1°C. 

Forecast for March 26th

Europe Zone

For 26 March, the MOi forecasting system predicts that the marine heatwave will decrease in intensity and surface area along the north-east Atlantic coast, with categories ranging from mostly moderate to locally severe.

In the Mediterranean, the marine heatwave is decreasing in intensity in the western Mediterranean basin, with categories that are mainly moderate. In the eastern Mediterranean basin, the marine heatwave remained stable.

Global Ocean  

For 26 March, MOI forecasts that the marine heatwave in the North Tropical Atlantic will decrease in intensity in the eastern part of the basin.

In the Caribbean, the marine heatwave is increasing in intensity with the appearance of strong and locally severe categories.

At the equator, the marine heatwave is increasing in intensity, with more areas in the extreme category in the centre of the basin, and all the moderate categories becoming severe.

In the Southern Tropical Atlantic, the marine heatwave remains stable overall. 

The marine heatwave in the Southern Ocean to the south-west of South Africa (between 30°W and 30°E) remains stable overall.

In the Tropical Pacific, the marine heatwave linked to El Niño conditions continues to increase in intensity, with more areas in the strong category. The marine heatwave is also increasing in extent and developing around Papua New Guinea and Indonesia.

In the South-East Asian seas, a severe marine heatwave is appearing to the south of Papua New Guinea, as well as to the north of Indonesia and Malaysia.

The marine heatwave to the east of New Zealand in the South Pacific remains stable overall.

In the Indian Ocean, the marine heatwave remains moderate overall, but its extent is increasing.

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.

[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