The marine heatwave bulletin provides forecasts and analysis of marine heatwave events across the globe and throughout the year. Used datasets include observations (satellite sea surface temperature maps) and numerical model analyses (assimilating satellite and in situ observations) to derive marine heatwave forecasts for a 10 day period. [1] This week’s forecasts were produced using as a comparison the marine heatwave situation on the 26th November.
Forecasts for 7 December
North Atlantic Ocean
The marine heatwave expanding from the south of Iceland to the coast of Morocco is stable in extend and in intensity with moderate and strong category.
North Tropical Atlantic Ocean
The extent and the intensity of the marine heatwaves in the Gulf of Mexico and the Caribbean Sea is decreasing. A few strong and severe categories are still present off the coast of West Indies. At the East, the extent and intensity of marine heatwaves are decreasing in the Gulf of Guinea and as far as the coasts of Guinea, with moderate and strong categories.
South Tropical Atlantic Ocean
The last marine heatwave traces remain stable. The extent of the marine heatwave in the Gulf of Guinea is decreasing but increasing in intensity with moderate and strong categories.
Southern Ocean
The marine heatwave in the Southern Ocean between 30°E and 75°E increases in extent and intensity with strong and severe and locally extreme categories.
Tropical Pacific Ocean
The moderate to strong marine heatwaves along the coasts of Chile and Peru is decreasing in extent. The marine heatwave from Papua New Guinea to off the coast of North America is decreasing in extent and intensity, with moderate to strong categories still being observed.
North Pacific Ocean
The marine heatwave around 170°W decreases in intensity. The marine heatwave in the Sea of Japan, the East China Sea, the Yellow Sea and to the east of Japan is decreasing in extent and intensity.
South-East Asian Seas
The marine heatwave is decreasing in intensity as well as in extent. The area remains in moderate and strong categories.
Indian Ocean
Forecasts show a strong decrease in the extent and intensity of marine heatwaves affecting the Bay of Bengal and the Arabian Sea. The extent and intensity of marine heatwaves is decreasing in the middle of the basin and in the west of Indonesia. The marine heatwave to the east of Madagascar decreases in extent, with moderate to strong categories and locally severe. At the south of Australia there is an ongoing marine heatwave with strong to severe categories and locally extreme.
European Zone
For December 7, forecasts show a global increase of the intensity of the marine heatwaves with category moderate and strong. The extent remains stable.
Weekly Temperature Anomalies
| Atlantic Ocean | North 1°C to 3°C | North Tropical 0°C to 2.5°C | South Tropical 0°C to 3°C | |
| Southern Ocean | -1°C to 3°C | |||
| Pacific Ocean | North 0°C to 3°C | Tropical -1°C to 2°C | South -1°C to 2°C | Southeast Asian Seas 0.5°C to 2°C |
| Indian Ocean | 0.5°C to 2°C | |||
| European Zone | 0.5 to 3°C |
Access the Daily Global Physical Bulletin for a 9-day forecast here.
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 an episode during which the ocean temperature is abnormally warm for at least 5 consecutive days.
Adapted from Hobday et al. (2018)
For any location in the ocean, the normal temperatures are defined for every day of the year using a climatological period (here 1993–2016). A heatwave is identified when the measured daily temperature is within the top 10% of the highest recorded values for that day (i.e., above the 90th percentile, see diagram), and with this condition persisting for at least five consecutive days.
The intensity of the heatwave on any given day is measured as the number of degrees above the climatological average (represented by the bold black line) indicated by the blue arrow. We can either calculate the cumulative intensity over the entire heatwave or record the maximum intensity.
Heatwaves are classified based on their intensity level. To do this, the intensity is compared to the difference between the climatological value and the 90th percentile value. A mhw intensity between 1 and 2 times this difference corresponds to a heatwave of moderate category; between 2 and 3 times, to a strong category; between 3 and 4 times, to a severe category; and a difference greater than 4 times corresponds to an 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