Marine heatwave bulletin - 20 October 2023

Marine heatwave bulletin – 20 October 2023

Mercator Ocean International (MOi) oceanographers examine marine heatwaves across the global ocean. They analyse a variety of datasets from observation analyses (satellite sea surface temperature maps) to model analyses (assimilating satellite and in situ observations) and model forecasts.^¹

Assessment for October 17

Europe zone

the marine heatwave along the Atlantic coast is still present, with moderate to severe categories, and even locally severe categories, covering a large area from the Iberian Peninsula to the English Channel.
The marine heatwave in the western Mediterranean is intensifying, with more extensive coverage of severe categories.
Weekly temperature anomalies range from 1.5°C to over 3°C along the Atlantic coast and in the western Mediterranean.

Figure 1: Marine heatwave category map for the global ocean for October 17, 2023. GLO12 analysis. Source: Mercator Ocean International

Global zone

In the Tropical Atlantic, the marine heatwave that stretches from the Iberian Peninsula to the equator is intensifying, becoming severe over the whole area between 30°N and the equator. The associated temperature anomalies are of the order of 1°C to 2°C. The marine heatwave zone covering the Gulf of Mexico and the Caribbean Sea remains at a high level, in the strong to severe category, with extreme categories locally. Weekly temperature anomalies in these areas are around 1.5°C.
In the Tropical Pacific, the marine heatwave linked to the formation of El Niño remains stable at a moderate intensity. The weekly temperature anomaly in the eastern part of the basin is in the region of 1.5°C to 3°C.
The marine heatwave in the North Pacific is still present but is decreasing to a moderate category. Weekly temperature anomalies are around 1 to 2°C.
In the western Indian Ocean, the marine heatwave off Madagascar and in the Arabian Sea is intensifying, with more areas affected by strong categories. Weekly temperature anomalies are in the region of 1°C to 2°C.

*Figure 2: Marine heatwave category map for the global ocean for October 24, 2023. GLO12 analysis. Source: Mercator Ocean International*

Forecasts for October 24

Europe zone

For October 24, Mercator Ocean International’s (MOI) forecasting system predicts a very sharp reduction in the marine heatwave along the Atlantic coast and around the Iberian Peninsula, where only moderate category areas remain.
On the other hand, the marine heatwave around Corsica remains stable at strong to locally severe levels.

Global zone

For 24 October, MOI forecasts a marine heatwave that will be stable in extent in the Tropical Atlantic, but which will become less intense with strong to moderate category passages. The marine heatwave in the Caribbean Sea and the Gulf of Mexico will remain severe to extreme locally.

In the Tropical Pacific, the situation remains stable.

The marine heatwave in the North Pacific continues to diminish, falling essentially to the moderate category.

The marine heatwave in the western Indian Ocean is still present. The area to the west of the Arabian Sea is decreasing in intensity from severe to strong; further east along the west coast of India, MOI forecasts an intensification of the marine heatwave to a severe category.

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 4: 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.

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