Marine heatwave bulletin – 5 March 2024

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 5th

Marine heatwave categories

Marine Heatwave Map
Figure 1: Map of categories of marine heatwaves for March 5, 2024, for the entire global ocean. GLO12 analysis. Source: Mercator Ocean International
North Atlantic Ocean (Europe zone)

On the Atlantic seaboard, 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 the majority of events in the moderate and strong categories.

Mediterranean Sea

In the whole Mediterranean basin, the marine heatwave is diminishing in intensity and in surface area, with the vast majority of moderate, and locally strong, categories being observed.

 North Tropical Atlantic Ocean

Caribbean Sea and Gulf of Mexico – The intensity of the marine heatwave is still present, with an increase in the severe categories.

Equator – The marine heatwave is becoming more intense, with an increase in the surface area of severe and extreme categories in the centre and west of the basin.

South Tropical Atlantic Ocean

In the Southern Tropical Atlantic, the marine heatwave is intensifying, with a more pronounced development of severe categories to the west of the basin.

The marine heatwave in the Southern Ocean off South Africa (between 30°W and 30°E) remains stable overall.

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 intensifying as the surface area of strong categories increases.

Indian Ocean

In the Indian Ocean, the marine heatwaves remain moderate overall.

Weekly temperature anomalies

North-east Atlantic Ocean – 0.5°C to 1.5°C

Mediterranean Sea 0.5°C to 1.5°C

Tropical North Atlantic Ocean 1°C to 2°C (off the Equator 1.5 °C to 2.5°C)

South Tropical Atlantic Ocean 1°C tot 3°C

Tropical Pacific Ocean 1°C to 2.5°C

South Pacific Ocean 1.5 °C to 3°C

Indian Ocean 0.5°C to 2°C

Forecast for March 12th

Sea surface temperature
Figure 2: Map of weekly surface temperature anomalies for the week of March 5, 2024 to March 12, 2024, for the entire global ocean. GLO12 Analysis. Source: Mercator Ocean International System

Europe Zone

North Atlantic Ocean – For March 12th, the Mercator Ocean International (MOi) forecasting system predicts that the marine heatwave will continue to diminish in intensity along the North-East Atlantic seaboard, with moderate categories predominating.

Mediterranean Sea – The marine heatwave will diminish in extent and intensity across the whole basin, with the majority of the basin moving into moderate categories. Strong categories will remain locally in the west, however.

Global Ocean

North Tropical Atlantic – For March 12th, MOi predicts that the marine heatwave in the Tropical North Atlantic will intensify in the western part close to the West Indies, with a pronounced development of severe and locally strong categories in this region.

Equator – The marine heatwave will decrease slightly in intensity and will reach the strong and severe categories.

South Tropical Atlantic – The surface area of the marine heatwave will remain stable.
In the Southern Ocean to the south-west of South Africa (between 30°W and 30°E), the marine heatwave will remain stable.

Tropical Pacific – The marine heatwave linked to El Niño conditions will remain stable overall. 

South Tropical Pacific The marine heatwave to the east of New Zealand in the South Pacific will increase in extent but decreasing in intensity.

Indian Ocean – Marine heatwaves will disappear in the eastern part of the region, but will expand in the western part to the north of Madagascar and off the Horn of Africa in 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