Marine heatwave bulletin – 23 January 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 23 January

Marine heatwave categories

Figure 1: Marine heatwave category map in the global ocean for January 23, 2024. GLO12 analysis. Source: Mercator Ocean International
North Atlantic Ocean (Europe zone)

Stable situation lasting for several months

  • Marocco and Iberian Peninsula – strong
  • Bay of Biscay – moderate
Mediterranean Sea

Generally stable with moderate to locally strong categories

North Tropical Atlantic Ocean

Marine heatwave present for several months with strong categories decreasing in surface area.

South Tropical Atlantic Ocean
Brésil et la Namibie – increased intensity from strong to locally extreme along the Brazilian coast at the Equator level

15°S et 30°S et ver 20°W – generally decreased intensity to moderate categories.

Tropical Pacific Ocean
The marine heat wave linked to the formation of an El Niño event in the region is generally stable with moderate categories.

Coral Sea and Eastern Australia – Increase of the area occupied by strong categories.

Indian Ocean
East of Madagascar and Arabian Sea – decreased intensity and surface area with moderate to locally strong categories.

Sourth East Asia Sea
Decreased intensity now ranging moderate categories.

Weekly temperature anomalies

  • North Atlantic (Europe zone) Ocean – 1.5 °C to 3°C
  • Mediterranean Sea – 1.5 °C to 3°C
  • North Tropical Atlantic Ocean – 1° C to 2° C
  • South Tropical Atlantic  Ocean– 1 °C to 2°C and 3°C locally
  • Tropical Pacific Ocean– 1.5°C to 3°C
  • Indian Ocean (west) – 2°C to 3°C
  • South East Asian Seas – 1°C

Forecasts for 30 January

Figure 2: Weekly surface temperature anomalies for the global ocean, for the week of 23 to 30 January, 2024. GLO12 analysis. Source: Mercator Ocean International.

Europe zone

North Atlantic Ocean – Increase in intensity to the southwest of the Iberian Peninsula and Moroccan coast.

Mediterranean Sea – Increase in surface area to the west of the basin.

Global Ocean

North Tropical Atlantic Ocean

According to Mercator Ocean International’s forecasting system the marine heat wave in the North Tropical Atlantic Ocean will remain stable until January 30.

Southern Tropical Atlantic
Increase in intensity with strong and extreme categories developing.

Eastern Tropical Pacific

the situation remains stable with mostly moderate categories and locally strong categories

Western Tropical Pacific
Coral Sea – decrease in intensity

Indian Ocean
Bay of Bengal – increase in intensity with 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 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