Mercator Ocean Bulletin: Marine heatwave forecasts – August 3rd 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 marine heatwave forecasts for a 7 day period.[1]

Forecasts for August 3rd

carte catégories vagues de chaleur marine — *Figure 1: Map of marine heatwave categories for August 3rd 2024 forecast for the world ocean as a whole. GLO12 analysis. Source : Mercator Ocean International*

Europe Zone

For the 3^rd of august, Mercator Ocean International forecasts a decrease in spatial extend and intensity of marine heatwaves in the eastern Mediterranean, with moderate and severe categories. In the Norwegian Sea, marine heatwaves are intensifying, going from moderate to strong categories.

Global Ocean

Atlantic Ocean

North Atlantic Ocean – MOi forecasts that marine heatwave in the mid North Atlantic remains stable overall.

North Tropical Atlantic Ocean – In the Tropical North Atlantic, the marine heatwave remains stable overall. In the Gulf of Mexico, the marine heatwave is increasing in intensity, with most of it now in strong category.

South Tropical Atlantic Ocean – In the Southern Tropical Atlantic, the marine heatwave is diminishing in intensity and area. In the central South Tropical Atlantic, the marine heatwave is in moderate category, with only a few strong categories persisting along the Brazilian coast.

Southern Ocean

The marine heat wave in the Southern Ocean, off the South African coast (between 30°W and 30°E) continues to decrease in spatial extend.

Pacific Ocean

Tropical Pacific Ocean – In the Tropical Pacific, the marine heatwave continues to decrease in spatial extend in the Western part, moderate and strong categories remain. On the Eastern side, the marine heat wave remains stable overall with moderate and strong categories.

North Pacific Ocean – In the North Pacific, between the coast of Japan and around 180°W, the marine heatwave continues to decrease in area. A moderate marine heatwave between southern Japan and Papua New Guinea. A second marine heatwave in the central North Pacific Ocean is moderate to strong and locally extreme. Between the Canadian coast and around 150°W, the marine heatwave continues to diminish in extent.

South-East Asian Seas – A marine heatwave with strong categories appears to the north of Malaysia.

South Pacific, to the east of New-Zealand – The marine heatwave remains stable.

Indian Ocean

In the Indian Ocean, Mercator Ocean forecasts that the marine heatwave decreases in spatial extend, moderate and strong categories remain mainly in the center of the basin.

Weekly temperature anomalies

Cartes des anomalies hebdomadaires de température des vagues de chaleur marine — *Figure 2 : Weekly surface temperature anomaly map for the week of 27 July to August 3rd 2024. GLO12 forecast. Source : Mercator Ocean International*

Atlantic Ocean	North 2°C to 3°C	North Tropical 1°C to 2°C	South Tropical 0°C to 1.5°C
Southern Ocean	0°C to 2°C
Pacific Ocean	North 1°C to 3°C	Tropical 0°C to 1°C	South 1°C to 2.5°C	South-East Asian Seas 0.5°C to 1°C
Indian Ocean	0.5°C to 2°C

Consult our 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 a heat episode during which the temperature is significantly higher than a certain threshold for at least 5 consecutive days.

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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 https://www.ipcc.ch/report/ar6/syr/downloads/report/IPCC_AR6_SYR_LongerReport.pdf