Marine Heatwave Forecast Monthly Report

Global Marine Heatwave Forecast Discussion

Observed and forecasted values include the effects of long-term warming. Values with the long-term warming trend removed are in brackets.

Current marine heatwave conditions:

In June 2024, 36% [22%] of the global ocean experienced MHWs, which ranked 8th [16th] among all months since 1991.

While the fraction of the globe with marine heatwave (MHW) conditions remained relatively unchanged from May, those along the equator in the tropical Pacific decreased as El Niño weakened. MHWs are currently found in the western tropical Pacific, across much of the North Pacific to the east of Japan, the Sea of Japan and South China Sea, the tropical Atlantic, the Gulf of Mexico, the Caribbean Sea, parts of the Norwegian and Mediterranean Seas, much of the Indian Ocean, and portions of the Atlantic, Indian and Pacific sectors of the Southern Ocean.

Marine heatwave forecasts:

Forecasts predict that MHW coverage and overall intensity will continue to decline in the coming months, dropping to ~31% [15%] of the global oceans experiencing MHW conditions by October 2024. Below is a regionally refined focus:

• Pacific - Probabilities of MHWs in the eastern/central tropical Pacific and eastern extratropical Pacific are predicted to decline through Boreal fall/winter (0-10% [0-10%]; high confidence), associated with a transition from El Niño to La Niña. In the Western tropical Pacific and central/western extratropical Pacific, elevated MHW probabilities are more widespread, reaching 50-90% ([10-50%]; low-medium confidence) by the end of the year.
• Tropical Atlantic - MHW conditions in the tropical North Atlantic are likely to persist through October 2024 (80-100% [30-60%]; low-medium confidence), with slightly lower MHW likelihoods in the Gulf of Mexico (40-80% [20-50%]; medium confidence).
• North Atlantic - Likelihood of MHWs in the central North Atlantic is predicted to fall over the next several months, though remaining elevated (30-60% [40-70%]; low confidence) through October.
• Indian Ocean - Elevated chances of MHW conditions are predicted through October in the central Indian Ocean and Indo-Pacific region, along the southeast African coast, and in the northern Arabian Sea and Bay of Bengal (50-80% [30-50%]; medium confidence).
• Southern Ocean - Elevated MHW likelihoods in the Southern Ocean are forecasted to continue through at least October in regions south of Africa, south of Australia and in the central Pacific (70-100% [40-90%]; low-medium confidence).

Marine Heatwave Maps

Marine Heatwave Area Percentage - Monthly Forecast

Marine Heatwave Area Percentage - Historical Record

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What has been changed to calculate the global area percentage of MHW?

The methodology employed prior to September 2023 for global marine heatwaves area percentage was a probabilistic approach. This approach involved identifying ocean areas with a forecasted probability exceeding 30%. Subsequently, this identified region was utilized to compute the percentage of the ocean area encompassed by marine heatwaves.

In contrast, the new methodology utilizes a deterministic approach to calculate the area percentage. It involves the identification of marine heatwaves in each ensemble member and model over the ocean (i.e., a binary 0 or 1). The area covered by marine heatwaves is then employed to determine the percentage of ocean area affected by marine heatwaves in each ensemble member and model. To provide a consolidated view, a multi-model mean of the area percentage is calculated, which offers insight into the overall percentage of ocean area affected by marine heatwaves.

A comparison of the two approaches can be seen below.

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Ocean Basin Area

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Remove long-term temperature trends?

No Yes

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