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Marine Heatwaves How wind - or lack thereof - compounds extreme events

Winds are at the heart of many extreme events that impact our society.

Some types of ocean wind-related extreme events are familiar, such as tropical cyclones that are fueled by ocean heat. According to the United States Environmental Protection Agency (EPA), cyclone intensity in the North Atlantic Ocean has risen noticeably over the past 20 years, and eight of the 10 most active years since 1950 have occurred since the mid-1990s.

North Atlantic Tropical Cyclone Activity According to the Accumulated Cyclone Energy Index, 1950–2020.
Climate Change Indicators: Tropical Cyclone Activity (US EPA)

But some wind-related extreme events are less well known. One type is marine heatwaves.

Marine heatwaves are long periods of warmer-than-usual seawater temperatures. In the past decade, marine heatwaves have become more frequent, and more intense. And their impacts can be far-reaching. The warmer the ocean, the higher likelihood for more powerful tropical cyclones.

Some marine heatwaves are caused by ocean currents that move warm waters from one place to another. However, some marine heatwaves are the direct result of weak winds that prevent heat from escaping from the ocean. Differences between normal ocean conditions and those during marine heatwaves are depicted below.

How are winds related to marine heatwaves? LEFT: Normal conditions with strong winds and ocean mixing. RIGHT: Marine heatwaves are exacerbated by weaker winds and ocean mixing, increased solar radiation, and decreased evaporative cooling.

How big of a problem are marine heatwaves? According to the American Meteorological Society's State of the Climate report, the ocean experienced a global average of 49 marine heatwave days in 2021, which is fewer than the 2020 average of 58 days and the 2016 record of 61 days (graph below, left). In 2021, 57% of the surface of the ocean experienced at least one marine heatwave (graph below, right). This daily average equates to 13% of the surface of the ocean experiencing a moderate to extreme marine heatwave on any given day.

LEFT: Average count of marine heatwave days occurrence from 1982 to 2011. RIGHT: Percent of the surface of the ocean experiencing a marine heatwave on any given day from 1982 to 2021.
State of the Climate in 2021

Marine heatwaves happen all around the world and are becoming more frequent.

Check out resources from the National Oceanic and Atmospheric Administration (NOAA).

NOAA Global Marine Heatwave Seasonal Forecast System
NOAA: The ongoing marine heat waves in U.S. waters, explained
As Earth’s climate warms, record-setting marine heatwaves are becoming more frequent and severe.
Marine Heatwaves International Working Group

Learn more about the marine heatwave that occurred in the northeast Pacific Ocean in 2013-2015.

The Blob

"In 2013, a mysterious pool of warm water developed off of Alaska. This marine heat was so persistent and unusual that it initially defied explanation. The pool lingered in the sub-Arctic Bering Sea and Gulf of Alaska through winter, and then quickly expanded south along the Pacific Coast. By summer of 2014, the heated mass of water stretched from Alaska to Mexico and had been nicknamed 'the blob.'"

"As the blob spread, unusually warm waters triggered extended harmful algae blooms. The blob fueled longer-lasting and more pervasive blooms, which became toxic to marine life and humans. Higher ocean temperatures also increased warm water algae species, which were less nutritious for marine life."

Read the Full Story at NASA Earthdata

The Blob's Biological Impacts

"Scientists and the public alike noted widespread changes in the biological structure and composition of both open ocean and coastal ecosystems. Changes included geographical shifts of species such as tropical copepods, pelagic red crabs, and tuna; closures of commercially important fisheries; and mass strandings of marine mammals and seabirds."

The blob "caused significant changes in zooplankton and marine invertebrate populations, with many species shifting their distributions toward cooler, more northern waters. Sightings suggest that tropical invertebrates such as tuna crabs were followed northward by their predators, tuna, which were in turn followed by their predators, sharks. Increased proportions of less nutritious warm-water copepod species and decreased abundance of krill were observed. This loss of high-quality food caused population declines of many fish and seabird species and contributed to record marine mammal strandings."

Economically, the effects of the blob "led to millions of dollars in losses among fishing industries. This is worrisome because the (blob) may be a harbinger of things to come. As sea surface temperatures continue to rise with increasing global temperatures, many of the same scenarios observed during the (blob) may be repeated, with dramatic ecological and economic consequences."

Read the Full Story by Cavole et al. (2016)
"We found that economic losses of single marine heatwave events to date exceed US$800 million in direct losses and additional, substantial indirect losses. The true costs are likely to be even greater because many socioeconomic effects remain unknown and underreported, particularly in lower-income countries." – Katie Smith, Marine Biological Association

Marine Heatwaves & Coral Reefs

Coral reefs are among the most biologically diverse and valuable ecosystems on Earth. While they cover less than 0.1% of the ocean floor, more than a quarter of all marine life – including over 4,000 species of fish – are dependent on coral reefs at some point in their life cycle. An estimated 1 billion people worldwide benefit from the many ecosystem services coral reefs provide including food, coastal protection, and income from tourism and fisheries.

Coral bleaching is one of the most obvious visual signs of climate change in the marine environment. Why? Corals are sensitive to environmental changes, thus serve as a "canary in the coal mine," warning us when areas of the marine environment are unhealthy.

The following slides show how marine heatwaves can contribute to coral bleaching.

Healthy Coral – Coral and algae have a "symbiotic" relationship. In other words, they depend on each other to survive. The algae are the coral's primary food source and give them their color.
Stressed Coral – If stressed by a marine heatwave, algae can leave the coral.
Bleached Coral – Without the algae, the coral loses is major source of food, turns white or very pale. It is more susceptible to disease.

Bleaching, fortunately, is not necessarily a death sentence. If stressors like elevated temperatures are resolved in time, symbiotic algae can return to the coral's tissue. However, if corals cannot recover their algae within a few months, they are likely to die.

Bleaching was recorded in coral reefs of National Marine Sanctuary of American Samoa between 2014 and 2015. (Source: NOAA National Marine Sanctuaries)

Marine heatwaves are also threatening the survival of the Great Barrier Reef. Australia's Climate Council reported that nearly 30 percent of corals on the Great Barrier Reef died as a result of mass bleaching in 2016. The event was at least 175 times more likely to occur due to climate change.

If climate change continues unabated, the Great Barrier Reef could face bleaching conditions every year after 2044. These conditions would effectively destroy the Reef, and most shallow water tropical reefs worldwide.

In Hot Water: Climate Change, Marine Heatwaves & Coral Bleaching

NASA's Ocean Vector Winds Science Team is actively investigating the physics of marine heatwaves that harm our ocean's ecosystems and also intensify storms, high-wind events, and coastal flooding.

NASA Ocean Vector Winds Science Team
"Marine heatwaves have increased by 50% over the past decade and now last longer and are more severe. Projections suggest that by the year 2100, marine heatwaves will increase 20-50 times in frequency and 10 times in intensity. While these changes impact the entire ocean, the Arctic and tropical regions are expected to be most affected." –International Union for Conservation of Nature

References

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