Heat Waves Happen at the Bottom of the Ocean Too, thanks to Deep Impact

Heat Waves Happen at the Bottom of the Ocean Too, thanks to Deep Impact ...

The western Atlantic Ocean Basin, including the continental shelf, is shown in this satellite-borne map. Credit: NOAA's National Environmental Satellite and Information Service

The "The Blob" marine heat wave from 2013 to 2016 heated a vast area of surface waters in the northeastern Pacific, causing damage to marine ecosystems on the West Coast, reducing salmon numbers, and causing commercial fisheries to collapse.

According to new data from the National Oceanic and Atmospheric Administration (NOAA), marine heat waves can also occur deep underwater.

A group led by NOAA researchers created the first broad assessment of bottom marine heat waves in productive continental shelf waters surrounding North America on March 13.

Heat waves in the oceans are causing significant disturbance to ocean ecosystems across the globe, from plankton to whales. There is a significant effort to research, track, and anticipate the timing, intensity, duration, and physical consequences of these events.

"For the first time we've been able to thoroughly investigate and study how these extreme events unfold along shallow seafloors, we've been studying marine heat waves at the sea surface."

Heat waves from the oceans around the globe are causing havoc on ocean ecosystems, affecting the productivity and distribution of organisms as small as plankton and as large as whales. As a result, there has been considerable effort to research, track, and predict the timing, intensity, duration, and physical consequences of these events.

The majority of the study focused on temperature extremes at the ocean's surface, for which there are many more high-quality observations taken by satellites, ships, and buoys. Sea surface temperatures can also be indicators for many physical and biochemical ocean characteristics of sensitive marine ecosystems, making analyses simpler.

The ocean, which has heated by about 1.5 degrees over the past century, has absorbed 90% of global warming's excess heat, and marine heatwaves have become about half as frequent in the last decade.

Ling cod, like this one caught off the edge of the Humboldt Bay Jetty in California, are a member of Pacific groundfish communities that are susceptible to the effects of bottom marine heat waves.

Scientists have increased their efforts to investigate marine heat waves throughout the water column using the limited data available. Previous research focused on temperature extremes on the ocean bottom along continental shelves, which provide essential habitat for important commercial fish such as lobsters, scallops, crabs, flounder, cod, and other groundfish.

The researchers used a data product known as "reanalysis" to perform the assessment, which starts with available data and then uses computer models that simulate ocean currents and the atmosphere's influence to "fill in the blanks."

These graphs illustrate the average intensity of bottom heat waves ( heat anomalies) that occurred between 1993 and 2019 in each of the major marine ecosystems studied by a team of NOAA scientists. Credit: NOAA Physical Sciences Laboratory

Although ocean reanalysis has been around for a long time, they have only recently become skilled enough and have a high enough resolution to study ocean features, including bottom temperatures, near the coast.

The NOAA, CIRES, and NCAR research team found that bottom marine heat waves tend to last longer than their surface counterparts, and can send more warming signals than overlying surface waters at the same time. This is particularly true in shallower areas where surface and bottom waters mix.

Lionfish have become a poster child for invasive species problems in the western north Atlantic area. Their populations continue to expand, putting their livelihoods at jeopardy, particularly the commercially and recreationally important fish that depend on them.

However, bottom marine heat waves may also occur with little or no evidence of warming at the surface, which has significant implications for the management of commercially significant fisheries.

According to a 2021 research, destructive algae blooms and loss of kelp forest habitat off the West Coast of the United States led to shellfisheries that cost the economy more than $185 million. Recreational razor clam and abalone fisheries in Washington and California both lost more than $84 million in tourist expenditure.

According to a groundfish survey published by NOAA Fisheries in 2021, Gulf of Alaska cod was down for the first time between 2015 and 2017. On the other hand, young groundfish and other marine animals in the Northern California Current system flourished under unprecedented ocean conditions, according to a 2019 paper by Oregon State University and NOAA Fisheries.

Unusually high bottom water temperatures have been linked to the expansion of invasive lionfish in the southeast United States, coral bleaching and subsequent declines of reef fish, and changes in young Atlantic cod survival rates in southern New England.

According to the authors, maintaining existing continental shelf monitoring systems is critical, as well as developing new real-time monitoring capabilities to alert marine resource managers to bottom warming conditions.

"We know that early recognition of marine heat waves is required for proactive management of the coastal ocean," said co-author Michael Jacox, a research oceanographer who works at NOAA's Southwest Fisheries Science Center and the Physical Sciences Laboratory. "Now it's clear that we need to pay closer attention to the ocean bottom, where some of the most valuable species dwell, as well as when heat waves are different from those on the surface."

Dillon J. Amaya, Michael G. Jacox, Michael A. Alexander, Clara Deser, Antonietta Capotondi, and Adam S. Phillips, Nature Communications, DOI: 10.1038/s41467-023-36567-0

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