Scientists identify heat wave at bottom of ocean

The 2013-2016 marine heat wave known as “The Blob” warmed a vast expanse of surface waters across the northeastern Pacific, disrupting West Coast marine ecosystems, depressing salmon returns, and damaging commercial fisheries. It also prompted a wave of research on extreme warming of ocean surface waters.

But, as new NOAA research shows, marine heat waves also happen deep underwater.

In a paper published in the journal Nature Communications, a team led by NOAA researchers used a combination of observations and computer models to generate the first broad assessment of bottom marine heat waves in the productive continental shelf waters surrounding North America.

“Researchers have been investigating marine heat waves at the sea surface for over a decade now,” said lead author Dillon Amaya, a research scientist with NOAA’s Physical Science Laboratory. “This is the first time we’ve been able to really dive deeper and assess how these extreme events unfold along shallow seafloors.”

Marine heat waves dramatically impact the health of ocean ecosystems around the globe, disrupting the productivity and distribution of organisms as small as plankton and as large as whales. As a result, there has been a considerable effort to study, track and predict the timing, intensity, duration, and physical drivers of these events.

Most of that research has 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 more straightforward.

About 90% of the excess heat from global warming has been absorbed by the ocean, which has warmed by about 1.5C over the past century. Marine heatwaves have become about 50% more frequent over the past decade.

In recent years, scientists have increased efforts to investigate marine heat waves throughout the water column using the limited data available. But previous research didn’t target temperature extremes on the ocean bottom along continental shelves, which provide critical habitat for important commercial species like lobsters, scallops, crabs, flounder, cod and other groundfish.

Scientists identify heat wave at bottom of ocean

Bottom vs. surface marine heatwave spatial extent. a–i The fraction of each Large Marine Ecosystem’s (LME) area experiencing surface marine heatwave (SMHW) conditions for each month from 1993–2019. Shading denotes average SMHW intensity (°C) in a given month, as measured by sea surface temperature anomalies (SSTAs) averaged across all grid cells experiencing SMHW conditions. Black contours mark fraction of LME’s area in bottom marine heatwave (BMHW) conditions (i.e., bar height in Fig. 5). Horizontal gray lines mark areal extents of 0.5 and 1. Note only grid cells with bottom depths <400 m were used for areal percentage and intensity calculations. Numbers next to LME names indicate total number of months with areal extent greater than 0.5 for SMHWs and BMHWs, respectively. © Nature Communications (2023). DOI: 10.1038/s41467-023-36567-0

Due to the relative scarcity of bottom-water temperature datasets, the scientists used a data product called “reanalysis” to conduct the assessment, which starts with available observations and employs computer models that simulate ocean currents and the influence of the atmosphere to “fill in the blanks.” Using a similar technique, NOAA scientists have been able to reconstruct global weather back to the early 19th century.

While ocean reanalyses have been around for a long time, they have only recently become skillful enough and have high enough resolution to examine ocean features, including bottom temperatures, near the coast.

The research team, from NOAA, CIRES, and NCAR, found that on the continental shelves around North America, bottom marine heat waves tend to persist longer than their surface counterparts, and can have larger warming signals than the overlying surface waters. Bottom and surface marine heat waves can occur simultaneously in the same location, especially in shallower regions where surface and bottom waters mingle.

But bottom marine heat waves can also occur with little or no evidence of warming at the surface, which has important implications for the management of commercially important fisheries. “That means it can be happening without managers realizing it until the impacts start to show,” said Amaya.

In 2015, a combination of harmful algal blooms and loss of kelp forest habitat off the West Coast of the United States—both caused by The Blob—led to closures of shellfisheries that cost the economy in excess of $185 million, according to a 2021 study. The commercial tri-state Dungeness crab fishery recorded a loss of $97.5 million, affecting both tribal and nontribal fisheries. Washington and Californian coastal communities lost a combined $84 million in tourist spending due to the closure of recreational razor clam and abalone fisheries.

In 2021, a groundfish survey published by NOAA Fisheries indicated that Gulf of Alaska cod had plummeted during The Blob, experiencing a 71% decline in abundance between 2015 and 2017. On the other hand, young groundfish and other marine creatures in the Northern California Current system thrived under the unprecedented ocean conditions, a 2019 paper by Oregon State University and NOAA Fisheries researchers found.

Unusually warm bottom water temperatures have also been linked to the expansion of invasive lionfish along the southeast U.S., coral bleaching and subsequent declines of reef fish, changes in survival rates of young Atlantic cod, and the disappearance of near-shore lobster populations in southern New England.

The authors say it will be important to maintain existing continental shelf monitoring systems and to develop new real-time monitoring capabilities to alert marine resource managers to bottom warming conditions.

“We know that early recognition of marine heat waves is needed for proactive management of the coastal ocean,” said co-author Michael Jacox, a research oceanographer who splits his time between 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 live and can experience heat waves quite different from those on the surface.”

More information:
Amaya, D.J. et al, Bottom marine heatwaves along the continental shelves of North America, Nature Communications (2023). DOI: 10.1038/s41467-023-36567-0.

Scientists identify heat wave at bottom of ocean (2023, March 17)

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