'The Blob' deep below may be triggered by intense seafloor heatwaves

Compared to its surficial twin, it could be more powerful and last longer.
Sade Agard
The western Atlantic Ocean Basin

Between 2013-2016 a monstrous marine heat wave known as 'The Blob' warmed a vast expanse of surface waters across the northeastern Pacific. 

Around $90 million was lost by commercial fisheries as a result of its prolonged presence, which disrupted the marine ecosystems along the West Coast and decreased salmon returns.

But little did scientists know that marine heat waves also happen deep underwater- until now.

That is, researchers have now created the first comprehensive assessment of 'bottom marine heat waves' in the productive continental shelf waters encircling North America, according to a new paper published in Nature.

'The Blob' - deep under

"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, in a press release

"This is the first time we've been able to really dive deeper and assess how these extreme events unfold along shallow seafloors."

The so-called bottom marine heatwaves can be more powerful and continue longer than hot spells at the ocean surface. However, it varies from coast to coast, according to their examination. 

They may also occur together with heat waves on the surface and show little to no evidence of surface warming. "That means it can be happening without [fishing] managers realizing it until the impacts start to show," explained Amaya. 

The study comes amid concerns about increasingly common marine heat waves, which significantly negatively influence the global health of ocean ecosystems.

Over the past century, the ocean has warmed by roughly 1.5 degrees Celsius (2.7 degrees Fahrenheit), absorbing nearly 90 percent of the extra heat caused by global warming.

Abnormally warm bottom water temperatures have been linked to changes in the survival rates of young Atlantic cod, the spread of invasive lionfish along the southeast coast of the United States, coral bleaching, and subsequent declines in reef fish. Not to mention, the disappearance of near-shore lobster populations in southern New England.

'The Blob' deep below may be triggered by intense seafloor heatwaves
Ling cod, like this one caught off of Humboldt Bay, are vulnerable to impacts from bottom marine heat waves.

Shellfisheries were also shut down because of 'The Blob,' which off the US West Coast resulted in a combination of harmful algal blooms and a loss of kelp forest habitat. Around $185 million was lost by the economy as a result, according to a 2021 report.

"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. 

"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."

The full study was published in Nature and can be found here.

Study abstract:

Recently, there has been substantial effort to understand the fundamental characteristics of warm ocean temperature extremes—known as marine heatwaves (MHWs). However, MHW research has primarily focused on the surface signature of these events. While surface MHWs (SMHW) can have dramatic impacts on marine ecosystems, extreme warming along the seafloor can also have significant biological outcomes. In this study, we use a high-resolution (~8 km) ocean reanalysis to broadly assess bottom marine heatwaves (BMHW) along the continental shelves of North America. We find that BMHW intensity and duration varies strongly with bottom depth, with typical intensities ranging from ~0.5 °C–3 °C. Further, BMHWs can be more intense and persist longer than SMHWs. While BMHWs and SMHWs often co-occur, BMHWs can also exist without a SMHW. Deeper regions in which the mixed layer does not typically reach the seafloor exhibit less synchronicity between BMHWs and SMHWs.

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