Marine heat waves in surface waters, capable of altering ecosystems, also occur in the depths of the ocean.
In an article published in the journal Nature Communications, a team led by NOAA researchers used a combination of observations and computer modeling to generate the first comprehensive assessment of marine heat waves at the bottom of continental shelf productive waters that surround North America.
“Researchers have been investigating marine heat waves at the sea surface for more than a decade,” said lead author Dillon Amaya, a research scientist in NOAA’s Physical Sciences Laboratory. “This is the first time we’ve been able to really dive deeper and assess how these extreme events play out along the shallow seafloor.”
Marine heat waves dramatically impact the health of ocean ecosystems around the world, disrupting the productivity and distribution of organisms as small as plankton and as large as whales. As a result, there has been considerable effort to study, track, and predict the timing, intensity, duration, and physical drivers of these events.
Most of that research has focused on extreme ocean surface temperatures, for which there are many more high-quality observations taken by satellites, ships, and buoys. Sea surface temperatures can also be indicators of many physical and biochemical characteristics of the oceans of sensitive marine ecosystems, making analyzes easier.
Around 90% of the excess heat from global warming has been absorbed by the ocean, which has warmed by around 1.5°C over the last century. Marine heat waves have become 50% more frequent over the past decade.
In recent years, scientists have increased efforts to investigate marine heat waves across the entire water column using the limited data available. But previous research did not point to extreme temperatures at the bottom of the ocean along the continental shelves, which provide critical habitat for important commercial species such as lobsters, scallops, crabs, flounder, cod and other groundfish.
Due to the relative paucity of bottom water temperature data sets, the scientists used a data product called “reanalysis” to make 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 climate since the early 19th century.
While ocean reanalyzes have been around for a long time, it’s only recently that they’ve become dexterous enough, and have high enough resolution, to examine ocean features, including bottom temperatures, close to shore.
The research team found that on the continental shelves around North America, deep-sea heat waves tend to persist longer than their surface counterparts and may 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 mix.
But seafloor heatwaves can also occur with little or no evidence of surface warming, which has important implications for the management of commercially important fisheries. “That means it may be happening without fisheries managers realizing it until the impacts start to show.”Amaya said.