Instruments carried by migrating elephant seals measured deep warm-water anomalies that lasted much longer than surface warming

The North Pacific Blob, an obscure maritime heat that started late 2013 and continued until ‘ in 2015, was the largest and longest maritime heat wave on record. A new study using data collected by elephant seals finds that in addition to the well-documented global warming, the deeper warm water anomalies associated with the Blob were much more extensive than previously reported.

The new findings were reported by a team of biologists and oceanologists from UC Santa Cruz in a July 4 posting in the Journal of Geophysical Investigate: Oceans.

“Elephant seals collect data in places different from existing oceanographic platforms,” explained lead author Christopher Edwards, professor of ocean science at UC Santa Cruz. “This is an underutilized dataset that can tell us about important oceanographic processes and help biologists understand the ecology of northern elephant seals.”

For decades, elephant seal researchers at UCSC, led by co-author Daniel Costa, professor of ecology and evolutionary biology and director of the Institute of Marine Science at UCSC (IMS), used advanced tagging technology to track months-long migrations of elephant seals in the North Pacific Ocean.

“While the seals have been used for a this time to study the physical oceanography of the polar regions, this is one of the first studies to use data collected by seals to answer physical oceanographic questions in temperate regions, such as the North Pacific Ocean” , said Costa..

The sensors worn by the animals record nt depth, temperature and salinity as the animals repeatedly dive to great depths during migrations of approximately 6 000 miles across the Pacific North.

“Female elephant seals go out to the open sea where a ship may pass and only collect data once in a while, while we have elephants of sea collecting data everywhere,” said Rachel Holser, first author and IMS research biologist. “It’s unusual to have this sort of data at the resolution we have in time and space, and at depths less than several hundred meters.”

Elephant seal data collected during the Blob revealed abnormally warm temperatures extending to 1 000 meters (3 000 feet) below the surface. Subsurface warming persisted in 2017, long after area temperatures returned to normal.

The Blob was well studied with respect to the heating of the area, which was driven by atmospheric conditions and decreased at the end of 2015. The vast subsurface warming raises questions about the mechanisms underlying it, Edwards said.

“These temperature anomalies are so deep that they are unlikely to are due to a mixing of the floor,” he said. “A reasonable mechanism is that the exceptionally warm waters were transported north from further south. What we don’t yet know is whether this northward transport is directly or indirectly related to the warming of the area. Changes to the area may have transiently altered deeper currents to draw southerly waters northward.”

Marine heat waves are expected to increase in frequency, magnitude and in duration as global temperatures continue to rise. These events can have significant impacts on marine life, as well as economic consequences for local communities that depend on fisheries and marine ecosystems. Understanding the physical processes involved in marine heatwaves will help scientists predict their onset and development and enable people to anticipate and cope with ecological and economic consequences.

“Just as with terrestrial heat waves, we have seen a dramatic increase in the frequency and magnitude of marine heat waves over the past decade,” Holser said. “The more information we can gather, the better off we will be in terms of understanding what’s going on and meeting the challenges. This study shows the interest of collaborating with elephant seals to collect oceanographic data complementary to other methods.”

In addition to Holser, Edwards and Costa, graduate ocean science student Theresa Keates also contributed to the study and is a co-author of the paper. This work was supported by the US Office of Naval Investigation and the Central & Northern California Ocean Observing Aystem (CeNCOOS). With continued support from CeNCOOS, the Costa lab, in collaboration with Edwards and ocean science professor Raphael Kudela, go on to collect oceanographic data using elephant seals.

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