From Discovery: “Huge, Slow-Moving ‘Internal’ Waves Wreak Havoc”

Discovery News
Discovery News

Feb 6, 2015

Oceanographer Rob Pinkel unpacked crates of scientific instruments this week aboard the 272-foot research vessel Falkor while docked in the port of Hobart, on the island of Tasmania. He checked the weather and made preparations along with several dozen other scientific crew members to hunt for an elusive ocean phenomenon, massive “internal waves” that are born on the tidal straits of New Zealand, chug across the Tasman Sea, and bounce off the coastline of Tasmania.



“We are expecting to see a large wave in the interior of the Tasman Sea crossing and hitting the slopes of Tasmania,” said Pinkel, a physical oceanographer from the Scripps Institution of Oceanography. “We have some specialized instruments that can see what’s going on in a pretty fast time scale.”

Compared to fast-moving, wind-driven surface waves, internal waves are lumbering giants that can grow to more than 2,000 feet high and travel at 3 to six 6 miles per hour. They are found across the world’s oceans, including coastal California and the mid-Atlantic. These waves churn cold deep water with warm surface water like a gigantic washing machine. It takes a week for them to journey from New Zealand to Tasmania.

As they propagate, internal waves stir up sea bottom nutrients like nitrogen, a prime food source for tiny surface-dwelling plankton, small fish and the entire marine food chain above. Internal waves create havoc for submarines and oil rigs, drag ships across the surface like toy boats, and have reportedly killed unwary scuba divers by plunging them hundreds of feet deep in a matter of minutes.

To better understand how internal waves operate, about 60 researchers from the United States, Australia, New Zealand and several other nations have converged on Hobart and the Tasman Sea between now and mid-March for the Tasman Tidal Dissipation Experiment (TTIDE).

Scripps physical oceanographer Matthew Alford says that internal waves are an important link between the Earth’s climate and ocean. Climate modelers need to understand the mixing process in order to build more accurate forecasts of how much heat the ocean can store, and consequently how fast the Earth’s atmospheric temperature will rise in the future.

“They are like waves on the beach but they are down deep,” said Matthew Alford, a physical oceanographer at Scripps who just completed a 23-day expedition to the Tasman Sea above the R/V Revelle, an oceanographic research ship based in San Diego. “Understanding internal waves is a hot topic for oceanographers right now.”
VIDEO: Measuring Big Waves

To capture the waves, the scientists have erected a fenceline of oceanographic instruments called moorings across the Tasman Sea. The moorings are anchored to the seafloor on the bottom and have a floating buoy on top. Robotic instruments crawl up and down the mooring measuring ocean salinity, depth and temperature to track the passage of the turbulent internal waves.

“It’s almost like a firehose or a nozzle of wave energy that is directed and pointed directly at Tasmania,” Alford said from Hobart. “We want to see the birth of the waves, watch them across the sea, and reach this final stage. We are trying to watch that process underwater.”

Other researchers, like Peter Strutton, associate professor at the Institute for Marine and Antarctic Studies at the University of Tasmania, want to know what internal waves do to tiny marine plants (phytoplankton). He’s taking water samples from various depths to see how much nitrogen the plants consume as the waves pass.

“Our work will help determine the reasons for the high productivity in the Tasman Sea, which will in turn help us understand if productivity changes seasonally or on longer time scales,” Strutton said via e-mail from sea. “Ultimately we’d like to use this information to help predict how marine ecosystems may change in the future.”

See the full article here.

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