The small research vessel Bob and Betty Beyster recently spent a week near the submerged La Jolla Canyon, dragging sensitive marine instruments up and down the ocean’s water column.
UC San Diego's Scripps Institution of Oceanography (SIO) scientists are spending time examining the environment around the canyon hoping to unlock the ocean dynamics that make the abundant sea life there possible.
The team uses a winch system to raise and lower a device called a CTD which measures conductivity and temperature at different depths.
“We’re just yo-yoing it. Driving nonstop transects of the canyon back and forth,” said Sara Goheen, an oceanographic engineer at the SIO.
She helped build or improve much of the equipment being used to quantify what is happening in the water. The team is looking for traces of dye released a few days earlier near the canyon.
“Letting us measure basically how the seafloor here is shaping what the water does when it’s hitting there and how the energy is dissipating or how energy is building up more. Or basically just what the canyon does to the area,” Goheen said.
The 4-foot-long tube that was gently lowered into the ocean resembles a model rocket.
It is attached to a winch that can raise and lower the roughly 100 pound CTD device quickly through the different ocean layers.
The first device drops as quickly as gravity can pull it down and it comes up as fast as the winch can pull it.
That gives researchers a snapshot of the water column from surface to ocean floor.
A second CTD deployed later in the day is designed to move much slower through the water column.
“It falls at a half a meter a second,” Goheen said. “It’s got these umbrella wings that actually open up in the water column so that it has a nice steady descent rate and it free falls.”
As the weight of the device pulls it down, the umbrella-like canopy opens up and slows it down.
“And we need it to free fall nice and slow so that the sensors on the tip of it — we have two different ones, one for sheer and one for micro temperature — can go down through the water column almost like a phonograph needle,” Goheen said.
And the information gathered on the vessel will help researchers understand the different layers of the ocean and how turbulence affects them.
“This information is providing us with fundamental fluid mechanics about how the ocean works that is super climate relevant.”Matthew Alford, Scripps Institution of Oceanography
The warmer water near the surface is separated from deeper, colder water by the thermocline. And it is also where the dyes have been sitting for nearly a week.
In addition to the CTD devices, researchers are adding audio snapshots of the water column thanks to an acoustic device strapped to the side of the ship.
“We can’t use lasers, things that use light in water,” said post-doctoral researcher Liz Weidner. “Because as you know probably by looking into the ocean, you can’t see very far down, so light attenuates very, very quickly in water.”
The acoustic signals are sent to the bottom of the ocean every few minutes. The return echo helps fill in the gaps that the larger instruments miss.
“We can also track the position of the thermocline. We can probably track it with very high resolution,” Weidner said. “We’re probably having an acoustic profile every one to five meters, which is much higher resolution than the CTD that we’re dropping and bringing back up. That’s maybe every hundred meters.”
And for a solid week the vessel spent every day with instruments in the water tracking the dye in the constantly moving ocean water.
In the Multiscale Ocean Dynamics lab on the SIO campus, researcher Matthew Alford hopes the data gathered around the canyon will help explain how underwater turbulence works there.
“This information is providing us with fundamental fluid mechanics about how the ocean works that is super climate relevant,” Alford said.
Oceanographers think the ocean near La Jolla is so vibrant with life because underwater turbulence brings nutrients from the deeper waters to the warmer surface layer of the ocean. That feeds all manner of plant and animal life.
Alford points to a simulation playing on a laptop computer.
“Simulations like this one have been done for years, but for this one I really like the structures in the dye,” Alford said. “You can see how incredibly complicated the turbulent structures are.”
Alford compared the underwater waves hitting the submerged slope to the ones surfers might see on the surface. The tidal and wind-driven ocean forces hitting the slope of the canyon drive the water’s movement.
“This dye begins at the boundary, but very quickly, due to convergences in this process get shot out into the interior,” Alford said. “This three-dimensional aspect of the turbulence is not captured in the models that we have right now.”
In the week-long experiment, the dye can tell researchers about the vigorous and complex turbulence that feeds life near the canyon. It is a small step toward understanding larger ocean processes that can affect things like climate.
“Climate models have grid cells, you know, they divide the ocean into grid cells and the smallest grid cell in a state-of-the-art climate model is San Diego County sized so everything that happens in La Jolla Canyon is not represented properly. So we have to teach them how to represent that in what’s called parameterizations,” Alford said.
Better climate models, Alford said, will lead to better understanding of the changes that are coming as the oceans warm with the rest of the planet.
