The Delicate, Detailed, and Chilling Dynamics of Underwater Waves

Swimmers at Scripps Beach in La Jolla sometimes experience a water temperature drop of about 11 degrees Celsius (12 degrees Fahrenheit) in a few seconds.

A sharp boundary between the leading edge of warm and cold water delivers that bracing change. If the swimmers were to look beneath the surface, they might also see clouds of fine sediments kicked up by the turbulent core of the gently flowing mass that delivered the chilling wave.

“Everybody who comes to the beach in La Jolla has seen these beautiful, organized bands of rough and smooth water,” said Drew Lucas, an oceanographer at UC San Diego’s Scripps Institution of Oceanography.

Known as internal waves, they are the undersea cousin of the well-known ocean surface waves that crash along the coast. For more than a decade, Lucas and fellow Scripps physical oceanographer Robert Pinkel have studied how internal waves are born, live, and die in the shallow waters near Ellen Browning Scripps Memorial Pier.

Research into internal waves from the Scripps Pier has a nearly 50-year history, beginning with work led by Clint Winant, now an emeritus professor at Scripps Oceanography.

“But exactly how these internal waves evolved in shallow water and how this evolution fits in with the pantheon of physical processes we are aware of was unclear,” Pinkel said.

Now Lucas and Pinkel, who are both Scripps Oceanography PhD alumni, have cleared up many of the murky details lurking beneath the waters of Scripps Beach using a new technology known as fiber optic distributed temperature sensing (DTS). Lucas and Pinkel reported their findings in a recent issue of the Journal of Physical Oceanography.

The innovative use of the temperature-sensing technology led to the discovery of intricate, small-scale three-dimensional patterns in the internal waves as they approached the Scripps shoreline. 

Developed for industry, DTS fiber-optic cable systems are often deployed in oil wells or along oil pipelines to monitor the temperature of their equipment. Scientists have used them in streams to make temperature measurements along the stream path. Pinkel also took the system to the equator where he used its ability to monitor ocean surface temperatures to detect the propagation of internal waves far beneath the surface.

“These were the first saltwater uses of DTS to do ocean science,” he said.

Deployed off Scripps Pier, the DTS system measured water temperature variability along the seafloor every meter at two-second intervals over the length of the two-kilometer (1.2-mile)-long fiber-optic cables. The cables, anchored to the pier and the seafloor, ran both parallel and at a right angle to the shore.

The system documented how the patterns of temperature on the seabed reflected passing internal waves. To complement the seabed observations, the team also deployed a moored Wirewalker wave-powered vertical profiler. The two in combination brought the focus Lucas and Pinkel needed.

Underwater waves crashing on the underwater beach

When ocean surface waves approach the beach, they evolve from their offshore, smoothly varying shape, steepening and gaining size. When the water is shallow enough, the waves break, losing their energy and creating turbulence. This wave breaking, well known to surfers and beachgoers, is important for internal waves as well. Previous research by Scripps scientists has documented how breaking internal waves in the deep ocean contribute to the ocean circulation and climate.

In the coastal ocean, internal waves surging up the seabed stirs up sediments and contribute to nearshore dynamics by providing the nutrients necessary for phytoplankton to carry out photosynthesis. La Jolla Submarine Canyon, just offshore of Scripps, is a known hotspot for internal waves and turbulence that facilitates an incredible richness in marine productivity.

“The nutrients are supplied from the bottom up via turbulent mixing,” said Lucas, who also is a faculty member at UC San Diego’s Jacobs School of Engineering Department of Mechanical and Aerospace Engineering.

“Understanding the transformation of energy from internal waves to turbulence is key to understanding how the coastal ecosystem functions,” he said.

Before this study, most of what was known regarding the details of shallow water breaking of internal waves came from supercomputer simulations. The results of those simulations were difficult to validate with experiments because the transformations occurred quickly and manifested as complex spatial patterns that were much smaller than those resolved by typical oceanographic measurements.

“We knew that these internal waves coming up on the underwater beach must evolve and ultimately break down, so we tried to shed light on the process using the coherent, continuous, and detailed sensing permitted by the DTS system,” Lucas said.

What the researchers found – an organized structure to the surging underwater waves – surprised them.

“It took me several years to believe it,” Lucas added, whose effort was supported by a Office of Naval Research Young Investigator Fellowship.

Unlike breaking surface waves that collapse into a turbulent mess on beaches, the unstable internal waves shoaling on Scripps Beach leave regularly structured vortices in their wake instead.

“As a shoaling internal wave got steeper and steeper, it didn’t go chaotic.” Pinkel said. “Instead, it developed a whole new side to its personality.” 

Said Lucas, “I don’t think we can say categorically that turbulent overturning similar to surface wave breaking never happens here, but we don’t see much evidence of it in our measurements.”

Instead, the newly discovered trailing vortex-like wake appears to drain energy from the wave, reducing its tendency for catastrophic breaking. At the same time, the currents caused by these waves have profound implications for how and where nutrients and sediments are exchanged.

“The beauty in this technique is the opportunity to get a glimpse of internal waves transforming in shallow water,” Lucas said. “It’s the first time we have been able to unambiguously track an individual wave from offshore to the shallow waters at the end of the pier. It is the end-stage of a much larger, multiscale process. But the details matter to the entire coastal ecosystem.”

It has become an adage among scientists that they know more about the surface of the moon and Mars than they do about the undersea domain. But Lucas and Pinkel have filled some of that gap by probing the waters offshore of Scripps Beach.

“Throughout my lifetime it’s been intriguing to develop tools to sense a greater and greater number of ocean phenomena and to be able to make sense of them,” said Pinkel, who received his PhD from Scripps in 1975. Given that the oceans measure 10,000 miles across and average three miles in depth, “That’s an accomplishment for our field.”

Added Lucas, “We’re still discovering how the ocean works. There’s a lot to learn, even in our own front yard.”