Waves within waves

by Leslie G. Baehr on 21 Dec 2014 The Waves Within the Waves - If the 30-foot wave we were looking for had tumbled across the ocean’s surface that July day, it might have been mistaken for a monstrous rogue wave but that’s not where this wave rolled. This wave surged within the ocean—deep beneath the surface.

Passing underneath us, the giant wave would reveal itself to our trained eyes only in the form of subtle alternating bands of rough and smooth water on the sea surface. But to most people, it would seem that the Atlantic was acting like its usual sleepy Monday self—irritable, but nothing very much out of the ordinary.

Scientists have a name for this phenomenon—internal waves. By definition, internal waves occur in the deep, well beyond people’s purview. Scientists have generated miniature internal waves in small tanks in the lab, but out in the wild, they’re as elusive as a white horse in a snowstorm.

Our boat rocked back and forth in the sticky midmorning sun as we waited for the sinuous beast to slither beneath our boat. It was 10:40 a.m., just thirty minutes into the four-hour time window when we expected to see the wave. Massachusetts Bay glistened—silver confetti on the navy blue sea—as though to celebrate the watery behemoth’s pending arrival.

'There’s a slick here, Jesús,' said Vicke Starczak, a researcher at Woods Hole Oceanographic Institution (WHOI), 'A couple of them, one at least.'

A 'slick,' or line of smooth water, usually preceded by a rough line of water, is the subtle footprint of an internal wave that manifests itself on the sea surface. Satellites see them as mammoth U-shaped forms radiating outward, but on the water we’d see only the rough-smooth bands stretching for miles until they bumped up against the horizon and faded into the sky.

'It does look like an internal wave slick. It might be,' said Jesús Pineda, a biologist at WHOI. 'It might not.'

Layers in the ocean
Pineda had set up camp on the port side of the boat, staring intently at his laptop, as he would do for the rest of the day. The screen displayed a picture of the ocean, gleaned from a scientific instrument called an echosounder that was hung off the back of the boat. Like an ultrasound device in a hospital, an echosounder uses sound waves to paint a picture of objects and structure in the ocean. Denser things, such as the seafloor, showed up in yellow, orange, or red colors on the computer screen. Most of the ambient water appeared a sapphire blue. The screen and its echosounder readings were our only real-time way to 'see' the internal waves, save for the slicks.

Other than some yellow-green blobs (fish), and the orange seafloor, one notable detail caught my eye on Pineda’s computer screen: a mysterious and diffuse turquoise layer from the surface down to about 15 feet. It gave the echosounder image the appearance of a bottle of unmixed salad dressing—turquoise oil sitting atop sapphire vinegar. The turquoise was warmer, less dense water sitting atop a sapphire layer of cooler, heavier water.

Where these layers met was known as the interface, Pineda explained. Off New England, this stratification of the ocean occurs only in the warm months of the year when the penetrating sun heats the top part of the sea.

Pineda glanced out the cabin window. Slicks appeared to be approaching. 'There’s one, there’s two, and there’s three,' he said. A large green blob appeared on Pineda’s screen. 'Oh, wow,' Pineda said. The blob got bigger. 'I just really hope this is the wave,' he said. The blob continued its journey left across the screen, but never developed the telltale signature squiggle of an internal wave.

'So maybe that was not the wave,' Pineda said.

'It was a lot of action whatever it was,' Starczak said.

‘Dead water’
In 1893 the Norwegian arctic explorer Fridtjof Nansen found his three-masted schooner held at a near-stop in Russia’s Kara Sea, 'as if by some mysterious force,' he wrote in his expedition diaries published as Farthest North. The ship would not answer to the helm. 'We made loops in our course, turned sometimes right around, tried all sorts of antics to get clear of it,' but to little effect. The ship would not budge. 'Dead water,' he called it.

The mysterious force beneath Nansen’s schooner was an internal wave. The water directly below his ship was less salty because melting sea ice added lighter fresh water at the ocean surface. A saltier, denser layer of water below completed the cocktail.

Then, the interface between the two layers did something sneaky. It stole the energy from Nansen’s ship.

'Put another way, some of the energy from the ship’s propulsion system was siphoned off into internal waves, rather than used to move the boat forward efficiently,' WHOI physical oceanographer Karl Helfrich said.

Internal waves are triggered when some energy is applied to the interface between different-density water layers, Helfrich explained. The water layers don't mix. The interface remains intact. The waves move along this interface much the way waves on the ocean surface move along the interface between two other fluids with different densities—air and water. For waves at the surface, winds often apply the trigger, lifting water that gravity forces downward again, creating a rippling wave.

Within the ocean, the trigger can be the propulsion energy from a ship, as it was in Nansen’s case, or it could be tidal motions. Or the interface can flow into a protruding seafloor feature like a mountain and run upward or dip downward like a car on a roller coaster.

Internal waves are so much larger than surface waves because it takes far more energy to 'lift' a mass of water into the air, than it does to lift that same mass of water into another less-dense mass of water, Helfrich said. Underwater, you get more bang for your energetic buck. In the China Sea, these waves can be on the order of 200 feet.

Because they are so large and globally ubiquitous, internal waves are hidden cogs in the inner clockworks of the ocean. They play powerful and still unknown roles in transferring heat, energy, water masses, and nutrients throughout the ocean.

Twelve decades after Nansen, here we were in Massachusetts Bay awaiting internal waves triggered by a large, deep feature sitting right below us in Massachusetts Bay. It wasn’t until fairly recently that anyone knew what that feature was.

Time and tides
Scientists began studying the internal waves of Massachusetts Bay in the 1970s. But while they could detect the presence of internal waves via echosounders, it was difficult to find the waves’ source. In the late ’70s, using satellite images, scientists discerned that internal waves emanating through the area were generated in the vicinity of (and were likely triggered by) Stellwagen Bank, a large underwater plateau. For almost four decades, Stellwagen Bank was the only known source of internal waves in Massachusetts Bay.

Then in 2008, WHOI visiting scientist José da Silva and Helfrich took another look at the satellite archives and noticed different waves. 'At first, he was looking at the waves from Stellwagen Bank,' Pineda said of da Silva, 'but eventually looking at all these images, it hit him that there were these waves that had not been described before.'

Internal waves were churning out of Race Point Channel—a cut in the ocean floor, 160 feet deep and several miles long, sandwiched by Stellwagen Bank to the north and the tip of Cape Cod to the south. Some of those waves intersected the southwestern tip of Stellwagen Bank, where we were looking for them on that Monday in July.

'More slicks,' said Pineda shortly before noon. 'This one looks real. This one has ripples behind it.'

If we did see the wave, Pineda said, it would appear on the screen as depression, because the wave didn’t veer upward, but moved downward like a U. Whether an internal wave moves upward or downward depends on the depth of the interface. If the interface is shallower than the total water depth, it leaves no room for the wave to go up, so it has to move down.

So, we were waiting for a U-shape to appear on the screen four to eight hours after low tide.

It would go like this: The outgoing tide would force water through the narrow Race Point Channel, converging like a ten-car pile up in a spot where four lanes merge into one. When the tide changed to incoming—6:10 a.m. on this particular day—the pileup would suddenly release, creating bumps or wiggles in the interface between density layers that would radiate outward and head back our way reaching us some time between about 10:10 a.m. to 2:10 p.m.

Where the internal wave swooped down, waters at the surface flattened and smoothed; where the internal wave rebounded up, waters in the top layer scrunched into less space, ruffling the surface. That’s what created those bands of rough and slick water on the surface that we were looking for.

'Oh, this looks like a slick. It’s just coming, right Vicke?' Pineda said. 'It might be, hmm?'

'It might be,' Starczak said.

'This might be it,' Pineda said excitedly, then quickly added, 'It’s better not to say anything.'

A waiting game
To spot an internal wave requires scientific equipment, of course, but also a boatload of persistence, patience and, perhaps most critically, luck. To the poet’s delight and the scientist’s lament, the ocean has always been the paradigm of impossible complexity. Science has never quite been able to predict ocean surface movements, much less the movements of internal waves.

Yes, tidal forces generate the internal waves and tides are predictable, but once the waves form, they are subject to an assembly line of ocean dynamics that can perturb them as they roll along. In the lab, internal waves can be generated and variables isolated, 'but the ocean doesn’t respect that,' Helfrich said. 'It says, ‘OK, you’ve got this theory. That’s fine, but I’m more complicated than that.’ '

'Once they generate, they are free to move,' Pineda said. 'Depending on many things, the waves go faster and slower.'

The speed of the waves (and therefore their arrival times) can differ depending on assorted factors: the net density difference of the layers above and below the interface, how deep the water is, the size of the wave (larger waves travel faster), the currents, etc.

Today, the internal waves appeared to be taking their sweet time. Starczak came over to glance at Pineda’s screen.

'Nada,' Pineda said.

'Nada,' Starczak said.
'But it did look like something,' he said.

'It did,' she replied as the slick that looked like something passed underneath us and the interface on the screen sat mockingly still.

Where the waves are ...

What was Pineda, a biologist, doing investigating physics and oceanography?

Around 2008, two things came across Pineda’s desk. The first was a map of the newly discovered Race Point internal waves, the waves studied by da Silva and Helfrich. The second were some figures on whale aggregations in the area. It appeared that both the waves and the whales had received an invitation to the same party, because both inhabited the exact same area.

'When I saw the map, and I saw the internal waves of my friend (da Silva), I said ‘Wow, that is a very interesting coincidence,’ ' Pineda said. Could the two be linked? he wondered. Internal waves concentrating plankton. Plankton attracting fish. Fish enticing whales.

It appeared that Pineda and his team were on to something. 'The first year we came out, it was like being in a circus. Whales off the side. Sand lance, and birds,' Starczak said. Sand lance are thin, silvery fish—a prey for humpback whales. 'We thought we had nailed it.'

That happened for two days straight. Seven years ago. Then it all stopped.

There were still internal waves, but there were hardly any whales or fish. After a lengthy bout of next to nothing on the whale count, and years of sitting in the same spot for about six days every year, something else caught Pineda's eye: dogfish.

'Lots of dogfish,' he said, or small sharks. And the sharks appeared to be responding to the internal waves. When the waves came, the dogfish huddled toward the bottom. Once the waves passed, the fish redistributed themselves, dispersing more evenly throughout the water column.

'When I started to be a scientist, I knew you have to have all these hypotheses and predictions and test the predictions,' Pineda said. 'When I grew up, I learned that in some cases, yes, that’s very powerful, but in some other cases, unexpected observations hit you in the head.'

Just passing through
So far today, there were no waves, few fish, and even fewer whales.
'It’s just the doldrums,' Starczak said, 'It’s so variable from year to year.'

High tide came and went with little action except for imitation internal wave slicks, a whale or two, and a few dogfish. I was beginning to think that maybe, as some of Pineda’s guest students liked to joke, there were no such thing as internal waves at all. 'Not today, I guess,' Pineda said.

It was about 3:20 p.m., an hour past our window for internal waves.

The heat had given way to a thick milky fog. Pineda got up to get his lunch from the boat’s miniature kitchen, then sat back down at his seat in front of the computer to guard the interface. Maybe it was because we had awakened at 4 a.m. to get here, or maybe it was the constant vigilance juxtaposed with disappointment, but time seemed to be passing in slow motion. And then …

'There’s sort of a slick out here,' Starczak said pointing over to starboard. We all glanced over and sure enough, on Pineda’s screen, the turquoise top layer seemed to be dipping into the sapphire bottom one in what would have scaled to a 30-foot U-shape. Then another one, almost an echo of the first. Looking outside, the boat appeared to be sandwiched between slicks.

Pineda scurried excitedly up the ladder to the bridge where two crew members and three volunteer whale spotters kept watch. 'It’s rougher, right?!' Pineda asked the crew as we rocked back and forth as though on a seesaw.

'It changed dramatically!' one volunteer said with wonderment.

'We really started to rock,' said another.

It felt as though a mythical beast had past through our midst. We had seen and felt an internal wave.
But we had seen few whales. 'I wish there was a relationship with internal waves and the whales,' but there wasn’t, Pineda said. 'Not here.'

That didn’t mean Pineda was done with internal waves. 'We have so much data. Beautiful data!' he said—that he and his colleagues will begin to investigate why Stellwagen Bank is such a famously fecund commercial fishing ground. 'It’s physics,' said Pineda, the biologist, 'but what physics?'

How internal waves work on Stellwagen Bank and how they may affect the region’s productivity are still uncharted waters.

'How can you even begin to start looking at something with so many variables?' asked one of the volunteers aboard.
'Start with one thing at a time,' Pineda said, sitting back into his chair to keep a watchful eye on the interface.


This research was supported by WHOI Sea Grant, the WHOI Marine Mammal Center, the Stellwagen Bank National Marine Sanctuary, and volunteer marine mammal observers who participated in the cruises.