"Long before it's in the papers"
January 08, 2014


Study said to explain giant underwater waves

Jan. 8, 2013
Courtesy of the Massachusetts Institute of Technology
and World Science staff

You can’t see them on a tur­bu­lent ocean sur­face, where they pro­duce a rise of just inches. But in­ter­nal waves, hid­den to­tally with­in the ocean, can tow­er as high as sky­scrap­ers, with pro­found ef­fects on cli­mate and on ocean ecosys­tems, sci­en­tists say.

Now new re­search, both in the ocean and in the larg­est-ever lab­o­r­a­to­ry ex­pe­ri­ments to in­ves­t­i­gate them, is said to solve a long­stand­ing mys­tery about just how the larg­est known in­ter­nal waves, in the South Chi­na Sea, form.

They’re shaped like sur­face waves. But in­stead of form­ing where wa­ter meets air, they form where dif­fer­ent lay­ers of wa­ter meet. The dif­fer­ence be­tween an un­derwa­ter wave and the wa­ter around it is its dens­ity, due to tem­pe­r­a­ture or salin­ity dif­fer­ences that cause ocean wa­ter to form lay­ers.

In­stru­ments can de­tect the bound­a­ry be­tween colder, salt­i­er wa­ter be­low and warm­er, less-salty wa­ter above. That bound­a­ry can re­sem­ble the ocean’s sur­face, pro­duc­ing waves that reach tow­ering heights, trav­el vast dis­tances, and can play a key role in the mix­ing of ocean wa­ters, help­ing drive warm sur­face wa­ters down­ward and draw­ing heat from the at­mos­phere, sci­en­tists say.

The new find­ings come from a team in­volv­ing the Mas­sa­chu­setts In­sti­tute of Tech­nol­o­gy and oth­er in­sti­tu­tions, and co­or­di­nated by the U.S. Of­fice of Na­val Re­search. 

Be­cause in­ter­nal waves are hard to de­tect, it’s of­ten hard to study them di­rectly in the ocean. Thom­as Pea­cock, a me­chan­i­cal en­gi­neer at MIT, joined oth­er re­search­ers in the stu­dy, pub­lished in the jour­nal Geo­phys­i­cal Re­search Let­ters. They did lab­o­r­a­to­ry ex­pe­ri­ments to study the pro­duc­tion of in­ter­nal waves in the Lu­zon Strait, be­tween Tai­wan and the Phil­ip­pines. “These are the most pow­er­ful in­ter­nal waves dis­cov­ered thus far in the ocean,” Pea­cock said. “These are skyscrape­r-scale waves.”

The sol­i­tary waves have been meas­ured to reach heights of 170 me­ters (more than 550 feet) and can trav­el at a lei­surely pa­ce of a few centime­ters per sec­ond. “They are the lum­ber­ing gi­ants of the ocean,” Pea­cock said.

The large-scale lab­o­r­a­to­ry ex­pe­ri­ments on the genera­t­ion of such waves used a de­tailed mod­el of the Lu­zon Strait’s seafloor, mount­ed in a 50-foot-wide ro­tat­ing tank in Gre­no­ble, France, the larg­est such facil­ity in the world. The tests in­di­cat­ed the waves are gen­er­at­ed by the en­tire ridge sys­tem on that ar­ea of seafloor, and not a lo­cal­ized hotspot with­in the ridge.

The last ma­jor field pro­gram of re­search on in­ter­nal-wave genera­t­ion took place off the coast of Ha­waii in 1999. In the years since, Pea­cock said, sci­en­tists have come to a great­er ap­precia­t­ion of the sig­nif­i­cance of these gi­ant waves in the mix­ing of ocean wa­ter—and there­fore in glob­al cli­mate.

“It’s an im­por­tant mis­sing piece of the puz­zle in cli­mate mod­eling,” Pea­cock said. “Right now, glob­al cli­mate mod­els are not able to cap­ture these pro­cess­es,” he said, but it’s im­por­tant to do so: “You get a dif­fer­ent an­swer … if you don’t ac­count for these waves.” To help in­cor­po­rate the new find­ings in­to these mod­els, the re­search­ers plan to meet this month with a cli­mate-mod­eling team as part of an ef­fort spon­sored by the Na­t­ional Sci­ence Founda­t­ion to im­prove cli­mate mod­eling.

These waves may be “the key mech­an­ism for trans­fer­ring heat from the uppe­r ocean to the depths,” Pea­cock said.

In­ter­nal waves have been known for well over a cen­tu­ry, Pea­cock said, but re­mained poorly un­der­stood be­cause of the dif­fi­cul­ty of ob­serva­t­ions. Among the new tech­niques that have helped is the use of sat­el­lite da­ta: While the sub­merged waves raise the wa­ter sur­face by less than an inch, long-term sat­el­lite da­ta can clearly dis­cern this dif­fer­ence.

“From 15 years of da­ta, you can fil­ter out the noise,” Pea­cock ex­plains: Many loca­t­ions, such as the Lu­zon Strait, gen­er­ate these waves in a steady, predicta­ble way as tides flow over sub­merged ridges and through nar­row chan­nels. A re­sult­ing 12-hour cy­cle is clearly vis­i­ble in sat­el­lite da­ta, he added.

In­ter­nal waves can al­so play a sig­nif­i­cant role in sus­tain­ing coral-reef ecosys­tems, by bring­ing nu­tri­ents up from ocean depths, Pea­cock said.

* * *

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You can’t see them on a turbulent ocean surface, where they produce a rise of just inches. But internal waves, hidden totally within the ocean, can tower hundreds of feet, with profound effects on climate and on ocean ecosystems, scientists say. Now new research, both in the ocean and in the largest-ever laboratory experiments to investigate them, is said to solve a longstanding mystery about just how the largest known internal waves, in the South China Sea, form. They’re shaped like surface waves. But instead of forming where water meets air, they form where different layers of water meet. The difference between an underwater wave and the water around it is its density, due to temperature or salinity differences that cause ocean water to form layers. Instruments can detect the boundary between colder, saltier water below and warmer, less-salty water above. That boundary can resemble the ocean’s surface, producing waves that reach towering heights, travel vast distances, and can play a key role in the mixing of ocean waters, helping drive warm surface waters downward and drawing heat from the atmosphere, scientists say. The new findings come from a team involving the Massachusetts Institute of Technology and other institutions, and coordinated by the U.S. Office of Naval Research. Because internal waves are hard to detect, it’s often hard to study them directly in the ocean. Thomas Peacock, a mechanical engineer at MIT, joined other researchers to in the new study, published in the journal Geophysical Research Letters. They did laboratory experiments to study the production of internal waves in the Luzon Strait, between Taiwan and the Philippines. “These are the most powerful internal waves discovered thus far in the ocean,” Peacock said. “These are skyscraper-scale waves.” The solitary waves have been measured to reach heights of 170 meters (more than 550 feet) and can travel at a leisurely pace of a few centimeters per second. “They are the lumbering giants of the ocean,” Peacock said. The large-scale laboratory experiments on the generation of such waves used a detailed model of the Luzon Strait’s seafloor, mounted in a 50-foot-wide rotating tank in Grenoble, France, the largest such facility in the world. The experiments indicated the waves are generated by the entire ridge system on that area of seafloor, and not a localized hotspot within the ridge. The last major field program of research on internal-wave generation took place off the coast of Hawaii in 1999. In the years since, Peacock said, scientists have come to a greater appreciation of the significance of these giant waves in the mixing of ocean water — and therefore in global climate. “It’s an important missing piece of the puzzle in climate modeling,” Peacock said. “Right now, global climate models are not able to capture these processes,” he said, but it’s important to do so: “You get a different answer … if you don’t account for these waves.” To help incorporate the new findings into these models, the researchers plan to meet this month with a climate-modeling team as part of an effort sponsored by the National Science Foundation to improve climate modeling. These waves may be “the key mechanism for transferring heat from the upper ocean to the depths,” Peacock said. Internal waves have been known for well over a century, Peacock said, but remained poorly understood because of the difficulty of observations. Among the new techniques that have helped is the use of satellite data: While the submerged waves raise the water surface by less than an inch, long-term satellite data can clearly discern this difference. “From 15 years of data, you can filter out the noise,” Peacock explains: Many locations, such as the Luzon Strait, generate these waves in a steady, predictable way as tides flow over submerged ridges and through narrow channels. A resulting 12-hour cycle is clearly visible in satellite data, he added. Internal waves can also play a significant role in sustaining coral-reef ecosystems, by bringing nutrients up from ocean depths, Peacock said.