Tides, Earth's rotation among sources of giant underwater waves

Scientists at the University of Rhode Island are gaining new insight into the mechanisms that generate huge, steep underwater waves that occur between layers of warm and cold water in coastal regions of the world's oceans.David Farmer, a physical oceanographer and dean of the URI Graduate School of Oceanography, together with student Qiang Li, said that large amplitude, nonlinear internal waves can reach heights of 150 meters or more in the South China Sea, and the effects they have on surface wave fields ensure that they are readily observable from space.Farmer and Li will report results of their research at the Ocean Sciences Meeting of the American Geophysical Union in Portland, Ore., on February 25."The large waves in the South China Sea have attracted a fair bit of attention in recent years," Farmer said, "but much of this has been directed at the interaction of the waves with the sloping continental shelf of mainland China where they break, overturn and produce intense mixing. Our focus is on the way in which they are generated in Luzon Strait, between Taiwan and the Philippines, and the way they evolve as they propagate westwards across the deep ocean basin of the South China Sea."Farmer and Li studied the evolution of large internal waves occurring at tidal periods generated by currents traversing submarine ridges in Luzon Strait. As these waves travel west through the South China Sea, they steepen and evolve into packets of steep, energetic waves occurring at periods of 20-30 minutes. It is these energetic short period waves that modulate the ocean surface roughness, making their presence observable from satellites in space.The URI scientists' observations showed that the Earth's rotation modifies internal waves as they travel cross the deep basin. This effect mainly influences the internal waves that form on the 24-hour period of diurnal tides, dispersing the energy and inhibiting the steepening process. Internal waves that form on the semi-diurnal tides are not affected in this way, are more readily steepened and then break into the energetic, short period waves.Farmer and Li studied internal waves in the South China Sea using pressure equipped inverted echo-sounders, instruments developed by scientists at the University of Rhode Island. From the seafloor, the device transmits an acoustic pulse and then listens for the echo from the sea surface. Sound travels faster through warm water than it does through cold water, so changes in the echo delay allow measurement of the thickness of the warm surface layer, enabling the shape and size of passing internal waves to be recorded.According to Farmer, nonlinear internal waves impact the ocean in many ways: stirring up sediment on the sea floor, creating hazards to offshore engineering structures, interfering with submarine navigation, and greatly affecting propagation of underwater sound. Internal waves also appear to have significant, if not fully understood, biological impacts, and in shallow water environments they can mix water masses and modify coastal circulation. University of Rhode Island

Sea Star Swells With Tides

A species of sea star has figured out a novel way of keeping cool on rocky shorelines. The animal literally soaks up chilly water during high tides to protect itself from the blazing temperatures that persist when the tide goes out, scientists announce today. Sea stars live at the ocean edge on rocky shorelines, and so they endure rapid changes in temperature as the tide comes in, covering them with chilly water, and then recedes to leave them bare to the sun's rays. "Sea stars were assumed to be at the mercy of the sun during low tide," said the lead study researcher Sylvain Pincebourde of François Rabelais University in Tours, France. "This work shows that some sea stars have an unexpected back-up strategy." Until now, scientists were not sure how the stars beat the heat. But Pincebourde suspected that perhaps fluid-filled cavities in the star's arms might play a role. So they placed so-called ocher sea stars, or Pisaster ochraceus, into aquariums kept at different temperatures and changed the water level to mimic tides. The animals exposed to higher temperatures were essentially bigger, or had a larger body mass, after the following high tide. The researchers figured that since the stars hadn't eaten, the mass must have been from the water. "This reservoir of cool water keeps the sea star from overheating when the tide recedes again the next day," Pincebourde said. The sea stars are likely cued during low tide that it's a hot day, the researchers say, and that signals them to soak up more water during the next high tide. "It would be as if humans were able to look at a weather forecast, decide it was going to be hot tomorrow, and then in preparation suck up 15 or more pounds of water into our bodies," said study researcher Brian Helmuth of the University of South Carolina in Columbia. That's fine and dandy for now, but the researchers say they worry that this cooling process may not hold up in a warming world. "This strategy only works when the sea water is colder than the air," said study researcher Eric Sanford of the University if California, Davis. "Ocean warming might therefore break down this buffering mechanism, making this sea star susceptible to global warming. There are likely limits to how much this mechanism can buffer this animal against global change." Another recent study, however, found that P. ochraceus might grow faster as the water warms. The new research is published in the December issue of The American Naturalist.

Oregon researcher says West tides increasing

An Oregon researcher says the tides are getting bigger along the West Coast, possibly speeding up erosion.Portland State University engineering professor David Jay says the amplitude of tides from Alaska to Mexico has increased - meaning the change between the highest and lowest tides is bigger.Jay says global climate change is likely to blame - along with rising sea levels.But scientists had previously believed the tides remained constant because they are driven by gravitational forces between the Earth and the moon.The Portland State study showed one of the biggest changes in the tides has been in Astoria - with the range increasing by nearly a foot a century.