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All quiet on the El Nino front.

All quiet on the El Nino front

When meteorologists gazed into their crystal ball of Pacific Ocean surface temperatures earlier this year, they thought they saw the signs of an impending El Nino--a warming of the Pacific waters that every several years disrupts climate patterns worldwide (SN:3/22/86, p. 184). But now the signs have died off and "the probabilities [of having an El Nino] this year are getting lower and lower every day,' says Eugene M. Rasmusson of the Climate Analysis Center at the National Oceanic and Atmospheric Administration (NOAA) in Silver Spring, Md.

In February, the Climate Analysis Center established an El Nino watch after the Pacific waters off the coast of South America began to warm. Had an El Nino developed, meteorologists would have expected this warming of the eastern Pacific to continue through May or June. Instead, the warming, which was rather mild, stopped after the first weeks in March.

In the event of an El Nino, scientists also would have expected to see certain changes occurring in the western Pacific in April, such as a drop-off in easterly winds, a shifting of the heavy rainfall areas near Indonesia and a warming of the ocean near the date line. "There have been some little intermittent pushes in that direction,' says Rasmusson. In addition, high rainfall, high mortality among young birds and the appearance of warm-water-loving Portuguese man-of-wars and sea snakes in the Galapagos Islands during March hinted at the onset of an El Nino. "But so far there's been nothing at all that's strong enough to indicate that this event was continuing,' he says.

Because the 1982-83 El Nino developed in June--unusually late for an El Nino-- scientists will probably wait a month to give their final prognosis. "So we can't write if off completely yet, but the time is getting short,' says Rasmusson.

While the symptoms of an El Nino were quieting down, a group of researchers at NOAA's Atlantic Oceanographic and Meteorological Laboratory in Miami, Fla., has been developing a technique for using the sound level in a layer of the Pacific as an indicator for El Ninos. Physicist David R. Palmer and his colleagues believe that during an El Nino a layer of water 50 to 280 meters deep becomes very quiet--that few low-frequency sound waves, such as those generated by ships, travel through it. The researchers presented their findings at the recent meeting in Cleveland of the Acoustical Society of America.

The speed at which a sound wave travels in the ocean depends on the temperature and pressure of the water, each of which changes with depth. Normally the temperature and pressure profiles in the equatorial Pacific are such that sound rays moving toward either the surface or the bottom are deflected--just as light rays are bent when they pass from air into a prism--and head back in the other direction before they reach their original destination. As a result, the sound waves can travel long distances without losing energy by crashing into the bottom.

During an El Nino, however, the additional warming of the upper layers of the ocean causes sound waves near the surface to be deflected downward at much steeper angles than usual, sending them into the bottom. Palmer's group used temperature and salinity measurements, collected by NOAA researchers over a three-year period that included the 1982-83 El Nino, to calculate in a computer model the El Nino's effect on low-frequency sound propagation. The researchers conclude that the intensity of sound in the upper Pacific layer should have decreased by as much as 1,000 times during the last El Nino.

They are now designing an experiment to check their findings. Palmer believes that ocean sound measurements will provide a new way to monitor an El Nino and to gauge its strength. The advantage of a system based on acoustics, he says, is that, unlike satellite measurements of sea-surface temperature, it would give scientists a sense of what's happening in the ocean interior. And, compared with moored instruments that measure temperature and other ocean properties only at isolated points, it would offer a less expensive and more complete picture of how an El Nino affects the ocean.

Because sound waves are extremely sensitive to changes in ocean conditions and because low-frequency sound can travel such great distances in the ocean, Palmer and a number of other scientists have been working for the last several years on using sound to probe the temperatures and currents of the ocean. "Studies of low-frequency sound in the ocean are analogous to remote sensing of the atmosphere with infrared and visible light and radar,' says Palmer. Because the upper ocean layer is so quiet, scientists won't be able to use "acoustic tomography' to study its dynamics during an El Nino. But Palmer says they may still learn something about El Ninos from sound traveling in lower layers.

Palmer's group is probably just as happy that an El Nino does not appear likely this year; it gives them more time to prepare for their experiment. As for whether an El Nino will develop next year, no one can say. Rasmusson notes, however, that a pattern of very warm water in the extreme western Pacific and slightly below-normal temperatures in the eastern Pacific--a pattern that typically precedes El Ninos and that first made meteorologists wary last fall--is still in place.

"We're always hesitant to call these things either way,' he says, "because the minute we do, nature crosses us up.'
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Author:Weisburd, Stefi
Publication:Science News
Date:Jun 7, 1986
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