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News Flash Photonics Assesses El Niño's Devastating Path

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Anyone who turned on CNN this winter remembers the images: expensive homes teetering on the edge of rocky cliffs, vulnerable to the Pacific's waves. Those familiar images were the result of El Niño -- the weather phenomenon blamed for uncharacteristic ocean warming and ruinous storms. A group of scientists now has the task of assessing the aftermath -- and photonics technology is at the forefront of the effort.

"El Niño is not a disaster and not an aberration; it's a phenomenon that has biological consequences." -- Gene Feldman, a NASA oceanographer involved with sea color mapping.

Researchers from NASA Goddard Space Flight Center on Wallops Island, Va., the National Oceanic and Atmospheric Administration Coastal Services Center in Charleston, S.C., and the US Geological Survey's Centers for Coastal Geology in St. Petersburg, Fla., and Menlo Park, Calif., have joined to produce detailed maps of the West Coast from Washington state to Southern California and, coincidentally, of sections of Maryland. Led by William Krabill of NASA, they mapped the coastline with a system that employs a laser altimeter in conjunction with the satellite-based Global Positioning System to determine precise coordinates.

The monitoring system is mounted aboard a two-engine Twin Otter plane and relies on lasers for topographic measurement: a Spectra-Physics laser emitting at 523 nm with 200 mJ or a Continuum EPO 5000 emitting at 532 nm. The lasers target the coastline from a very low altitude -- between 350 and 700 m -- mapping a small area at a time. A Hamamatsu photomultiplier tube aboard the plane measures the backscatter, collecting between 3000 and 5000 spot elevations per second. Combining the laser's range data with the known position supplied by Global Positioning System equipment enables the scientists to map the coastline changes to an accuracy of four inches.

Laser trade-offs

Crucial to the performance of the system are the laser types incorporated in the airborne topographic mapper. Each laser possesses advantages and disadvantages, said Krabill. The air-cooled Continuum laser is the less powerful one and therefore consumes less on-board power. It also has a 2.5-ns pulse width, which is suitable for ranging at shorter distances. The Spectra- Physics laser is more powerful for longer-distance ranging.

"We're always watching the technology, and it seems there are lasers with high power and low repetition or low power and high repetition. For our requirements we need something in between," said Krabill.

Since 1991, he has used the system to map changes in Greenland's ice cap that serve to measure global warming. As a tuneup for one of its Greenland missions, the NASA team mapped nearby Assateague Island off the Maryland coast and parts of the National Seashore that extend down to Virginia. By happenstance, two northeasters early this year pounded the same seashore the team had mapped in the fall. This provided the first opportunity to compare "before and after" El Niño maps.

The team returned to the coast in March and remapped the same section. They found that an inlet on the Maryland side had been destroyed, turning up some surprising results: The sand dunes of the inlet had been resting on a fossilized forest, with its old root structure intact.

Krabill said the before-and-after topographic maps would be used to predict the amount of erosion in future El Niño years. This could prove a powerful tool for researchers attempting to identify high-risk areas in California. The method also replaces the old measurement technique of taking multiple photographs of the same location, which was expensive and labor-intensive.

"It's probably the largest scale investigation that has ever been done on the coastlines," said Abby Sallenger, senior research oceanographer at the US Geological Survey. "We're already starting to see some patterns that we hadn't been able to see before. We can develop relationships of how beaches and cliffs respond during El Niños."

Ocean changes

Aside from the well-documented coastal damage, El Niño has had subtler, more far-reaching effects. Satellite images caught the abnormal temperature shifts in ocean currents this year, as they have every year since 1982. "The El Niño itself starts below the surface in the central and eastern Pacific normally undergoing cooling," said NASA oceanographer David Adamec.

When cooling currents below sections of the Pacific failed to get under way over the winter, the water in regions off the coast of South America became 5° to 6° warmer than normal winter water. This abnormally warm water indirectly caused the devastating storms and high tides that afflicted the West Coast.

Sensors aboard several satellites owned by the National Oceanic and Atmospheric Administration monitored these changes. Measuring emitted and reflected radiation in several channels in the mid-IR between 3.55 and 3.93 µm, the sensor maps sea surface temperature. Red indicates the hottest ocean water temperatures, whereas orange and yellow signify cooler waters. This year's images reveal a larger number of red patches than in previous years.

Adamec said another method of tracking temperature changes is analyzing buoys moored in the Pacific. Attached to those buoys are thermistor chains -- instruments similar to conventional thermometers that provide temperature readings at different depths. The problem is that these devices provide only snapshots of a particular area. "The moorings are located, in some instances, 1500 km apart," he said. The multispectral imager provides a better overall picture of how the different currents act.

El Niño's effect on ocean currents hurt the phytoplankton population -- marine vegetation that makes up the bottom rung of the ocean's food chain. The cold currents off the coasts of Peru and Ecuador contain important nutrients necessary for the plant growth. Because of El Niño, those currents never mixed with warmer currents off the coast of California, adversely affecting phytoplankton populations.

To gather information on these populations, NASA scientists used a multispectral line scanner from the SeaStar satellite, launched in August 1997. The scanner took readings from five 20-nm channels between 412 and 865 nm. Chlorophyll in the vegetation absorbs in the blue region of the spectrum and reflects in the green, so areas of the sea that have thriving phytoplankton concentrations have a greenish hue. Areas with small phytoplankton populations appear blue. Therefore, images from the multispectral line scanner captured an accurate picture of where phytoplankton dwindled last winter.

El Niño's return

As El Niño's effects have waned and the television images of blooming desert flowers and coastal destruction fade from the world's collective memory, photonics may better prepare us for future problems.

Knowing how phytoplankton is affected during El Niño years may cause governments to manage their fisheries differently -- preparing for the worst. Builders on coastal areas may think twice before they construct on oceanfront property.

"El Niño is not a disaster and not an aberration; it's a phenomenon that has biological consequences," said Gene Feldman, a NASA oceanographer involved with sea color mapping. "We're going to predict more accurately when El Niño will take place -- the duration and the magnitude."

Photonics Spectra
Jun 1998
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