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Winter 1997

Science Bits

University of Washington
Decades-long climate cycle influences salmon fisheries

Researchers at the University of Washington think that a decades-long climate variation in the Pacific Ocean may explain the changes in salmon harvests off the U.S. West Coast and Alaska over the course of the century. The scientists call this climatic phenomenon the Pacific decadal oscillation, or PDO. David Battisti described the PDO in a paper in the Journal of Climate, and Nathan Mantua and coauthors also wrote a paper about it for the June 1997 Bulletin of the American Meteorological Society.

The researchers found the oscillation by examining climate records for the Pacific basin over the past century in connection with records of salmon catches from Alaska and the Pacific Coast states. In the more recent decades, El Niño quickly emerged as the dominant recurring pattern of year-to-year climate variability. But when records were studied back to 1900, with a focus on the region north of Hawaii in the Pacific Basin, the PDO revealed itself as parallel fluctuations in air pressure at sea level and in sea surface temperature, with phases lasting from 10 to 30 years.

For about the last 20 years, there has been a large pool of cooler-than-average surface water in the central North Pacific Ocean, with a narrow belt of warmer-than-average sea surface temperatures near the west coast of the Americas. These phenomena, the researchers believe, are distinctive features of the positive phase of the PDO.

The ocean temperatures, the authors think, also explain why Alaskan salmon harvests are bountiful during the positive PDO phase and sparse in the negative phase. Records of salmon catches from the Columbia River and the Washington, Oregon, and California coasts show something of an opposite pattern, though the correspondence is less clear than in Alaska. The researchers believe the weaker connection is the result of stronger human influence in the southern waters.

Stanford University
Scientists discover massive jet streams flowing inside the sun

Scientists at Stanford University, using the Solar and Heliospheric Observatory (SOHO) spacecraft, have discovered "jet streams" or "rivers" of hot, electrically charged gas called plasma flowing beneath the surface of the Sun. They also have found features similar to trade winds that transport gas beneath the Sun's fiery surface.

"We have detected motion similar to the weather patterns in the Earth's atmosphere," said Jesper Schou of Stanford. "Moreover, in what is a completely new discovery, we have found a jetlike flow near the poles. This flow is totally inside the Sun. It is completely unexpected, and cannot be seen at the surface."

"These polar streams are on a small scale, compared to the whole Sun, but they are still immense compared to atmospheric jet streams on the Earth," added Philip Scherrer, the SOI principal investigator at Stanford. "Ringing the Sun at about 75 degrees latitude, they consist of flattened oval regions about 17,000 miles across where material moves about 10% faster than its surroundings. Although these are the smallest structures yet observed inside the Sun, each is still large enough to engulf two Earths."

Additionally, there are features similar to the Earth's trade winds on the surface of the Sun. The Sun rotates much faster at the equator than at the poles. However, Stanford researchers Schou and Alexander Kosovichev have found that there are belts in the northern and southern hemispheres where currents flow at different speeds relative to each other. Six of these gaseous bands move slightly faster than the material surrounding them. The solar belts are more than 40,000 miles across, and they contain "winds" that move about ten miles per hour relative to their surroundings.

Finally, the solar physicists have determined that the entire outer layer of the Sun, to a depth of at least 15,000 miles, is slowly but steadily flowing from the equator to the poles. The flow rate is relatively slow, about 50 miles per hour, compared to the Sun's rotation speed, about 4,000 miles per hour; however, this is fast enough to transport an object from the equator to the pole in a bit more than a year. This polar flow is in the opposite direction from that of sunspots and the zonal belts.

"It is intriguing to speculate that these streams may affect solar weather like the terrestrial jet stream impacts weather patterns on Earth, but this is completely unclear right now," said Douglas Gough, of Cambridge University, U.K. "It will be especially helpful to make observations as the Sun enters its next active cycle, expected to peak around the year 2001."

McGill University, University of Maryland
Simulation captures growth, landfall of Hurricane Andrew

A team of researchers from McGill University (Yubao Liu and M.K. Yau) and the University of Maryland (Da-Lin Zhang) have produced a successful 72-hour explicit simulation of Hurricane Andrew (1992) on NCAR supercomputers using a movable, triply nested grid version of the Penn State/NCAR weather model (MM5). The hurricane was explicitly resolved using a grid size of six kilometers and a sophisticated cloud microphysics package containing prognostic equations for cloud water, ice, rainwater, snow, and graupel. The simulation covers the storm's deepening to near-Category-5 hurricane intensity and its landfall over Florida.

Compared with observations and the best hurricane track analysis, the model captured very well the evolution and inner-core structures of the storm. In particular, the model reproduced the track, the explosive deepening rate, the shoreline and maximum winds, the eye, the eyewall, the spiral rainbands, and other cloud features. The model's simulation of the core regions compares favorably to observations of hurricanes.

These results suggest that it may be possible to predict reasonably well the track, intensity, and inner-core structures of hurricanes from tropical synoptic conditions if a model has high grid resolution and realistic physics and incorporates the storm's initial conditions (depth, size, and intensity) in relation to larger-scale atmospheric conditions.

University of Alaska
About face: Arctic ice moves counterclockwise, too

Sea ice in the Arctic Ocean is decreasing, and temperatures in the arctic are climbing. Is this a sign of global warming, or is it a natural change?

Andrey Proshutinsky and Mark Johnson (University of Alaska) tend toward the latter view. Using model data, they theorize that ice in the Arctic Ocean circulates under the influence of wind in a regular pattern during a roughly 15-year oscillation. During the first five to seven years of the oscillation, the ice moves around the North Pole more or less clockwise; in the next five to seven years, it goes counterclockwise. In the counterclockwise phase, arctic temperatures and humidity are naturally higher. According to their model, the arctic is currently in its counterclockwise phase.

The researchers recognize that this circulation shift is a radical idea. "Scientists have been educated to believe that ice in the Arctic Ocean circulates in one way only--in a clockwise direction," Proshutinsky said. "This is something we were all taught in school as a fact, like the earth is round."

The computer model designed by Proshutinsky and Johnson shows that when high atmospheric pressure prevails over the Arctic Ocean, wind and ice move clockwise, generating conditions considered typical of the arctic, such as cold, dry air. The few research expeditions that have gathered data on arctic ice and wind have occurred in such high-pressure periods. When air pressure is low, however, wind and ice move counterclockwise, generating warm, moist, air, such as has been observed recently. If Proshutinsky and Johnson's theory is accurate, wind and ice in the Arctic Ocean will revert from counterclockwise to clockwise in a year or so, with accompanying changes in atmospheric pressure and circulation.

Proshutinsky said the theory does not eliminate the possibility of an arctic global warming signal. The 15-year oscillation in the model seemed to exist in a context of increasing overall temperatures for the arctic. The researchers also point out that they lack observational data to test their theory.

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Edited by Carol Rasmussen, carolr@ucar.edu
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Last revised: Tue Apr 4 14:43:58 MDT 2000