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National Geographic : 1977 Dec
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of the Weather Service's Long Range Predic tion Group, made such a forecast, based in part on the severe cold of that autumn and the likelihood it would persist. A similar prediction came from oceanographer and meteorologist Joseph Chase of Woods Hole, Massachusetts. He based his forecast on atmo spheric pressures and a calculation that our winter air would flow in from Siberia. Simultaneously, climatologists Jerome Namias and his British colleague Robert R. Dickson pored over data that Dr. Namias had amassed at the Scripps Institution of Oceanography in La Jolla, California. Inde pendently they arrived at similar forecasts. Pacific May Shape U. S. Weather Dr. Namias, a pioneer of long-range fore casting, believes that North America's weath er is shaped largely by surface temperatures of the North Pacific. These temperatures, he contends, strongly influence the air masses above them, and this air eventually flows eastward to become our weather. Analyzing some 20,000 temperature read ings transmitted by ocean vessels each month, Dr. Namias observed the buildup of an im mense pool of cool water in the western Pacific. By autumn this surface water was the coldest ever recorded. The ship reports also revealed regions of abnormally warm water off California and Central America. "Those adjacent areas of cool and warm surface water," the climatologist told me, "formed a pattern that seemed likely to an chor a high-pressure system over the coast. The sharp temperature gradient between the two Pacific pools would tend to accelerate the upper-air winds flowing north toward Alas ka, and these in turn would loop south again, bringing Arctic air down to the East. This, in oversimplified terms, set the stage for last winter's weather." Dr. Namias's forecasts still are experi mental, as is far-reaching research at the Weather Service's parent agency, the Na tional Oceanic and Atmospheric Adminis tration, which focuses on numerically sim ulating the weather in huge computers.* Clues may exist in the striking similarity of last winter and the vicious one of 1917-18. "Before both winters," notes NOAA meteor ologist A. James Wagner, "the distribution of Pacific surface temperatures was much the same. Also, both autumns were abnormally cool east of the Rockies. Even Arctic air pat terns seemed to be similar. The weather never repeats itself exactly, but the parallels suggest something to look for." Another important clue may lie in the amount of early-winter snow cover. Studying 11 years of satellite records, NOAA's Donald R. Wiesnet discovered that heavy snows in December presage more of the same for the entire winter. Predictable or not, extremes of temperature and precipitation will be with us for a while, contend many meteorologists. They believe that the recent decades of benign weather-a period most of us regard as "normal"-were an era of exceptional mildness. "The unusual thing," says Dr. J. Murray Mitchell, Jr., a NOAA senior climatologist, "is not the variability we have now, but the lack of it between 1950 and 1970." Tree Rings Tell of Past Droughts What is the cause of this increasing vari ability? Here I found far less agreement. But a large body of experts believe it relates to a gradual cooling of the Northern Hemisphere, particularly in the Arctic, since 1940. "With cooling," asserts Dr. George Kukla of the Lamont-Doherty Geological Observa tory in New York, "the fluctuations of the seasons extend farther south, introducing a new uncertainty into the weather. You also see a greater north-south swing of the jet stream, increasing the likelihood of persistent, looping weather systems such as last winter's." From tree rings comes new evidence to support another intriguing theory: that western droughts tie in with the sunspot cycle. Scientists have long pointed out the syn chronization between drought on the Great Plains and the 22-year "double cycle" when sunspots reach their minimum. Now Murray Mitchell of NOAA and Dr. Charles W. Stock ton of the University of Arizona claim that tree rings confirm a 22-year drought cycle west of the Mississippi. Another exhaustive analysis of tree rings, by the University of Ari zona's Dr. Harold C. Fritts, discloses that weather patterns like the one that plagued the U. S. last winter occurred frequently in *See "We're Doing Something About the Weather," by Walter Orr Roberts, NATIONAL GEOGRAPHIC, April 1972, and "What's Happening to Our Climate?" by Samuel W. Matthews, November 1976. The Year the Weather Went Wild 823
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