1995-18 Release at Will: September-November 1995 Contact: Joan Vandiver Frisch Manager, NCAR Media Relations Boulder, CO 80307-3000 Telephone: 303-497-8607 E-mail: jfrisch@ucar.edu THE NORTH ATLANTIC OSCILLATION: NEW FINDINGS ON A KEY PLAYER IN WINTER CLIMATE For many of the past 15 winters, a recurring pressure pattern has kept parts of the North Atlantic cooler than normal and Europe warmer and wetter than usual. Two recent papers from scientists at the National Center for Atmospheric Research (NCAR) shed new light on this pattern-- called the North Atlantic Oscillation (NAO)--and how it may be related to industrial sulfate emissions. Jim Hurrell analyzes decadal trends in the NAO in the August 4 issue of Science. The NAO is one of the primary factors in the earth's year-to- year atmospheric variability. It is a north-south oscillation in atmospheric mass, with centers of action near the semipermanent Icelandic low and Azores high. When the NAO is in a positive phase, pressures are higher than normal across the North Atlantic south of 55 degrees N and lower than normal across the Arctic. The resulting westerly flow brings maritime warmth to Europe during the winter and allows the northwest Atlantic to cool below normal. Using a data set for the years 1899-1993, Hurrell compared surface pressures with sea-surface and continental temperatures. The comparison revealed a strong correlation between the NAO pressure signature and the presence of European warmth and northwest Atlantic cold. The highest positive values of the NAO index in this century have occurred since 1980. Hurrell then linked the decadal variations in the mean circulation patterns to changes in the atmospheric moisture budget that agree with observed variations in regional precipitation. The message from these and other analyses performed by Hurrell is that the NAO is largely responsible for the unusually warm and wet winters observed across much of Europe over the past 15 years or so. What might be causing these unusually intense positive phases of the NAO? Hurrell notes that the large, warm wintertime anomalies over Europe resemble some results obtained by computer models when they couple atmospheric and oceanic processes and incorporate steadily increasing greenhouse gases. However, the models do not tend to project the cold anomalies observed over the North Atlantic. One reason may be the absence of sulfate aerosols in the models. Along these lines, NCAR's David Erickson has put the effects of sulfate aerosols into an NCAR global climate model. The results appear in the August 1 Geophysical Research Letters. With coauthors Robert Oglesby (Purdue University) and Susan Marshall (University of North Carolina at Charlotte), Erickson used a function of cloud reflectivity that varies with industrial sulfate emissions. Sulfate emissions are mainly confined to the industrial centers of Europe, China, and eastern North America. One might expect cooling over these regions due to the increased reflection of sunlight by the sulfate aerosols. However, Erickson and colleagues found otherwise. The model's results are remarkably similar to the NAO pattern analyzed by Hurrell: wintertime warming over Europe and cooling across the northwest Atlantic and eastern United States. "The model results suggest that it is possible that the way the North American continent responds to the [sulfate] forcing is fundamentally different than how Eurasian climate evolves under the same forcing," write Erickson and his coauthors. They note that the competing influences of greenhouse gases and sulfates may be producing a persistent change in the hemispheric "wave train" of storm systems. Hurrell and Erickson now plan to collaborate and examine how well Erickson's model simulates the NAO itself and its recent variations. NCAR is managed by the University Corporation for Atmospheric Research under sponsorship of the National Science Foundation. ÑThe EndÑ Writer: Bob Henson, UCAR Communications