NSF PR 95-64 - October 2, 1995
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New Findings on a Key Player in Winter Climate
For much 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.
Research by scientists at the National Science Foundation-supported
National Center for Atmospheric Research (NCAR) in
Boulder, Colorado is shedding new light on this pattern
-- called the North Atlantic Oscillation (NAO) --
and how it may be related to industrial sulfate emissions.
Researcher Jim Hurrell has analyzed decadal trends
in the NAO. The NAO is one of the primary factors
in the earth's yearto-year atmospheric variability.
A north-south oscillation in atmospheric mass, the
NAO's centers of action are 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, 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 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 variations in circulation
patterns to changes in atmospheric moisture that agree
with variations in 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.
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.
To answer this question, NCAR's David Erickson has
put the effects of sulfate aerosols into a global
climate model. With researchers 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.
Cooling might be expected over these regions due to
the increased reflection of sunlight by the sulfate
aerosols, say scientists. 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 sulfates
is fundamentally different than how Europe's climate
evolves under the same conditions," says Erickson.
He notes that the influences of greenhouse gases and
sulfates may be producing a persistent change in the
hemispheric "wave train" of storm systems.
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