The
Eastern U.S. Keeps Its Cool
While surface temperatures across
most of the globe are on the rise, the eastern U.S.
appears to be slowly cooling. Scientists say the trend
could be a result of increasing cloud cover triggered
by warming Pacific waters.
NASA
Science News
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January 18, 2001 -- Much of the
Earth has warmed over the last half-century, but the
eastern half of the United States has shown a cooling
trend. NASA-funded research indicates cooler
temperatures in the eastern U.S. are caused by an
increase in sun-shielding clouds produced by warmer
ocean temperatures in the Pacific.
Eastern
U.S. temperatures have displayed a cooling trend of
0.1 degrees Celsius per decade, while global
temperatures warmed by that same amount from 1950 to
1997. The researchers used a computer climate model to
see if this regional cooling could be caused by
changes in sea surface temperature.
Right: Clouds appear
bright white when viewed from above because they
reflect sunlight back into space. The ground and
atmosphere below these clouds absorb less solar
radiation and therefore release less heat into the
environment. Image taken by NASA's SeaWiFS
satellite.
In the computer model, "warmer
sea surface temperatures in the tropical Pacific cause
greater cloud cover over the eastern United States.
This increased cloud cover is directly responsible for
the cooling," said researcher Walter
A. Robinson, an associate professor of atmospheric
sciences at the University of Illinois at
Urbana-Champaign.
The brightness of a cloud causes a
large percentage of incoming solar radiation to be
reflected back into space, thus keeping the atmosphere
cooler than if the cloud wasn't there.
Using the climate simulations,
Robinson found the amount of water vapor in the Gulf
of Mexico follows closely the water vapor released by
the warm sea surface temperatures in the Pacific
Ocean. Water vapor from the Pacific moves east to the
Gulf of Mexico and is then carried over the eastern
U.S. by the clockwise circulation around an Atlantic
subtropical high pressure system. When the water vapor
arrives over the U.S. it condenses and generates more
cloud cover, allowing less solar radiation to reach
and warm the Earth's surface.
Robinson's research utilized the
NASA Goddard Institute for Space Studies (GISS)
"general circulation model" (GCM), which
simulates the circulation of the atmosphere around the
world, and used sea surface temperatures from around
the globe.
Above: Models in Walter
Robinson's research showed that an increase of almost
1 degree Celsius in tropical Pacific Ocean (TPac)
temperatures (black curve) correspond to an increase
in Eastern U.S. (EUS) low cloud cover (red and blue
curves) by almost 5 percent. The blue curve represents
increased cloud cover using only tropical (TO) Pacific
water temperatures, while the red curve shows the
increase in Eastern U.S. clouds using global (GO)
water temperatures. In the model, this increase in
cloud cover keeps the Eastern U.S. from warming along
with the rest of the globe.
In order to create a focus on
sea-surface temperatures in the model runs, three
components that can contribute to warming or climate
forcing, were "fixed." These are aerosols,
solar irradiance, and greenhouse gases such as carbon
dioxide. Because these factors were held constant,
they could be ruled out as the cause of cooling in the
model, leaving only sea surface temperatures as a
variable.
The GISS model used ocean
temperature data over a 47-year span, from 1950 to
1997, and considered global sea surface temperatures
in different areas. The only time the model showed
significant cooling in the eastern United States was
when the tropical Pacific waters warmed.
Walter A. Robinson of the
University of Illinois at Urbana-Champaign, James
Hansen of NASA Goddard Institute for Space Studies,
and Reto Reudy of Science Systems and Applications,
Inc. presented these findings in a paper entitled
"Where's the Heat? Insights From GCM Experiments
into the Lack of Eastern U.S. Warming" at the
American Meteorological Society Annual Meeting in
Albuquerque, N.M. on January 15.
Related Links
Globally
averaged atmospheric temperatures -- a look at
the temperature trends of the troposphere and the
stratosphere
Is
Earth's Temperature Up or Down or Both? --
Science@NASA article investigating reasons for
temperature trend "disagreements" between
layers of the atmosphere
NASA's
Goddard Institute for Space Studies -- Home page
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