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Krishna Ramanujan
Goddard Space Flight Center, Greenbelt, Md.     March 12, 2003
Kramanuj{at}pop900.gsfc.nasa.gov
(Phone: 301/286-3026)

RELEASE: 03-27

THE 1991 MT. PINATUBO ERUPTION PROVIDES A NATURAL TEST
FOR THE INFLUENCE OF ARCTIC CIRCULATION ON CLIMATE

A recent NASA-funded study has linked the 1991 eruption of the Mount 
Pinatubo to a strengthening of a climate pattern called the Arctic 
Oscillation. For two years following the volcanic eruption, the Arctic 
Oscillation caused winter warming over land areas in the high and 
middle latitudes of the Northern Hemisphere, despite a cooling effect 
from volcanic particles that blocked sunlight.

One mission of NASA's Earth Science Enterprise, which funded this 
research, is to better understand how the Earth system responds to 
human and naturally-induced changes, such as large volcanic eruptions.

"This study clarifies the effect of strong volcanic eruptions on 
climate, important by itself, and helps to better predict possible 
weather and short-term climate variations after strong volcanic 
eruptions," said Georgiy Stenchikov, a researcher at Rutgers 
University's Department of Environmental Sciences, New Brunswick,
N.J., and lead author on a paper that appeared in a recent issue of 
the Journal of Geophysical Research.

A positive phase of the Arctic Oscillation has slowly strengthened 
over the few last decades and has been associated in prior research 
with observed climate warming.

"The study has important implications to climate change because it 
provides a test for mechanisms of the Arctic Oscillation," Stenchikov 
said.

A positive phase of the Arctic Oscillation is associated with 
strengthening of winds circulating counterclockwise around the North 
Pole north of 55 deg N, that is, roughly in line with Moscow, Belfast, 
and Ketchikan, Alaska. In winter these winds pull more warm air from
oceans to continents causing winter warming, and like a top spinning 
very fast, they hold a tight pattern over the North Pole and keep 
frigid air from moving south. 

According to this research, temperature changes caused by a radiative 
effect of volcanic aerosols in two lower layers of the atmosphere, the 
troposphere and the stratosphere, can lead to a positive Arctic 
Oscillation phase. The troposphere extends from Earth's surface to an 
altitude of 7 miles in the polar regions and expands to 13 miles in 
the tropics. The stratosphere is the next layer up with the top at an 
altitude of about 30 miles.

The study uses a general circulation model developed at the National 
Oceanic and Atmospheric Administration's Geophysical Fluid Dynamics 
Laboratory to simulate how volcanic aerosols following the Pinatubo 
eruption impacted the climate.

In the troposphere, volcanic aerosols reflect solar radiation and cool 
the Earth's surface, decreasing temperature differences between the 
equator and the North Pole in the bottom atmospheric layer. These
changes end up inhibiting processes that slow counterclockwise winds 
that blow around the North Pole mostly in the stratosphere. This in 
turn strengthens a positive phase of the Arctic Oscillation.

In the stratosphere, volcanic aerosols absorb solar radiation, warm 
the lower stratosphere (about 15 miles above the Earth's surface) and 
increase stratospheric temperature differences between the equator and 
the North Pole. These changes strengthen westerly winds in the lower 
stratosphere and help to create a positive phase of the Arctic 
Oscillation. 

In previous research, an observed positive Arctic Oscillation trend 
has been attributed to greenhouse warming that led to an increase of 
stratospheric temperature differences between equator and pole. But
this study finds that tropospheric temperature change in the course of 
climate warming may play an even greater role.

In one type of computer simulation, Stenchikov and colleagues isolated 
the contribution of a decreased temperature difference in the 
troposphere, and found that it could produce a positive phase of the 
Arctic Oscillation by itself. That's because greenhouse heating near 
the North Pole melts reflective sea ice and snow, and reveals more 
water and land surfaces.  These surfaces absorb the Sun's rays and 
increasingly warm the Earth's polar regions. Polar heating at the 
Earth's surface lessens the temperature differences between the 
equator and North Pole in the troposphere, which ultimately 
strengthens a positive phase of the Arctic Oscillation.

The study also finds that when aerosols get into the stratosphere, 
very rapid reactions that destroy ozone (especially in high latitudes) 
take place on the surfaces of aerosol particles. When ozone gets
depleted, less UV radiation is absorbed in the stratosphere. This 
cools the polar stratosphere, and increases the stratospheric 
equator-to-pole temperature difference, creating a positive phase of 
the Arctic Oscillation. Ozone data were obtained from NASA's Total 
Ozone Mapping Spectrometer (TOMS) satellite and ozonesonde 
observations.

For more information and images, see:
     http://www.gsfc.nasa.gov/topstory/2003/0306aopin.html

TOMS satellite:
     http://jwocky.gsfc.nasa.gov/

NOAA's SKYHI Atmospheric Computer Model:
     http://www.gfdl.noaa.gov/~gth/AR97/3MiddleAtmosphere.html#26583

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