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March 12, 2003
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°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.
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Contacts:
Krishna Ramanujan
Goddard Space Flight Center,
Greenbelt, Md.
(Phone: 301/28603026)
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 The
Arctic Oscillation (AO)
A positive phase of the Arctic Oscillation (top) is
associated with strengthening of winds circulating counterclockwise around
the North Pole north of 55°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. Cool winds sweep across eastern Canada while North
Atlantic storms bring rain and mild temperatures to Northern Europe.
Drought conditions prevail over the Mediterranean region.
During the negative phase of the Arctic Oscillation
(bottom), cool continental air plunges into the Midwestern United States
and Western Europe while storms bring rainfall to the Mediterranean region.
Credit: David W. J. Thompson, J. M. Wallace
 Eruption
of Mount Pinatubo, Philippines, July 1991
Strong explosive volcanic eruptions, like ones of the
Mt. Pinatubo in Philippines in June 1991, inject millions of ton of sulfur
dioxide gas at the altitudes of about 15 miles where it interacts with
water vapor producing a volcanic aerosol layer that consists of tiny
droplets of highly concentrated sulfuric acid.
As a result of the Pinatubo eruption, globally averaged
surface temperature decreased by about 0.3 Kelvin (0.3 Celsius) for two
years after the eruption and the temperature in the tropical lower stratosphere
increased by about 2-3 Kelvin (2-3 Celsius). The tropospheric response
over most land areas in the Northern Hemisphere is characterized by summer
cooling and winter warming. Credit: U.S. Geological Survey, J.N. Marso,
July 1991
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