From: LARRY KLAES (ljk4_at_msn.com)
Date: Sat Mar 15 2003 - 01:17:16 PST
----- Original Message -----
From: Mark Hess
Sent: Wednesday, March 12, 2003 2:27 PM
To: News Media list.serv
Subject: THE 1991 MT. PINATUBO ERUPTION PROVIDES A NATURAL TEST FOR THEINFLUENCE OF ARCTIC CIRCULATION ON CLIMATE
Krishna Ramanujan March 12, 2003
Goddard Space Flight Center, Greenbelt, Md.
(Phone: 301/286-3026)
Kramanuj_at_pop900.gsfc.nasa.gov
Release No: 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°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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