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NASA September 30, 2002
UNUSUALLY
SMALL ANTARCTIC OZONE HOLE THIS YEAR ATTRIBUTED TO EXCEPTIONALLY STRONG
STRATOSPHERIC WEATHER SYSTEMS
Scientists
from NASA and the Commerce Department's National Oceanic and Atmospheric
Administration (NOAA) have confirmed the ozone hole over the Antarctic this
September is not only much smaller than it was in 2000 and 2001, but has split
into two separate "holes."
Environment
Canada
Southern
Hemisphere Ozone Map
The
researchers stressed the smaller hole is due to this year's peculiar
stratospheric weather patterns and that a single year's unusual pattern does not
make a long-term trend. Moreover, they said, the data are not conclusive that
the ozone layer is recovering.
Paul
Newman, a lead ozone researcher at NASA's Goddard Space Flight Center,
Greenbelt, Md., said this year, warmer-than-normal temperatures around the edge
of the polar vortex that forms annually in the stratosphere over Antarctica are
responsible for the smaller ozone loss.
Estimates
for the last two weeks of the size of the Antarctic Ozone Hole (the region with
total column ozone below 220 Dobson Units), from the NASA Earth Probe Total
Ozone Mapping Spectrometer (EPTOMS) and the NOAA-16 Solar Backscatter
Ultraviolet instrument (SBUV/2), are around 15 million square kilometers (6
million square miles). These values are well below the more-than 24 million sq.
km. (9 million sq. mi.) seen the last six years for the same time of year.
The
stratosphere is a portion of the atmosphere about 6-to-30 miles above the
Earth's surface where the ozone layer is found. The ozone layer prevents the
sun's harmful ultraviolet radiation from reaching the Earth's surface.
Ultraviolet radiation is a primary cause of skin cancer. Without protective
upper-level ozone, there would be no life on Earth.
"The
Southern Hemisphere's stratosphere was unusually disturbed this year," said
Craig Long, meteorologist at NOAA's Climate Prediction Center (CPC). The unusual
weather patterns were so strong, the ozone hole split into two pieces during
late September. NOAA's CPC has been monitoring and studying the ozone since the
early 1970s. "This is the first time we've seen the polar vortex split in
September," said Long.
At
South Pole Station, balloon-borne ozone-measuring instruments launched by NOAA's
Climate Monitoring and Diagnostics Laboratory (CMDL) reveal the vertical
structure of the developing ozone hole. Bryan Johnson, a scientist with CMDL,
said the main ozone depletion region, from 7-to-14 miles above the Earth, has
large ozone losses, similar to the last few years. At more than 15 miles above
the Earth, surface measurements show higher-than-normal ozone concentrations and
higher temperatures.
The
combination of these layers indicate total ozone levels in a column of
atmosphere will be higher than observed during the last few years, Johnson said.
However, some layers may still show complete ozone destruction by early October,
when ozone depletion is greatest.
In
2001, the Antarctic ozone hole was larger than the combined area of the United
States, Canada and Mexico. The last time the ozone hole was this small was in
1988, and that was also due to warm atmospheric temperatures.
"While
chlorine and bromine chemicals cause the ozone hole, temperature is also a key
factor in ozone loss," Newman said. The Montreal Protocol and its
amendments banned chlorine-containing chlorofluorocarbons (CFCs) and
bromine-containing halons in 1995, because of their destructive effect on the
ozone layer. However, CFCs and halons are extremely long-lived and still linger
at high concentrations in the atmosphere.
The
coldest temperatures over the South Pole typically occur in August and
September. Thin clouds form in these cold conditions, and chemical reactions on
the cloud particles help chlorine and bromine gases to rapidly destroy ozone. By
early October, temperatures usually begin to warm, and thereafter the ozone
layer starts to recover.
NOAA
and NASA continuously observe Antarctic ozone with a combination of ground,
balloon, and satellite-based instruments.
National
Oceanic and Atmospheric Administration summary-Very
low ozone values were observed over Antarctica again in the Southern Hemisphere
winter of 2002. Ozone depletion of more than 40 percent was observed over
Antarctica compared to total ozone amounts observed in the early 1980's.
Vertical soundings over the South Pole during September and October 2002 again
showed strong destruction of ozone at altitudes between 15 and 20 km. However,
for the year 2002, the ozone hole declined rapidly in late September, and had
the shortest duration of any year since 1988. Lower stratosphere temperatures in
the winter and spring of 2002 over the Antarctic region were much higher than
average values. Associated with this, the ozone hole area was among the smallest
of recent years.
Observations of
chloroflourocarbons and of stratospheric hydrogen chloride support the view that
international actions are reducing the use and release of ozone depleting
substances ; Anderson et al., 2000). However, chemicals already in the
atmosphere are expected to continue to deplete ozone for many decades to come.
Further, changing atmospheric conditions that modulate ozone can complicate the
task of detecting the start of ozone layer recovery. The eruption of the
Pinatubo volcano provided an example of such a complication in the 1990s. Based
on an analysis of 10 years of South Pole ozone vertical profile
measurements, estimated that recovery in the Antarctic ozone hole may be
detected as early as the coming decade. Indicators include: 1) an end to
springtime ozone depletion at 22-24 km, 2) 12-20 km mid-September column ozone
loss rate of less than 3 DU per day, and 3) a 12-20 km ozone column of more than
70 DU on September 15. An intriguing aspect of recent observations of the
Antarctic stratosphere had been the apparent trend towards a later breakup of
the vortex in most recent years. However, the limited size and duration of the
2002 ozone hole is attributed to highly unusual meteorological conditions this
year. A full explanation of such meteorological anomalies is not yet available.
Continued monitoring and measurements, including total ozone and its vertical
profile, are essential to achieving the understanding needed to identify ozone
recovery.
http://www.cpc.ncep.noaa.gov/products/stratosphere/winter_bulletins/sh_02/index.html
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