GENEVA, 1 OCTOBER, 2002 (WMO) – The Antarctic ozone hole has split in two, which

is said to be ‘an unprecedented event’, according to experts of the World Meteorological Organization (WMO) today. The Antarctic ozone hole split into two smaller holes, last week, one centered west of the southern tip of South America and the other centered southeast of the tip of Africa.

Less than two weeks ago WMO reported that the 2002 ozone hole was one of the smallest in the

past decade, and that based upon previous years it would be expected to grow somewhat larger

during the following weeks. However, last week an unprecedented split of the Antarctic ozone

hole occurred. Although the two holes were relatively small, each contained a core depleted of

more than 50% of its ozone. Since that time, the hole south east of Africa has maintained its size

and intensity, while the hole near South America has weakened considerably. Together the

depleted areas cover only a small part of Antarctica.

Although the Antarctic ozone holes during 2000 and 2001 were the largest ever, this year’s ozone

hole is not matching those levels. Under full compliance with the present international

agreements that would phase out production of practically all ozone depleting compounds, it is

expected that the ozone layer will remain particularly vulnerable for another decade. The total

amount of ozone depleting substances in the stratosphere is believed to have reached or nearly

reached its maximum, and is expected to decrease to pre-ozone hole values in about 50 years.

However, during this period, the size, depth and persistence of the ozone hole will vary from year

to year due to natural changes in the meteorological conditions in the stratosphere. This year’s

unusual ozone hole is seen as a response to the present unusual meteorological conditions in the

stratosphere.

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."

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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

Ozone Measurement

1 Dobson Unit (DU) is defined to be 0.01 mm thickness at STP (standard temperature and pressure). Ozone layer thickness is expressed in terms of Dobson units, which measure what its physical thickness would be if compressed in the Earth's atmosphere.

  In those terms, it's very thin indeed. A normal range is 300 to 500 Dobson units, which translates to an eighth of an inch-basically two stacked pennies. In space, it's best not to envision the ozone layer as a distinct, measurable band. Instead, think of it in terms of parts per million concentrations in the stratosphere (the layer six to 30 miles above the Earth's surface).

NASA Earth Probe TOMS  Images

Earth Probe TOMS is currently the only NASA spacecraft on orbit specializing in ozone retrieval.

Comparison ozone hole year 2001 development and growth with ozone hole year 2002

August 2002 

September 2002 

October 2002 

November 2002

December 2002

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Southern Hemisphere Ozone Maps

Comparison ozone hole year 2001 development and growth with ozone hole year 2002

May 2002

June 2002

July 2002

August 2002

September 2002

October 2002

November 2002

December 2002

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The total ozone maps are based on ground-based measurements available from the World Ozone and Ultraviolet Radiation Data Centre. Preliminary near real-time data from ground-based observations were also used for the most recent maps. Total ozone values are given in Dobson Units. The numbers represent observations taken from ground stations situated at the bottom left corner of the number.

Maps of deviations represent total ozone deviations from the 1978-1988 level estimated using Total Ozone Mapping Spectrometer (TOMS) data for all areas except the Antarctic and from the pre-1980 level estimated using Dobson data over the Antarctic.

Over areas with poor data coverage adjustments are made according to TOMS on Nimbus-7, Meteor-3, ADEOS and Earth Probe satellites. Over the polar night area Dobson and Brewer moon observations and/or NOAA's TIROS Operational Vertical Sounder (TOVS) satellite data are used. TOVS data are also used when the more reliable TOMS data are not available. The mapping algorithm is similar to those used by the WMO Ozone Mapping Centre.

NOAA's TIROS Operational Vertical Sounder(TOVS) 

NOAA's TIROS Operational Vertical Sounder(TOVS) is a suite of three instruments: the Microwave Sounding Unit(MSU), the High resolution Infrared Radiation Sounder(HIRS), and the Stratospheric Sounding Unit(SSU). Each instrument measures radiation emmitted by the Earth at several different wavelengths. The HIRS channel 9 measures Earth's emmitted infrared radiation at 9.7 microns (10^-6 meters). This is a "window channel" meaning that the radiation measured by the HIRS instrument is emmited from the earth's surface (as opposed to radiation being emmitted at other levels of the earth's atmosphere). The amount of radiation reaching the HIRS instrument is dependant upon how much ozone is in the earth's atmosphere (less ozone = more radiation). Therefore, the TOVS Total Ozone algorithm uses this channel (along with information from other HIRS channels) to estimate the total amount of ozone in the earth's atmosphere. The greatest contribution of the emmitted radiation occurs in a region between 200 hPa and 30 hPa (13km to 27km). This "lower stratosphere" region is below the levels where the greatest contribution to the total ozone amount occurs(50hpa to 10hPa or 20km to 30km). Thus the ozone amount measured by the TOVS Total Ozone algorithm is not a true measure of the "total" amount of ozone in the earth's atmosphere. Rather it is a better measure of the ozone amount in the lower stratosphere. To obtain a "total" ozone amount, the TOVS Total Ozone algorithm adjusts the lower stratosphere ozone amount by a climatological amount that is variable with season and latitude.

This is in contrast with satellite instruments which measure the amount of backscattered radiation at various ultraviolet wavelengths. Backscattered radiation levels at wavelengths where ozone absorbtion does and does not take place are compared with the same wavelenghts measured directly from the sun to derive a "total ozone" amount in the earth's atmosphere. This methodology is used by the NASA TOMS and the NOAA SBUV/2 ozone monitoring programs. This methodology provides a truer measure of the total ozone amount in the earth's atmosphere. One drawback is that this method uses "backscattered" sunlight. Which means that data cannot be retrieved in the earth's shadow or polar night regions.

The TOVS Total Ozone algorithm can determine ozone amounts at all times since it is derived from the Earth's emmitted infrared radiation. There are drawbacks to the TOVS infrared methodolgy though. When the earth's surface is either too cold (e.g., the high Antarctic Plateau) too hot (e.g., the Sahara desert) or too obscured (e.g., by heavy tropical cirrus clouds) the accuracy of this methodolgy declines.