NRL's POAM III Measurements Confirm This Year's Shrinking Ozone Hole Caused by Unusual Stratospheric Weather Patterns

6/5/2002 - 51-02r
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Using data from the Naval Research Laboratory's Polar Ozone and Aerosol Measurement (POAM III) instrument, scientists have confirmed that despite the fact that this year's Antarctic ozone hole is shrinking prematurely, the chemistry that causes the ozone depletion in the ozone hole is unchanged from recent prior years. This corroborates the recent findings from NASA and NOAA scientists that the unique nature of this year's ozone hole is most probably the result of unusual stratospheric weather patterns.

Research since the discovery of the ozone hole in 1985 has shown that it results from chemical ozone destruction by reactions involving chlorine and bromine compounds. The stage is set for these chemical reactions by high thin clouds called polar stratospheric clouds (PSCs) that form within the polar vortex, a very cold mass of swirling air located over the polar cap each winter. During the winter, chemistry on the surface of these clouds converts chlorine into a form that readily destroys ozone once the sun rises over the southern polar cap in early spring, forming the ozone hole. The ozone hole persists until the vortex begins to dissipate later in the austral spring, letting in air that contains normal amounts of ozone. The ozone hole, thus, requires both the presence of chlorine and bromine compounds, and a persistent, cold, polar vortex which allows formation of PSCs while isolating air within the vortex.

Beginning in late May 2002, POAM observed the amount of ozone in the southern hemisphere polar stratosphere from 25 km up to 60 km to be unusually variable. This variability was presumably due to much larger than normal large-scale wave disturbances that affected the stratospheric wind flow throughout this southern hemisphere winter, perhaps setting the conditions for the smaller ozone hole. The enhanced wave activity likely weakened and warmed the polar vortex. As a result, the frequency with which POAM observed PSCs this year was smallest in the eight-year POAM record.

When the sun began to rise over the polar stratosphere in the late winter, ozone was observed to begin decreasing in a typical fashion. In fact, the average ozone loss rate at altitudes near 18 km (the center of the altitude region affected by ozone loss in the ozone hole) from mid-August through 20 September was virtually indistinguishable from that observed in other years. Although PSCs were less frequent this year, it appears that they were sufficient to prime the atmosphere for the large ozone chemical losses which occur in the ozone hole.

On September 24th, 2002, a stratospheric wave disturbance caused a poleward influx of much higher ozone abundances from mid-latitudes at altitudes above 20 km. However, the POAM observations show that at lower altitudes chemical processes caused ozone abundances to continue to decline to the very low typical values found in the ozone hole during this season. These observations confirm that unusual stratospheric weather patterns, rather than chemistry, are responsible for the unusually small Antarctic ozone hole this year.

The POAM III instrument was launched in March 1998 and is still operational. It is the successor to NRL's POAM II instrument, which operated from October 1993 to November 1996. The POAM measurement complement includes profiles of ozone, water vapor, nitrogen dioxide and aerosol particles in the polar stratosphere of both hemispheres. POAM is well suited for investigation of Antarctic ozone hole and Arctic ozone depletion events, and the POAM instruments have now monitored the evolution of the Antarctic ozone hole for eight years.

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