Unusually Low Ozone Detected Over South-Central U.S.

Unusually low total ozone was observed over the south-central United States during November 1994. The region of low ozone was associated with a persistent subtropical jet stream over the Southeastern United States, which was accompanied by significant above-normal temperatures over the eastern two-thirds of the country [NOAA Climate Analysis Center, 1994]. Although the ozone is apparently the lowest reported over the United States since satellite observations were begun, ultraviolet radiation measured in Texas was lower than during October because of the reduced Sun angle during November and, on some days, overcast skies

The low ozone was observed by two satellite instruments - Meteor-3 Total Ozone Mapping Spectrometer (TOMS) and the Tiros Operational Vertical Sounder (TOVS) - and by ground-based instruments in Texas (Microprocessor Total Ozone Portable Spectrometers (MicroTOPS)) and in North Carolina, Georgia, and Washington, D.C. (Brewer scanning spectrophotometers).

The ozone anomaly provided an opportunity to rapidly compare observations by these instruments. TOMS infers ozone by means of the differential absorption of backscattered solar ultraviolet light. TOVS monitors the Earth's radiance in the 9.6 micrometer ozone absorption band, and MicroTOPS and the Brewer spectrophotometer measure ozone by direct Sun differential absorption spectroscopy.

The region of low ozone moved into the United States from Mexico during early November, where it was observed by several MicroTOPS instruments at Seguin in south-central Texas. The significance of the reduced ozone amount can best be understood by a comparison with satellite measurements since 1978. The mean of Nimbus-7/TOMS observations at Seguin for November from 1978 to 1992 is 275 Dobson units (DU), while the mean during November 1994 by Meteor-3/TOMS is 240 (±5 DU). The means of the preliminary observations during November by TOVS and MicroTOPS are 242 and 243 DU, respectively. The minimum ozone at Seguin, 221 DU, was observed by a MicroTOPS on November 17, 1994.

Fig. 1. Total ozone over the United States on November 19, 1994, observed by Meteor-3/TOMS. (Image is provided courtesy of NASA TOMS Ozone Processing Team, Goddard Space Flight Center.)

Satellite observations by TOVS and TOMS on November 19 show that the total ozone was at or below 225 DU in northern Mexico, Texas, most of the southeastern United States, and portions of New York, Pennsylvania, and Ohio (Figure 1). TOVS observations show that the zonal mean of total ozone from 25 to 35° N for November 1994 is within 1 standard deviation of the mean for 1979-1993 (Figure 2). The total ozone at Seguin (29.6° N), during November is several standard deviations below the 1979-1993 mean, and this confirms that the ozone minimum we observed was a regional event. Previously, it was reported that during the winter of 1993-1994 the ozone measured at four Dobson stations across the United States had recovered to about the amounts measured prior to the eruption of Mount Pinatubo in 1991 [Hofmann et al., 1994]. For the local region of the Southeastern United States, satellite and ground observations have shown that the low ozone observed during November 1994 persisted through December, though not as low as during November. The total ozone over Texas observed by MicroTOPS through late February 1995 resembles the unusually low amounts observed during the same period in 1993, even though most of the volcanic aerosols resident in the stratosphere in 1993 are no longer present.

Fig. 2. Mean total zonal ozone from 25° to 35° North observed by TOVS for November from 1979 to 1993 and for 1994 (error bars denote 1 standard deviation).

In all likelihood, the unusual autumn ozone event that we observed was a natural phenomenon brought about by a combination of quasi-biennial oscillation westerly winds in the equatorial stratosphere and an air mass that brought record high surface temperatures to the southeastern United States. In view of the association of the low ozone with the subtropical jet and the frequent presence of the jet over the southern U.S. this winter, low ozone over this region may continue into spring 1995. Should this occur, higher Sun angles will allow increased levels of ultraviolet light.

We thank the TOMS Ozone Processing Team for providing Meteor-3/TOMS data. The Brewer observations were funded by the U.S. Environmental Protection Agency under an agreement with the Uinversity of Georgia

References

NOAA Climate Analysis Center, United States monthly climate summary, November 1994, Weekly Clim. Bull., 94, 5, 1994.

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