L. Froidevaux et al., GLOBAL OZONE OBSERVATIONS FROM THE UARS MLS - AN OVERVIEW OF ZONAL-MEAN RESULTS, Journal of the atmospheric sciences, 51(20), 1994, pp. 2846-2866
Global ozone observations from the Microwave Limb Sounder (MLS) aboard
the Upper Atmosphere Research Satellite (UARS) are presented, in both
vertically resolved and column abundance formats. The authors review
the zonal-mean ozone variations measured over the two and a half years
since launch in September 1991. Well-known features such as the annua
l and semiannual variations are ubiquitous. In the equatorial regions,
longer-term changes are believed to be related to the quasi-biennial
oscillation (QBO), with a strong semiannual signal above 20 hPa. Ozone
values near 50 hPa exhibit an equatorial low from October 1991 to Jun
e 1992, after which the low ozone pattern splits into two subtropical
lows (possibly in connection with residual circulation changes tied to
the QBO) and returns to an equatorial low in September 1993. The ozon
e hole development at high southern latitudes is apparent in MLS colum
n data integrated down to 100 hPa, with a pattern generally consistent
with Nimbus-7 Total Ozone Mapping Spectrometer (TOMS) measurements of
total column; the MLS data reinforce current knowledge of this lower-
stratospheric phenomenon by providing a height-dependent view of the v
ariations. The region of 30-degrees-S to 30-degrees-N (an area equal t
o half the global area) shows very little change in the ozone column f
rom year to year and within each year. The most striking ozone changes
have occurred at northern midlatitudes, with the October 1992 to July
1993 column values significantly lower than during the prior year. Th
e zonal-mean changes manifest themselves as a slower rate of increase
during the 1992/93 winter, and there is some evidence for a lower fall
minimum. A recovery occurs during late summer of 1993; early 1994 val
ues are significantly larger than during the two previous winters. The
se results are in general agreement with variations measured by the Ni
mbus-7 TOMS and Meteor-3 TOMS instruments at midlatitudes. However, th
e southern midlatitudes exhibit less of a column ozone decrease (relat
ive to the north) in the MLS data (down to 100 hPa) than in the TOMS c
olumn results. The timing and latitudinal extent of the northern midla
titude decreases appear to rule out observed ClO enhancements in the A
rctic vortex, with related chemical processing and ozone dilution effe
cts, as a unique cause. Local depletion from ClO-related chemical mech
anisms alone is also not sufficient, based on MLS ClO data. The puzzli
ng asymmetric nature of the changes probably requires a dynamical comp
onent as an explanation. A combination of effects (including chemical
destruction via heterogeneous processes and QBO phasing) apparently ne
eds to be invoked. This dataset will place constraints on future model
ing studies, which are required to better understand the source of the
observed changes. Finally, residual ozone values extracted from TOMS-
minus-MLS column data are briefly presented as a preliminary view into
the potential usefulness of such studies, with information on troposp
heric ozone as an ultimate goal.