INTERHEMISPHERIC ASYMMETRY IN THE 1 MBAR O-3 TREND - AN ANALYSIS USING AN INTERACTIVE ZONAL MEAN MODEL AND UARS DATA

Trends in O-3 calculated from solar backscattered ultraviolet (SBUV) o bservations near 1 mbar are more negative at high latitudes in the sou thern hemisphere than in the northern hemisphere [Hood et al., 1993]. A mechanism is presented that produces an interhemispheric O-3 trend a symmetry similar to the observed asymmetry in the Goddard Space Flight Center dynamically interactive zonal mean model. Upper Atmosphere Res earch Satellite (UARS) data. are then examined for evidence that the a tmospheric trend asymmetry is produced by a similar mechanism. The mod el O-3 trend asymmetry is mainly due to interhemispheric differences i n odd chlorine (Cl-y) partitioning. The asymmetry in Cl-y partitioning is caused primarily by lower amounts of CH4 and NO in the southern he misphere than the northern hemisphere high latitudes, due to differenc es in the dynamical behavior of the model hemispheres. Symmetric incre ases in Cl-y are accompanied by a southern hemisphere increase in ClO that is larger than in the northern hemisphere. Concentrations of CH4 and N2O retrieved by the cryogenic limb array etalon spectrometer aboa rd UARS during 1992 are lower in the southern hemisphere fall and wint er seasons at high latitudes in the upper stratosphere than in the nor thern hemisphere, favoring higher southern hemisphere ClO values. Howe ver, observations of ClO by the microwave limb sounder on UARS do not show consistently higher values in the southern hemisphere compared wi th the northern hemisphere in 1992, 1993, or 1994. The UARS data there fore do not confirm that a mechanism similar to the model mechanism oc curs in the real atmosphere and is the cause of the SBUV O-3 trend asy mmetry.