ZONAL VARIANCE OF NITRIC-ACID VAPOR AS AN INDICATOR OF MERIDIONAL MIXING IN THE SUBTROPICAL LOWER STRATOSPHERE

Several transport scenarios have been advanced in the literature to ac count for the exchange of tracers between the extratropics and tropics in the lower stratosphere. Each scenario involves a slow, net upward transport at low latitudes and varying degrees of mixing at middle lat itudes plus detrainment across a subtropical region of enhanced tracer gradients. In situ observations for differentiating among these scena rios are generally lacking above about 21 km altitude. Existing satell ite data sets can provide useful information throughout the lower stra tosphere. In particular, we have looked for such exchange by examining time series of the daily zonal variance of the Nimbus 7 limb infrared monitor of the stratosphere distributions of nitric acid vapor (HNO3) mixing ratio. This species is characterized by strong subtropical mer idional gradients between about 10 and 100 hPa, which develop in respo nse to extratropical wave activity and any associated mixing processes . The subtropical variance is a measure of that activity, its region o f maximum values moving equatorward during late autumn and then relaxi ng and retreating in late spring. At and below the 45-hPa level there is not a clear maximum of variance at a given latitude, perhaps a resu lt of the secondary circulation that was present during the 1978/1979 winter in response to the easterly phase of the quasi-biennial oscilla tion cycle. At the 30-hPa and 50-hPa levels, while we find occasions i n winter when tongues of subtropical air were transported to higher la titudes, we find no clear evidence for a large-scale transport of extr atropical air to lower latitudes through a subtropical region of enhan ced gradients.