Ml. Santee et al., Six years of UARS Microwave Limb Sounder HNO3 observations: Seasonal, interhemispheric, and interannual variations in the lower stratosphere, J GEO RES-A, 104(D7), 1999, pp. 8225-8246
We present an overview of the seasonal, interhemispheric, and interannual v
ariations in the distribution of HNO3 in the lower stratosphere based on me
asurements of gas-phase HNO3 made by the UARS Microwave Limb Sounder (MLS)
through six complete annual cycles in both hemispheres, Outside of the wint
er polar regions, zonal-mean HNO3 mixing ratios on the 465-K potential temp
erature surface are comparable in the two hemispheres in all latitude bands
and in all years examined. Except at high latitudes, interannual variabili
ty is minimal, and there is no significant hemispheric asymmetry in the ove
rall HNO3 distribution or its seasonal cycle. Although the Antarctic experi
ences widespread severe denitrification, the MLS data indicate that the den
itrification is not complete; that is, not all polar stratospheric cloud (P
SC) particles sediment out of the lower stratosphere. Replenishment of HNO3
at 465 K during the mid- to late-winter period (when temperatures, though
still low, are generally rising) is most likely achieved through a combinat
ion of PSC evaporation and continuing weak diabatic descent. Despite large
interhemispheric and interannual differences in the extent and duration of
PSC activity and denitrification, HNO3 recovers to similar values at the en
d of every winter in both the Arctic and the Antarctic. Zonal-mean HNO3 val
ues for the two hemispheres are virtually indistinguishable for the latitud
es equatorward of 65 degrees, even during the winter months, Thus the effec
ts of severe denitrification are confined in both space and time to the reg
ions poleward of 65 degrees S during the winter and early spring.