J. Eluszkiewicz et al., RESIDUAL CIRCULATION IN THE STRATOSPHERE AND LOWER MESOSPHERE AS DIAGNOSED FROM MICROWAVE LIMB SOUNDER DATA, Journal of the atmospheric sciences, 53(2), 1996, pp. 217-240
Results for the residual circulation in the stratosphere and lower mes
osphere between September 1991 and August 1994 are reported. This circ
ulation is diagnosed by applying an accurate radiative transfer code t
o temperature, ozone, and water vapor measurements acquired by the Mic
rowave Limb Sounder (MLS) onboard the Upper Atmosphere Research Satell
ite (UARS), augmented by climatological distributions of methane, nitr
ous oxide, nitrogen dioxide, surface albedo, and cloud cover. The sens
itivity of the computed heating rates to the presence of Mt. Pinatubo
aerosols is explored by utilizing aerosol properties derived from the
measurements obtained by the Improved Stratospheric and Mesospheric So
under instrument, also onboard UARS. The computed vertical velocities
exhibit a semiannual oscillation (SAG) around the tropical stratopause
, with the region of downward velocities reaching maximum spatial exte
nt in February and August. This behavior reflects the semiannual oscil
lation in temperature and ozone and mimics that seen in past studies o
f the October 1978-May 1979 period based on data from the Limb Infrare
d Monitor of the Stratosphere onboard the Nimbus 7 satellite. The SAO
vertical velocities are stronger during the northern winter phase, as
expected if planetary waves from the winter hemisphere are involved in
driving the SAG. A possible quasi-biennial oscillation (QBO) signal e
xtending from the middle into the upper stratosphere is also hinted at
, with the equatorial vertical velocities in the region 10-1 hPa signi
ficantly smaller (or even negative) in 1993/94 than in 1992/93. Despit
e the short data record, the authors believe that this pattern reflect
s a QBO signal rather than a coincidental interannual variability, sin
ce the time-height section of vertical velocity at the equator resembl
es that of the zonal wind. Wintertime high-latitude descent rates are
usually greater in the Northern Hemisphere, but they also exhibit sign
ificant variability there. In the three northern winters analyzed in t
his study, strong downward velocities are diagnosed in the lower strat
osphere during stratospheric warmings and are associated with enhanced
wave forcing (computed as the momentum residual) in the mid- and uppe
r stratosphere. The implications of the computed circulation for the d
istribution of tracers are illustrated by the example of the ''double-
peaked'' structure in the water vapor distribution measured by MLS.