Observations of middle atmosphere CO from the UARS ISAMS during the early northern winter 1991/92

Structure and kinematics of carbon monoxide in the upper stratosphere and l ower mesosphere (10-0.03 hPa) are studied for the early northern winter 199 1/92 using the Upper Atmosphere Research Satellite Improved Stratospheric a nd Mesospheric Sounder (ISAMS) measurements. The study is aided by data fro m a 6-week parameterized-chemistry run of the Goddard Space Flight Center 3 D Chemistry and Transport Model (CTM), initialized on 8 December 1991. Generally, CO mixing ratios increase with height due to the increasing sour ce contribution from CO2 photolysis. In the tropical upper stratosphere, ho wever, a local maximum in CO mixing ratio occurs. A simple photochemical mo del is used to show that this feature results largely from methane oxidatio n. In the extratropics the photochemical lifetime of CO is long, and therefore its evolution is dictated by large scale motion of air, evidenced by stron g correlation with Ertel potential vorticity. This makes CO one of the few useful observable tracers at the stratopause level and above. Thus CO maps are used to study the synoptic evolution of the polar vortex in early Janua ry 1992. Modified Lagrangian mean mixing diagnostics are applied to ISAMS and CTM da ta to examine the strength of the mixing barrier at the polar vortex edge. It is demonstrated that planetary wave activity weakens the barrier, promot ing vortex erosion. The vortex erosion first appears in the lower mesospher e and subsequently descends through the upper stratosphere, and is attribut ed to effects of planetary wave dissipation. Agreement between ISAMS and CTM is good in the horizontal distribution of C O throughout the examined period, but vertical CO gradients in the CTM weak en with time relative to the ISAMS observations.