12th Conference on Middle Atmosphere

Tuesday, 5 November 2002
Lower stratospheric equatorial downward motion during the westerly shear quasi-biennial oscillation phase
Hyunah Lee, NCAR, Boulder, CO; and W. Choi and A. Lambert
During the transition period from the easterly to the westerly QBO, characteristic variations of ozone and aerosol are repeatedly observed by the HALOE in the equatorial lower stratospheric region: ozone increases abruptly while the aerosol decreases significantly from 10S to 10N. Ozone variations induced by the equatorial quasi-biennial oscillation (QBO) are simulated using a fully interactive NCAR two-dimensional chemical-dynamical-radiative model (SOCRATES). During the westerly shear phase of the QBO, the calculated absolute vertical velocity is negative for a short time period, which is consistent with the warm temperature anomaly related with the QBO. Due to the downward motion, the temporal variation of equatorial ozone shows a very sharp increase in the 10S to 10 N region, which is similar to the observed ozone variation. Budget analysis of the ozone variation supports the fact that the primary factor influencing the large ozone increase during the westerly shear QBO phase is not a chemical reaction or diffusion but a transport process. In addition to the ozone variation, the temporal variation of the lower stratospheric aerosol is analyzed to understand the mechanism causing sudden decrease. The calculated aerosol settling velocity using effective radius estimated from SAGE II surface area density is not large enough to explain the observed aerosol decrease, which fortifies the importance of the transport process induced by the QBO secondary circulation during westerly shear. Aerosol extinction distributions observed by HALOE during the same period show similar variations.

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