Tuesday, 15 January 2002: 10:30 AM
The role of the Quasi-biennial oscillation in Stratospheric Dehydration
Discovery of the quasi-biennial oscillation (QBO) in the zonal flow of the equatorial statosphere is among Professor Richard Reed's major achievements. Very early Reed realized that the secondary meridional circulation associated with the QBO would modulate the transport of ozone. The QBO also influences the distribution of other vertically stratospheric constituents. Stratospheric water vapor is unusual in that is influenced by the QBO not only through transport by the meridional circulation, but also through the modulation of the tropopause temperature by the QBO. A simple two-dimensional (longitude-height) model of the water vapor budget at the equator in the 14-20 km altitude range is used to study the role of the QBO in stratospheric dehydration. Wind and temperature fields are specified following Reed's simple analytic model of the QBO. Although the annual cycle in tropopause temperature is the major control on statsopheric water, the QBO signal is clearly apparent. The influence of the QBO depends strongly on the phase of the QBO tropopause temperature signal relative to the annual cycle. Lowest water vapor mixing ratios occur when easterly shear is a maximum below the 20 km level during Northern Hemisphere winter so that the negative temperature anomaly of the QBO near the tropopause is in phase with the cold phase of the annual cycle.