Interannual Variability of Trace Gases in the Subtropical Winter Stratosphere

Abstract

Measurements of water vapor and methane from the Halogen Occultation Experiment instrument on board the Upper Atmosphere Research Satellite are used to study the interannual variability of trace gas distributions in the atmosphere. Particular attention is paid to the mechanisms influencing trace gas distributions in the subtropics. The study highlights the quasi-biennial oscillation (QBO) dependence of subtropical tracer distributions more clearly than in previous studies. There is a strong correlation between the equatorial wind QBO and the slope of the tracer isolines in the Northern Hemisphere subtropics, with steeper subtropical isoline slopes in the easterly phase compared with the westerly phase. This is particularly so in the lower stratosphere. Two possible mechanisms for the QBO signal in subtropical isoline slopes are identified: advection by the mean circulation and isentropic mixing. A comparison between the QBO signal in the slope of the tracer isolines and the isentropic tracer gradients is proposed as a method of determining which process is dominant. The authors suggest that the behavior of these two data diagnostics provides a stringent constraint on computer models of the atmosphere. On the basis of these diagnostics three height regions of the subtropical atmosphere are identified. 1) Below 450?500 K isentropic mixing associated with tropospheric disturbances penetrating the lower stratosphere is dominant. 2) In the region 500?750 K the data suggest that advection by the mean meridional circulation is important and that the role of isentropic mixing by eddies is relatively small. 3) Above 750 K isentropic mixing becomes increasingly important with height, and both advection and mixing are influential in determining the subtropical tracer distributions.