Seasonal variations of a mixing layer in the lowermost stratosphere as identified by the CO‐O₃ correlation from in situ measurements

Abstract

Within the Stratosphere Troposphere Experiment by Aircraft Measurements (STREAM) project, airborne measurements of various trace species such as CO, CO2, N2O, and O3 have been performed in several years and seasons. Here we focus on a summer campaign conducted from Timmins (Canada, 47°N, 80°W) in July 1998 and winter measurements carried out in March 1997 from Kiruna (Sweden, 68°N, 20°E). From correlations between CO and O3 we identify a mixing layer in the lowermost stratosphere of which the vertical extent shows a significant seasonal variability. Troposphere to stratosphere exchange appears to be more vigorous in summer, although CO mixing ratios of the order of 15 ppbv indicate the presence of aged stratospheric air. While a significant tropospheric influence was observed up to potential temperatures of Θ = 330 K in March, the mixing layer in July increased to Θ = 360 K. In addition, its elevation above the tropopause also showed an increase. The CO‐O3 correlation indicates that the higher elevation in July is most likely caused by a stronger contribution of subtropical tropospheric air, which was mixed into the lowermost stratosphere at the subtropical jet.

On the structural changes in the Brewer-Dobson circulation after 2000

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Global atmospheric chemistry – which air matters

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The COST 723 Action

“…The extratropical tropopause is now recognized as a region where tropospheric and stratospheric air mix and, in doing so, gives rise to a transition layer of about 1 to 2 km, or 30 K in potential temperature, above the thermal tropopause (Hoor et al, 2002(Hoor et al, , 2004Sprenger and Wernli 2003;Pan et al, 2004;Hegglin et al, 2005). Deep exchanges across the tropopause occur due to tropopause folds combined with convection (Stohl et al, 2003;Reid and Vaughan, 2004;Brioude et al, 2006) and are most intense in the occluded region of cyclonic perturbations.…”