A barrier to vertical mixing at 14 km in the tropics: Evidence from ozonesondes and aircraft measurements

We use ozonesondes launched from Samoa (14 degrees S) during the Pacific Ex ploratory Mission (PEM) Tropics A to show that O-3 mixing ratios usually st art increasing toward stratospheric values near 14 km. This is well below t he tropical tropopause las defined either in terms of lapse rate or cold po int), which usually occurs between 16 and 17 km. We argue that the main rea son for this discrepancy in height between the chemopause and tropopause is that there is very little convective detrainment of ozone-depleted marine boundary layer air above 14 km. We conjecture that the top of the Hadley ci rculation occurs at roughly 14 km, that convective penetration above this a ltitude is rare, and that air that is injected above this height subsequent ly participates in a slow vertical ascent into the stratosphere. The observ ed dependence of ozone on potential temperature in the transitional zone be tween the 14-km chemopause and the tropical tropopause is consistent with w hat would be expected from this hypothesis given calculated clear-sky heati ng rates and typical in situ ozone production rates in this region. An obse rved anticorrelation between ozone and equivalent potential temperature bel ow 14 km is consistent with what would be expected from an overturning Hadl ey circulation, with some transport of high O-3/low theta(e) air from midla titudes. We also argue that the positive correlations between O-3 and N2O i n the transitional zone obtained during the 1994 Airborne Southern Hemisphe re Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft) (ASHOE/MAESA) campaign support the notion that air in this regio n does have trace elements of Stratospheric air las conjectured previously) , so that some of the ozone in the transitional zone does originate from th e stratosphere rather than being entirely produced in situ.