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

Abstract

We use ozonesondes launched from Samoa (14°S) during the Pacific Exploratory Mission (PEM) Tropics A to show that O3 mixing ratios usually start increasing toward stratospheric values near 14 km. This is well below the tropical tropopause (as defined either in terms of lapse rate or cold point), which usually occurs between 16 and 17 km. We argue that the main reason 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 circulation occurs at roughly 14 km, that convective penetration above this altitude is rare, and that air that is injected above this height subsequently participates in a slow vertical ascent into the stratosphere. The observed dependence of ozone on potential temperature in the transitional zone between the 14-km chemopause and the tropical tropopause is consistent with what would be expected from this hypothesis given calculated clear-sky heating rates and typical in situ ozone production rates in this region. An observed anticorrelation between ozone and equivalent potential temperature below 14 km is consistent with what would be expected from an overturning Hadley circulation, with some transport of high O3/low θe air from midlatitudes. We also argue that the positive correlations between O3 and N2O in the transitional zone obtained during the 1994 Airborne Southern Hemisphere Ozone Experiment/Measurements for Assessing the Effects of Stratospheric Aircraft) (ASHOE/MAESA) campaign support the notion that air in this region does have trace elements of stratospheric air (as conjectured previously), so that some of the ozone in the transitional zone does originate from the stratosphere rather than being entirely produced in situ. Copyright 1999 by the American Geophysical Union.