Quantifying the impact of the North American monsoon and deep midlatitude convection on the subtropical lowermost stratosphere using in situ measurements

The chemical composition of the lowermost stratosphere exhibits both spatial and temporal variability depending upon the relative strength of (1) isentropic transport from the tropical tropopause layer (TTL), (2) diabatic descent from the midlatitude and northern midlatitude stratosphere followed by equatorward isentropic transport, and (3) diabatic ascent from the troposphere through convection. In situ measurements made in the lowermost stratosphere over Florida illustrate the additional impact of equatorward flow around the monsoon anticyclone.

This flow carries, along with older stratospheric air, the distinct signature of deep midlatitude convection. We use simultaneous in situ measurements of water vapor (H₂O), ozone (O₃), total odd nitrogen (NOy), carbon dioxide (CO₂), and carbon monoxide (CO) in the framework of a simple box model to quantify the composition of the air sampled in the lowermost stratosphere during the mission on the basis of tracer mixing ratios ascribed to the source regions for these transport pathways.