Composite View of Ozone Stratosphere-Troposphere Exchange in Midlatitude Cyclones

I examine the role of midlatitude cyclones in stratosphere-troposphere exchange by compositing observations of O₃ from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES) onboard the Aura satellite and contrasting them to O₃ composites obtained with Modern Era Retrospective-analysis for Research and Applications (MERRA and MERRA-2) as well as with the GEOS-Chem chemical transport model. MERRA sea level pressure fields are used to identify 18,360 cyclones in the northern hemisphere between 2004 and 2012. In the lowermost stratosphere, cyclone-centric composites of MLS observations display a 50 ppbv mean enhancement in O₃ to the southwest of the cyclone center. The O₃ enhancements reach an 80 ppbv maximum in April. The magnitude and seasonality of this enhancement is captured by MERRA and MERRA-2 composites. In the middle troposphere, TES observations display a 5 ppbv mean O₃ enhancement reaching a 8 ppbv maximum in May, which is also reproduced by MERRA and MERRA-2. GEOS-Chem underestimates the O₃ enhancements by a factor of 2 and peak enhancements occur 1 month earlier than observed. MERRA-2 O₃ fields allow to map the composite 3-D structure of O₃ intrusions within midlatitude cyclones and examine how their magnitude and depth vary with season and potential vorticity. This framework will be used to assess the contributions of midlatitude cyclones to stratosphere-troposphere exchange of O₃ in the northern hemisphere.