Multiyear Composite View of Ozone Enhancements and Stratosphere-to-Troposphere Transport in Dry Intrusions of Northern Hemisphere Extratropical Cyclones

We examine the role of extratropical cyclones in stratosphere-to-troposphere (STT) exchange with cyclone-centric composites of O-3 retrievals from the Microwave Limb Sounder (MLS) and the Tropospheric Emission Spectrometer (TES), contrasting them to composites obtained with the Modern-Era Retrospective-analysis for Research and Applications (MERRA and MERRA-2) reanalyses and the GEOS-Chem chemical transport model. We identify 15,978 extratropical cyclones in the northern hemisphere (NH) for 2005-2012. The lowermost stratosphere (261hPa) and middle troposphere (424hPa) composites feature a 1,000km wide O-3 enhancement in the dry intrusion (DI) airstream to the southwest of the cyclone center, coinciding with a lowered tropopause, enhanced potential vorticity, and decreased H2O. MLS composites at 261hPa show that the DI O-3 enhancements reach a 210ppbv maximum in April. At 424hPa, TES composites display maximum O-3 enhancements of 27ppbv in May. The magnitude and seasonality of these enhancements are captured by MERRA and MERRA-2, but GEOS-Chem is a factor of 2 too low. The MERRA-2 composites show that the O-3-rich DI forms a vertically aligned structure between 300 and 800hPa, wrapping cyclonically with the warm conveyor belt. In winter and spring DIs, O-3 is enhanced by 100ppbv or 100-130% at 300hPa, with significant enhancements below 500hPa (6-20ppbv or 15-30%). We estimate that extratropical cyclones result in a STT flux of 11956 TgO(3)yr(-1), accounting for 4220% of the NH extratropical O-3 STT flux. The STT flux in cyclones displays a strong dependence on westerly 300hPa wind speeds.