ALBERT

Description: [1]  The chemical discontinuity at the extratropical tropopause (ExTP) and stratosphere-troposphere exchange (STE) pathways are investigated using the long-lived chemical species carbon monoxide (CO) and ozone (O 3 ) measured by the AuraMicrowave Limb Sounder (MLS). A relative coordinate, tropopause latitude (TpLat), is developed based on potential vorticity (PV) from the Goddard Earth Observing System Version 5 (GEOS-5) data assimilation system. TpLat is defined as theshortest geographic distance along an isentropic surfacefrom the extratropical tropopause (ExTP) to an observation location. Our results show that this coordinate highlights the sharp chemical discontinuities at the ExTP more clearly than the widely-used equivalent latitude coordinate (EqLat). Geographical distributions of STE pathways and barriers are investigated based on meridional gradients in O 3 abundances in the new TpLat coordinate in conjunction with analysis of Rossby wave-breaking between 330 K and 360 K. In northern hemispheric (NH) winter (Dec-Jan-Feb), NH STE pathways are seen mainly above the northeast Pacific. In NH summer (Jun-Jul-Aug), the NH pathway covers all longitudes at 330 K. However, it is mainly located above Asia at 340 K and above the Atlantic and the North Pacific at 350 K and 360 K.In the southern hemisphere (SH), there is a weaker STE region above the Eastern Indian Ocean and the southwestern Pacific,at and above 350 K in SH winter, and a stronger STE region overthe Southeastern Pacificat these levels during SH summer. In addition, this study shows NH PV gradients are slightly stronger near the ExTP in summer than in winter even though the subtropical jet is weaker and Rossby-wave breaking is stronger in summer than in winter.