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A sudden stratospheric warming (SSW) in early January 2013 caused theArctic polar vortex to split and temperatures to rapidly rise above thethreshold for chlorine activation. However, ozone in the lowerstratospheric polar vortex from late December 2012 through earlyFebruary 2013 reached the lowest values on record for that time ofyear. Analysis of Aura Microwave Limb Sounder (MLS) trace gasmeasurements and Cloud-Aerosol Lidar and Infrared Pathfinder SatelliteObservations (CALIPSO) polar stratospheric cloud (PSC) data shows thatexceptional chemical ozone loss early in the 2012/13 Arctic winterresulted from a unique combination of meteorological conditionsassociated with the early-January 2013 SSW: unusually low temperaturesin December 2012, offspring vortices within which air remained wellisolated for nearly 1 month after the vortex split, and greater-than-usual vortex sunlight exposure throughout December 2012 and January2013. Conditions in the two offspring vortices differed substantially,with the one overlying Canada having lower temperatures, lower nitricacid (HNO3), lower hydrogen chloride, more sunlightexposure/higher ClO in late January, and a later onset of chlorinedeactivation than the one overlying Siberia. MLS HNO3 andCALIPSO data indicate that PSC activity in December 2012 was moreextensive and persistent than at that time in any other Arctic winter inthe past decade. Chlorine monoxide (ClO, measured by MLS) rose earlierthan previously observed and was the largest on record throughmid-January 2013. Enhanced vortex ClO persisted until mid-Februarydespite the cessation of PSC activity when the SSW started. VortexHNO3 remained depressed after PSCs had disappeared; passivetransport calculations indicate vortex-averaged denitrification of about4 parts per billion by volume. The estimated vortex-averaged chemical ozone loss,~ 0.7–0.8 parts per million by volume near 500 K (~21 km), was the largestDecember/January loss in the MLS record from 2004/05 to 2014/15

Polar processing in a split vortex: Arctic ozone loss in early winter 2012/2013