Inorganic chlorine variability in the antarctic vortex and implications for ozone recovery

Abstract

We infer the interannual variability of inorganic chlorine in the Antarctic lower stratospheric vortex using 9 years of Aura Microwave Limb Sounder (MLS) nitrous oxide (N2 O) measurements and a previously measured compact correlation. Inorganic chlorine (Cly) is the sum of the destruction products of long-lived chlorine-containing source gases. Its correlation with N2O, derived from observations in the year 2000, is scaled to the years 2004–2012 to account for subsequent N2O growth and chlorofluorocarbon decline. The expected annual Cly change due to the Montreal Protocol is 20 ppt/yr, but the MLS-inferred Cly varies year-to-year from 200 to +150 ppt. Because of this large variability, attributing Antarctic ozone recovery to a statistically significant chlorine trend requires 10 years of chlorine decline. We examine the relationship between equivalent effective stratospheric chlorine (EESC) and ozone hole area. Temperature variations driven by dynamics are a primary contributor to area variability, but we find a clear linear relationship between EESC and area during years when Antarctic collar temperatures are 1σ or more below the mean. This relationship suggests that smaller ozone hole areas in recent cold years 2008 and 2011 are responding to decreased chlorine loading. Using ozone hole areas from 1979 to 2013, the projected EESC decline, and the inferred interannual Cly variability, we expect ozone hole areas greater than 20 million km2 will occur during very cold years until 2040. After that time, all ozone hole areas are likely to be below that size due to reduced EESC levels.