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We present 20 years (1996–2015) of austral springtime measurements ofchlorine monoxide (ClO) over Antarctica from the Chlorine Oxide Experiment(ChlOE1) ground-based millimeter wave spectrometer at Scott Base, Antarctica,as well 12 years (2004–2015) of ClO measurements from the Aura MicrowaveLimb Sounder (MLS). From August onwards we observe a strong increase in lowerstratospheric ClO, with a peak column amount usually occurring in earlySeptember. From mid-September onwards we observe a strong decrease in ClO. Inorder to study interannual differences, we focus on a 3-week period from28 August to 17 September for each year and compare the average column ClOanomalies. These column ClO anomalies are shown to be highly correlated withthe average ozone mass deficit for September and October of each year. Wealso show that anomalies in column ClO are strongly anti-correlated with30 hPa temperature anomalies, both on a daily and an interannual timescale.Making use of this anti-correlation we calculate the linear dependence of theinterannual variations in column ClO on interannual variations intemperature. By making use of this relationship, we can better estimate theunderlying trend in the total chlorine (Cly  =  HCl + ClONO2+ HOCl + 2  ×  Cl2 + 2  ×  Cl2O2+ ClO + Cl). The resultant trends in Cly, which determine thelong-term trend in ClO, are estimated to be −0.5 ± 0.2,−1.4 ± 0.9, and −0.6 ± 0.4 % year−1, for zonal MLS,Scott Base MLS (both 2004–2015), and ChlOE (1996–2015) respectively. Thesetrends are within 1σ of trends in stratospheric Cly previouslyfound at other latitudes. The decrease in ClO is consistent with the trendexpected from regulations enacted under the Montreal Protocol

20 years of ClO measurements in the Antarctic lower stratosphere