ClO in the upper stratosphere is affected by variations in CH4 and by the solar cycle, hence seasonal and interannual variations are not necessarily representative of changes in chlorine, but since 1995 the ChlOE measurements near the peak of the ClO mixing ratio at Mauna Kea (20N) do show a decline in ClO of ~0.6%/year. Since the ChlOE measurements span the gap between the UARS and Aura MLS measurement periods they have also been used to show that, within the uncertainty of the ChlOE and MLS measurements, there is no significant bias between the two sets of MLS measurements.
At Scott Base (78S) we measure the large seasonal changes in lower stratospheric ClO which occur from mid-August through mid-October during the ‘ozone hole' period, when temperatures are low enough for the formation of polar stratospheric clouds, and sunlight is available. The interannual variations in the measured ClO column over Scott Base are shown to be closely correlated with overall ozone mass depletion in September and October. They are also anti-correlated with the local interannual variations in 30 hPa temperatures from the MERRA reanalysis. Because of the complicated coupling of seasonal and diurnal variations of column ClO at these high latitudes a direct comparison of MLS and ChlOE measurements is difficult, but they do show similar sensitivities to interannual temperature variations. We make use a simple parameterization of the interannual 30 hPa temperature and column ClO variations in order to obtain chlorine trend estimates in this region.