Merging OSIRIS, SAGE II and MLS Vertical Ozone Profiles for the Determination of Long Term Trends

Abstract:

The OSIRIS instrument has been in operation onboard the Odin spacecraft since the autumn of 2001. Since that time OSIRIS has routinely measured spectrally dispersed limb-scattered sunlight from which vertical ozone profiles have been derived. The length of the OSIRIS ozone data record makes it a valuable resource for the analysis of long term trends. However, on its own it is not of sufficient length to capture all of the relevant signatures required to completely understand important changes in the vertical distribution of ozone. In particular the 1997 turnaround is not captured within the OSIRIS time series. For this reason it is important that the OSIRIS measurements be combined, or merged, with other data records like the SAGE II time series before a full analysis can be performed. This paper is focused on the merged SAGE II – OSIRIS time series and the resulting trends. It has been recently noted that the post 1997 positive trends at around 40 km that have been derived from this time series are higher than those calculated using other merged data sets. In this paper we will detail our efforts to understand why this is so. We will present analyses related to instrument drift, sampling biases and sensitivity to the method used to determine long term trends. Included along with the thorough analysis of the SAGE II – OSIRIS merged time series is an analysis of a SAGE II – MLS time series combined using the exact same method. Similarities and differences in the results derived from the two merged time series will also be discussed.