Articles | Volume 17, issue 15
Research article
04 Aug 2017
Research article |  | 04 Aug 2017

Trends and annual cycles in soundings of Arctic tropospheric ozone

Bo Christiansen, Nis Jepsen, Rigel Kivi, Georg Hansen, Niels Larsen, and Ulrik Smith Korsholm

Abstract. Ozone soundings from nine Nordic stations have been homogenized and interpolated to standard pressure levels. The different stations have very different data coverage; the longest period with data is from the end of the 1980s to 2014.

At each pressure level the homogenized ozone time series have been analysed with a model that includes both low-frequency variability in the form of a polynomial, an annual cycle with harmonics, the possibility for low-frequency variability in the annual amplitude and phasing, and either white noise or noise given by a first-order autoregressive process. The fitting of the parameters is performed with a Bayesian approach not only giving the mean values but also confidence intervals.

The results show that all stations agree on a well-defined annual cycle in the free troposphere with a relatively confined maximum in the early summer. Regarding the low-frequency variability, it is found that Scoresbysund, Ny Ålesund, Sodankylä, Eureka, and Ørland show similar, significant signals with a maximum near 2005 followed by a decrease. This change is characteristic for all pressure levels in the free troposphere. A significant change in the annual cycle was found for Ny Ålesund, Scoresbysund, and Sodankylä. The changes at these stations are in agreement with the interpretation that the early summer maximum is appearing earlier in the year.

The results are shown to be robust to the different settings of the model parameters such as the order of the polynomial, number of harmonics in the annual cycle, and the type of noise.

Short summary
Ozone soundings in the troposphere from nine Arctic stations covering the period 1984–2014 have been analyzed. Stations with the best data coverage show a consistent and significant temporal variation with a maximum near 2005 followed by a decrease. Some significant changes are found in the annual cycle in agreement with the notion that the ozone summer maximum is appearing earlier in the year. Such changes in Arctic ozone in the free troposphere have not been reported before.
Final-revised paper