Intraseasonal oscillations of stratospheric ozone above Switzerland

GROMOS, the ground-based millimeter-wave ozone spectrometer, continuously measures the stratospheric ozone profile between the altitudes of 20 and 65 km above Bern (46°57′N46°57′N, 7°27′E7°27′E) since November 1994. Characteristics of intraseasonal oscillations of stratospheric ozone are derived from the long-term data set. Spectral analysis gives evidence for a dominant oscillation period of about 20 days in the lower and middle stratosphere during winter time. A strong 20-day wave is also found in collocated geopotential height measurements of the microwave limb sounder onboard the Aura satellite (Aura/MLS) confirming the ground-based observations of GROMOS and underlining the link between ozone and dynamics. Remarkably, the ozone series of GROMOS show an interannual variability of the strength of intraseasonal oscillations of stratospheric ozone. The interannual variability of ozone fluctuations is possibly due to influences of planetary wave forcing and the quasi-biennial oscillation (QBO) on the meridional Brewer–Dobson circulation of the middle atmosphere. In detail, time series of the mean amplitude of ozone fluctuations with periods ranging from 10 to 60 days are derived at fixed pressure levels. The mean amplitude series are regarded as a measure of the strength of intraseasonal oscillations of stratospheric ozone above Bern. After deseasonalizing the mean amplitude series, we find QBO-like amplitude modulations of the intraseasonal oscillations of ozone. The amplitudes of the intraseasonal oscillations are enhanced by a factor of 2 in 1997, 2001, 2003, and 2005. QBO-like variations of intraseasonal oscillations are also present in wind, temperature and other parameters above Bern as indicated by meteorological reanalyses of the European Centre for Medium-range Weather Forecasts (ECMWF). Further, intercomparisons of interannual variability of intraseasonal tropospheric and stratospheric oscillations are performed where the NAO index (North-Atlantic oscillation) and the MJO index (Madden–Julian oscillation) are taken as proxies for tropospheric oscillations. As a result, interannual variability of intraseasonal oscillations of the stratosphere seems not to be well correlated with respect to that of the troposphere.

► We derived mean characteristics of intraseasonal oscillations of stratospheric ozone above Bern. ► The vertical ozone profiles were measured by the microwave radiometer GROMOS. ► Mean spectra of ozone oscillations have a dominant spectral component at a period of 20 days in winter. ► We discovered QBO-like interannual modulations of intraseasonal oscillations in ozone and dynamical parameters. ► Indication of a relationship between intraseasonal oscillations and sudden stratospheric warmings seems to exist.