Variability of age of air derived from MIPAS SF₆ measurements

A new and improved setup of the SF6 retrieval together with a newly calibrated version of MIPAS-ENVISAT level 1b spectra (version 5, ESA data version 5.02/5.06) was used to obtain a new global data set of vertically resolved SF6 mixing ratios, covering the total observational period of MIPAS from July 2002 to April 2012 for the first
time. This data set was validated with balloon-borne in-situ measurements as well as with data from the other renowned satellite instrument measuring SF6, the Atmospheric Chemistry Experiment Fourier Transform Spectrometer (ACE-FTS). Monthly and zonally averaged SF6 vertical profiles were converted into mean age of air (AoA) using a tropospheric SF6 reference curve. The obtained data set of age of air was compared to airborne age of air measurements. The temporal evolution of the mean age of air was then investigated in 10° latitude and 1-2 km altitude bins. A regression model consisting of a constant and a linear trend term, two proxies for the quasi-biennial oscillation variation, sinusoidal terms for the seasonal and semiannual variation and overtones was fitted to the age of air time series. The annual cycle of age of air for particular regions in the stratosphere was investigated and compared to other studies.
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The age of air trend over the total MIPAS period consisting of the linear term was assessed and compared to previous findings of Stiller et al. [2012]. While the linear increase of mean age is confirmed to be positive for the northern midlatitudes and southern polar middle stratosphere, differences are found in the northern polar upper
stratosphere, where the mean age is now found to increase as well. The magnitude of trends in the northern midlatitude middle stratosphere is slightly lower compared to the previous results and the trends fit remarkably well to the trend derived by Engel et al. [2009] for northern midlatitudes. Negative age of air trends found by
Stiller et al. [2012] are confirmed for the lowermost tropical and southern midlatitudinal stratosphere. Differences to the previous data versions occur in the middle tropical stratosphere around 25 km, where the age trends are now negative. Overall, the new
latitude-altitude distribution of trends appears to be less patchy and more coherent than the previous one. In addition, different sensitivity studies on the calculation of age of air have been carried out, including a further developed non-linearity correction with simulated age of air spectra as opposed to the traditional method where an inverse Gaussian function was used to parametrise the age spectrum as a function of the mean age. Age of air trend patterns were found to be robust with respect to these variations of the analysis method. Applying the same methods as for MIPAS, it was tried to infer decadal age of air trends also from ACE-FTS SF6 data. Even though data coverage is very sparse, significant positive decadal trends for the northern midlatitudes were found also for this data set. The hemispheric asymmetry of negative age of air trends in the Southern Hemisphere, and positive trends in the Northern Hemisphere
was found to be consistent with simulations by the Lagrangian chemistry transport model CLaMS driven by ERA-Interim data. The decadal trends are also compared to trends calculated from SF6 mixing ratios simulated by the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) and good agreement is found.
While CLaMS model calculations for the MIPAS period have shown that trends in AoA can often be attributed to trends in mixing processes and not necessarily to circulation changes, the negative AoA trends in the southern midlatitudinal lowermost stratosphere could unambiguously be attributed to an accelerating shallow branch of the Brewer-Dobson circulation.