Abstract
Abstract. 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 SF6 data set, covering the total observational period of MIPAS from July 2002 to April 2012 for the first time. Monthly and zonally averaged SF6 profiles were converted into mean age of air 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 for particular regions in the stratosphere was investigated and compared to other studies. 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 version and the trends fit remarkably well to the trend derived by Engel et al. (2009). Negative age of air trends found by Stiller et al. (2012) are confirmed for the lowermost tropical stratosphere and lowermost southern midlatitudinal stratosphere. Differences to the previous data versions occur in the middle tropical stratosphere around 25 km, where the trends are now negative. Overall, the new latitude–altitude distribution of trends appears to be less patchy and more coherent than the previous one. The new data provide evidence of an accelerating shallow branch of the Brewer–Dobson circulation, at least in the Southern Hemisphere. Finally the age of air decadal trends are compared to trends calculated with simulated SF6 values by the Karlsruhe Simulation Model of the Middle Atmosphere (KASIMA) and good agreement is found. The hemispheric asymmetry in the trends found in the MIPAS data is also indicated in the trends calculated with simulated SF6 values by the KASIMA model.
Highlights
While it is widely accepted that climate change with enhanced greenhouse-gas abundances leads to a warming of the troposphere and a cooling of the stratosphere, the secondary effects, in particular on the global circulation in the stratosphere, the Brewer–Dobson circulation (BDC), are still an issue of current research (Butchart, 2014)
The mean age of air comprises both information on the speed of the advection and the amount of mixing and stirring exerted on the air parcel
Zonal means were converted into mean age of air, from which decadal trends were inferred for latitude and altitude bins
Summary
While it is widely accepted that climate change with enhanced greenhouse-gas abundances leads to a warming of the troposphere and a cooling of the stratosphere, the secondary effects, in particular on the global circulation in the stratosphere, the Brewer–Dobson circulation (BDC), are still an issue of current research (Butchart, 2014). The non-homogeneity of trends was reported by Monge-Sanz et al (2013), who found a significant increasing trend in the mean age of air over northern midlatitudes in an multiannual CTM simulation driven by ERA-Interim winds over the period 1990– 2009 and confirmed the measurements by Stiller et al (2012) and Engel et al (2009). In their model study they already noticed a hemispheric asymmetry, which was later found by Mahieu et al (2014) with SLIMCAT model calculations. MIPAS could observe atmospheric parameters in the altitude range from 5 to 160 km with minimum and maximum steps of 1 and 8 km respectively (Fischer et al, 2008)
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