Observed temporal evolution of global mean age of stratospheric air for the 2002 to 2010 period

Abstract

An extensive observational data set, consisting of more than 10<sup>6</sup> SF<sub>6</sub> vertical profiles from MIPAS measurements distributed over the whole globe has been condensed into monthly zonal means of mean age of air for the period September 2002 to January 2010, binned at 10° latitude and 1–2 km altitude. The data were analysed with respect to their temporal variation by fitting a regression model consisting of a constant and a linear increase term, 2 proxies for the QBO variation, sinusoidal terms for the seasonal and semi-annual variation and overtones for the correction of the shapes to the observed data set. The impact of subsidence of mesospheric SF<sub>6</sub>-depleted air and in-mixing into non-polar latitudes on mid-latitudinal absolute age of air and its linear increase was assessed and found to be small. <br><br> The linear increase of mean age of stratospheric air was found to be positive and partly larger than the trend derived by Engel et al. (2009) for most of the Northern mid-latitudes, the middle stratosphere in the tropics, and parts of the Southern mid-latitudes, as well as for the Southern polar upper stratosphere. Multi-year decrease of age of air was found for the lowermost and the upper stratospheric tropics, for parts of Southern mid-latitudes, and for the Northern polar regions. Analysis of the amplitudes and phases of the seasonal variation shed light on the coupling of stratospheric regions to each other. In particular, the Northern mid-latitude stratosphere is well coupled to the tropics, while the Northern lowermost mid-latitudinal stratosphere is decoupled, confirming the separation of the shallow branch of the Brewer-Dobson circulation from the deep branch. We suggest an overall increased tropical upwelling, together with weakening of mixing barriers, especially in the Northern Hemisphere, as a hypothetical model to explain the observed pattern of linear multi-year increase/decrease, and amplitudes and phase shifts of the seasonal variation

Earth System Chemistry integrated Modelling (ESCiMo) with the Modular Earth Submodel System (MESSy) version 2.51

“…SF 6 is inert in most parts of the stratosphere, but photochemically destroyed in the mesosphere. The decent of SF 6 -poor air from the mesosphere into the stratosphere raises the calculated mean AoA (Stiller et al, 2012). This might explain some of the deviations between EMAC and MIPAS data in the upper stratosphere.…”

“…Moreover, data of atmospheric SF 6 concentrations from July 2002 to April 2012 are available from the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS; Fischer et al, 2008) instrument for the calculation of mean AoA. Here, we use mean AoA from the MIPAS-ENVISAT (Environmental Satellite) level 1b spectra version 5 (Haenel et al, 2015), which is an update of Stiller et al (2012). Figure 23 shows the time evolution of the annual mean tropical upward mass flux from 1960 to 2100 from the simulations in the lower (around 70 hPa, top), middle (around 10 hPa, middle), and upper (around 1 hPa, bottom) stratosphere.…”

Quantifying contributions to the recent temperature variability in the tropical tropopause layer

“…However, the strengthening or weakening of the BDC is still under debate (Butchart, 2014, and references therein). Results from observations indicate that the BDC may have slightly decelerated (Engel et al, 2009;Stiller et al, 2012), while estimates from a number of chemistry-climate models (CCMs) show in contrast a strengthening of the BDC (Butchart et al, 2010;Li et al, 2008;Butchart, 2014). The reason for the discrepancy between observed and modelled BDC changes, as well as the mechanisms of the BDC response to climate change, is still under discussion (Oberländer et al, 2013;Shepherd and McLandress, 2011).…”

Mean age of stratospheric air derived from AirCore observations

“…It relies mainly on very sporadic balloon-borne observations of CO 2 and SF 6 dating back to 1975 ) and on satellite observations of SF 6 (Stiller et al, 2012;Haenel et al, 2015). The balloon-borne observations used in Engel et al (2009) were taken in a region between 24 and 35 km where the vertical gradient in mean age at Northern Hemisphere midlatitudes was found to be very small, leading to little variability in this region.…”

“…The balloon data were limited to a total of 28 flights and showed a positive trend of 0.24 years per decade for this region, which was, however, estimated to be non-significant. Satellite observations of SF 6 used in Stiller et al (2012) and Haenel et al (2015) were limited to the lifetime of the Envisat satellite of about 10 years. They show an uneven distribution of trends with positive trends in the middle stratosphere of the Northern Hemisphere but negative trends in the Southern Hemisphere.…”