Mean Age Of Stratospheric Air Research Articles

Effects of missing gravity waves on stratospheric dynamics; part 1: climatology

Energy and momentum deposition from planetary-scale Rossby waves as well as from small-scale gravity waves (GWs) largely control stratospheric dynamics. Interactions between these different wave types, however, complicate the quantification of their individual contribution to the overall dynamical state of the middle atmosphere. In state-of-the-art general circulation models (GCMs), the majority of the GW spectrum cannot be resolved and therefore has to be parameterised. This is commonly implemented in two discrete schemes, one for GWs that originate from flow over orographic obstacles and one for all other kinds of GWs (non-orographic GWs). In this study, we attempt to gain a deeper understanding of the interactions of resolved with parameterised wave driving and of their influence on the stratospheric zonal winds and on the Brewer–Dobson circulation (BDC). For this, we set up a GCM time slice experiment with two sensitivity simulations: one without orographic GWs and one without non-orographic GWs. Our findings include an acceleration of the polar vortices, which has historically been one of the main reasons for including explicit GW parameterisations in GCMs. Further, we find inter-hemispheric differences in BDC changes when omitting GWs that can be explained by wave compensation and amplification effects. These are partly evoked through local changes in the refractive properties of the atmosphere caused by the omitted GW drag and a thereby increased planetary wave propagation. However, non-local effects on the flow can act to suppress vertical wave fluxes into the stratosphere for a very strong polar vortex. Moreover, we study mean age of stratospheric air to investigate the impact of missing GWs on tracer transport. On the basis of this analysis, we suggest that the larger ratio of planetary waves to GWs leads to enhanced horizontal mixing, which can have a large impact on stratospheric tracer distributions.

Extratropical age of air trends and causative factors in climate projection simulations

Abstract. Climate model simulations show an acceleration of the Brewer–Dobson circulation (BDC) in response to climate change. While the general mechanisms for the BDC strengthening are widely understood, there are still open questions concerning the influence of the details of the wave driving. Mean age of stratospheric air (AoA) is a useful transport diagnostic for assessing changes in the BDC. Analyzing AoA from a subset of Chemistry–Climate Model Initiative part 1 climate projection simulations, we find a remarkable agreement between most of the models in simulating the largest negative AoA trends in the extratropical lower to middle stratosphere of both hemispheres (approximately between 20 and 25 geopotential kilometers (gpkm) and 20–50∘ N and S). We show that the occurrence of AoA trend minima in those regions is directly related to the climatological AoA distribution, which is sensitive to an upward shift of the circulation in response to climate change. Also other factors like a reduction of aging by mixing (AbM) and residual circulation transit times (RCTTs) contribute to the AoA distribution changes by widening the AoA isolines. Furthermore, we analyze the time evolution of AbM and RCTT trends in the extratropics and examine the connection to possible drivers focusing on local residual circulation strength, net tropical upwelling and wave driving. However, after the correction for a vertical shift of pressure levels, we find only seasonally significant trends of residual circulation strength and zonal mean wave forcing (resolved and unresolved) without a clear relation between the trends of the analyzed quantities. This indicates that additional causative factors may influence the AoA, RCTT and AbM trends. In this study, we postulate that the shrinkage of the stratosphere has the potential to influence the RCTT and AbM trends and thereby cause additional AoA changes over time.

Mean age of stratospheric air derived from AirCore observations

Abstract. Mean age of stratospheric air can be derived from observations of sufficiently long-lived trace gases with approximately linear trends in the troposphere. Mean age can serve as a tracer to investigate stratospheric transport and long-term changes in the strength of the overturning Brewer–Dobson circulation of the stratosphere. For this purpose, a low-cost method is required in order to allow for regular observations up to altitudes of about 30 km. Despite the desired low costs, high precision and accuracy are required in order to determine mean age. We present balloon-borne AirCore observations from two midlatitude sites: Timmins in Ontario/Canada and Lindenberg in Germany. During the Timmins campaign, five AirCores sampled air in parallel with a large stratospheric balloon and were analysed for CO2, CH4 and partly CO. We show that there is good agreement between the different AirCores (better than 0.1 %), especially when vertical gradients are small. The measurements from Lindenberg were performed using small low-cost balloons and yielded very comparable results. We have used the observations to extend our long-term data set of mean age observations at Northern Hemisphere midlatitudes. The time series now covers more than 40 years and shows a small, statistically non-significant positive trend of 0.15 ± 0.18 years decade−1. This trend is slightly smaller than the previous estimate of 0.24 ± 0.22 years decade−1 which was based on observations up to the year 2006. These observations are still in contrast to strong negative trends of mean age as derived from some model calculations.

Direct inversion of circulation and mixing from tracer measurements – Part 1: Method

Abstract. From a series of zonal mean global stratospheric tracer measurements sampled in altitude vs. latitude, circulation and mixing patterns are inferred by the inverse solution of the continuity equation. As a first step, the continuity equation is written as a tendency equation, which is numerically integrated over time to predict a later atmospheric state, i.e., mixing ratio and air density. The integration is formally performed by the multiplication of the initially measured atmospheric state vector by a linear prediction operator. Further, the derivative of the predicted atmospheric state with respect to the wind vector components and mixing coefficients is used to find the most likely wind vector components and mixing coefficients which minimize the residual between the predicted atmospheric state and the later measurement of the atmospheric state. Unless multiple tracers are used, this inversion problem is under-determined, and dispersive behavior of the prediction further destabilizes the inversion. Both these problems are addressed by regularization. For this purpose, a first-order smoothness constraint has been chosen. The usefulness of this method is demonstrated by application to various tracer measurements recorded with the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS). This method aims at a diagnosis of the Brewer–Dobson circulation without involving the concept of the mean age of stratospheric air, and related problems like the stratospheric tape recorder, or intrusions of mesospheric air into the stratosphere.

Unraveling impact factors for future changes in the Brewer‐Dobson circulation

Climate models project an increase in the Brewer‐Dobson circulation (BDC) with increasing future greenhouse gas (GHG) concentrations. This study identifies the causes of future changes in the BDC from sensitivity simulations with the EMAC chemistry‐climate model, by changing the external forcings, like GHG concentrations, sea surface temperatures (SSTs) together with sea ice concentrations, and ozone‐depleting substances (ODS), separately. The particular influence of rising tropical SSTs is assessed. Contributions of different waves to changes in the residual circulation are calculated as well as changes in mean age of stratospheric air (AoA) to account for the effect of mixing processes. We find that in boreal winter the tropical upward mass flux increases by about 1%/dec in the upper and 2%/dec in the lower stratosphere until the end of the 21st century. Mean AoA decreases by up to 60 and 30 days/dec, respectively. Changes in transient planetary and synoptic waves account for the strengthening of the BDC in the lower stratosphere, whereas upper stratospheric changes are due to improved propagation properties for gravity waves in future climate. Regarding the external forcings, the radiative impact of rising GHG concentrations is detected to affect upper stratospheric layers only, whereas lower stratospheric signals are almost entirely due to rising SSTs. Changes in tropical SSTs influence not only the shallow but also the deep branch of the BDC as confirmed from both changes in residual circulation and mixing. Declining ODS will slightly counteract the BDC increase in the Southern Hemisphere.

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 106 SF6 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 SF6-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. 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.

Age of stratospheric air unchanged within uncertainties over the past 30 years

The rising abundance of greenhouse gases in the atmosphere has been associated with a change in the stratospheric circulation and a decrease in the mean age of stratospheric air. Balloon-borne measurements of trace gases over 30 years suggest that, in contrast to model predictions, there have been no significant trends (at a 90% confidence limit) in the age of stratospheric air. The rising abundances of greenhouse gases in the atmosphere is associated with an increase in radiative forcing that leads to warming of the troposphere, the lower portion of the Earth’s atmosphere, and cooling of the stratosphere above1. A secondary effect of increasing levels of greenhouse gases is a possible change in the stratospheric circulation2,3, which could significantly affect chlorofluorocarbon lifetimes4, ozone levels5,6 and the climate system more generally7. Model simulations have shown that the mean age of stratospheric air8 is a good indicator of the strength of the residual circulation9, and that this mean age is expected to decrease with rising levels of greenhouse gases in the atmosphere10. Here we use balloon-borne measurements of stratospheric trace gases over the past 30 years to derive the mean age of air from sulphur hexafluoride (SF6) and CO2 mixing ratios. In contrast to the models, these observations do not show a decrease in mean age with time. If models are to make valid predictions of future stratospheric ozone levels, and of the coupling between ozone and climate change, a correct description of stratospheric transport and possible changes in the transport pathways are necessary.

Global distribution of mean age of stratospheric air from MIPAS SF<sub>6</sub> measurements

Abstract. Global distributions of profiles of sulphur hexafluoride (SF6) have been retrieved from limb emission spectra recorded by the Michelson Interferometer for Passive Atmospheric Sounding (MIPAS) on Envisat covering the period September 2002 to March 2004. Individual SF6 profiles have a precision of 0.5 pptv below 25 km altitude and a vertical resolution of 4–6 km up to 35 km altitude. These data have been validated versus in situ observations obtained during balloon flights of a cryogenic whole-air sampler. For the tropical troposphere a trend of 0.230±0.008 pptv/yr has been derived from the MIPAS data, which is in excellent agreement with the trend from ground-based flask and in situ measurements from the National Oceanic and Atmospheric Administration Earth System Research Laboratory, Global Monitoring Division. For the data set currently available, based on at least three days of data per month, monthly 5° latitude mean values have a 1σ standard error of 1%. From the global SF6 distributions, global daily and monthly distributions of the apparent mean age of air are inferred by application of the tropical tropospheric trend derived from MIPAS data. The inferred mean ages are provided for the full globe up to 90° N/S, and have a 1σ standard error of 0.25 yr. They range between 0 (near the tropical tropopause) and 7 years (except for situations of mesospheric intrusions) and agree well with earlier observations. The seasonal variation of the mean age of stratospheric air indicates episodes of severe intrusion of mesospheric air during each Northern and Southern polar winter observed, long-lasting remnants of old, subsided polar winter air over the spring and summer poles, and a rather short period of mixing with midlatitude air and/or upward transport during fall in October/November (NH) and April/May (SH), respectively, with small latitudinal gradients, immediately before the new polar vortex starts to form. The mean age distributions further confirm that SF6 is destroyed in the mesosphere to a considerable degree. Model calculations with the Karlsruhe simulation model of the middle atmosphere (KASIMA) chemical transport model agree well with observed global distributions of the mean age only if the SF6 sink reactions in the mesosphere are included in the model.

On the decay of stratospheric pollutants: Diagnosing the longest‐lived eigenmode

On the basis of a one‐dimensional (1‐D) analysis the decay time of the lowest eigenmode, τ1, for the stratospheric distribution of a conserved tracer is derived from measured vertical profiles of the mean age of stratospheric air. Two case studies (a and b) give τ1,a = 3.8 ± 0.8 years and τ1,b = 5.3 ± 1.1 years. These semiobservational times are considerably longer than most of the τ1 derived from 2‐D and 3‐D models. At the same time they are shorter than the observational eigentime, τ1,HTO = 7.7 ± 2 years, determined from the decay of the tritium (T) content in stratospheric water vapor, following the thermonuclear test explosions in the early 1960s. Part of the differences among the observational eigentimes can be explained by the assumptions that had to be made to extract τ1,HTO from the trend in the T content of stratospheric water vapor (namely, the cosmogenic background of tritiated water vapor and the trend in stratospheric water vapor). This leads to a revised value τ1,HTO = 6.3 ± 0.9 years for the time period 1975–1983. Allowing for a possible temporal trend in Γ and hence τ1, the value for the current τ1 decreases to 5.3 ± 1 years.

Analysis of residual mean transport in the stratosphere: 2. Distributions of CO2 and mean age

Distributions of CO2 and the mean age of stratospheric air are examined using the interactive two‐dimensional model described in the companion paper. It is shown that the model can explain the distribution of age in the lower stratosphere and reproduce the correlations between CO2 and N2O observed at midlatitudes in the lower stratosphere. Seasonal characteristics of the measured correlations are captured by the model. The model uses externally specified coefficients Kyy to account for large‐scale isentropic mixing in the stratosphere. The sensitivity of calculated distributions of CO2 to Kyy is be examined by comparing model runs using three different distributions of Kyy. It is shown that the CO2 measurements require Kyy to exceed a threshold value of approximately 105 m2 s−1 for most of the year. Once this threshold is exceeded, the slope of the CO2/N2 correlations depends only weakly on the magnitude and the structure of the distribution of Kyy.

CO2 and SF6 as stratospheric age tracers: Consistency and the effect of mesospheric SF6‐loss

Mixing ratios of the conserved tracers SF6 and CO2 in air samples collected with cryogenic whole‐air samplers during two balloon flights in the Arctic and mid‐latitude stratosphere up to 30 km altitude in 1997 are presented. The data are used to derive the mean age of stratospheric air. The calculated CO2‐ages agree with the corresponding SF6‐ages to within 0.5 years. These results do not confirm the findings previously reported by Harnisch et al. [1998] but rather suggest that SF6 and CO2 can be regarded as equivalent age tracers. However, evidence is presented that this general agreement may not be valid for polar stratospheric air parcels that comprise fractions of mesospheric air.

Evaluation of transport in stratospheric models

We evaluate transport characteristics of two‐ and three‐dimensional chemical transport models of the stratosphere by comparing their simulations of the mean age of stratospheric air and the propagation of annually periodic oscillations in tracer mixing ratio at the tropical tropopause into the stratosphere to inferences from in situ and satellite observations of CO2, SF6, and water vapor. The models, participants in the recent NASA “Models and Measurements II” study, display a wide range of performance. Most models propagate annual oscillations too rapidly in the vertical and overattenuate the signal. Most models also significantly underestimate mean age throughout the stratosphere, and most have at least one of several unrealistic features in their mean age contour shapes. In the lower stratosphere, model‐to‐model variation in N2O, NOy, and Cly is well correlated with variation in mean age, and the magnitude of NOy and Cly variation is large. We conclude that model transport inaccuracies significantly affect simulations of important long‐lived chemical species in the lower stratosphere.

Influence of nonlocal chemistry on tracer distributions: Inferring the mean age of air from SF6

Sulfur hexafluoride (SF6) is nearly inert in the troposphere and stratosphere and has a documented, steady increase in the troposphere due to industrial sources, making it a useful tracer of atmospheric circulation. Studies using SF6 to estimate the mean age of stratospheric air have assumed the influence of mesospheric photochemical destruction is negligible. However, the mean age of an air parcel may be sensitive to small fractions of the air that have resided for long times in the upper atmosphere. Here we use two three‐dimensional chemical transport models to estimate the influence of mesospheric SF6 loss on mean age inferences in the stratosphere. Because the mechanisms of SF6 loss are uncertain, we perform a number of simulations employing a range of magnitudes of a simple constant mesospheric loss frequency, as well as a range of scenarios for SF6 time variation in the troposphere. Using loss rates producing plausible global lifetimes of SF6 (1000 to 3000 years), and tropospheric time variation matching observations, we find that age estimates inferred from SF6 mixing ratios may be significantly biased and that the bias is increasing in time. For example, the present‐day time lag of SF6 mixing ratio from the troposphere overestimates the mean age by up to 18% at 68°S and 20 km, and up to 65% at 68°S and 30 km, depending on the loss rate and model. Correcting for this bias would bring recent comparisons of modeled and measured mean age closer in line.