Sensitivity of Age of Air Trends on the derivation method for non-linear increasing tracers

Abstract

<p><strong>Abstract.</strong> Mean age of air (AoA) is a diagnostic of transport along the stratospheric Brewer-Dobson circulation. While models consistently show negative trends, long-term time series (1975–2016) of AoA derived from observations show non-significant positive trends in mean AoA in the northern hemisphere. This discrepancy between observed and modeled mean AoA trends is still not resolved. There are uncertainties and assumptions required when deriving AoA from trace gas observations. At the same time, AoA from climate models is subject to uncertainties, too.</p> <p> In this paper, we focus on the uncertainties due to the parameter selection in the method that is used to derive mean AoA from SF<sub>6</sub> measurements in Engel et al. (2009) and Engel et al. (2017). To correct for the non-linear increase in SF<sub>6</sub> concentrations, a quadratic fit to the time-series at the reference location, i.e. the tropical surface, is used. For this derivation, the width of the AoA distribution (age spectrum) has to be assumed. In addition, to choose the number of years the quadratic fit is performed for, the fraction of the age spectrum to be considered has to be assumed. Even though the uncertainty range due to all different aspects has already been taken into account for the total errors on the AoA values, the systematic influence of the parameter selection on AoA trends is described for the first time in the present study. </p><p> In addition, a method to derive mean AoA is evaluated that applies a convolution to the reference time series. The resulting mean AoA and its trend only depend on an assumption about the ratio of moments. Also in that case, it is found that the larger the ratio of moments, the more the AoA trend gravitates towards the negative. The linear tracer and SF<sub>6</sub> AoA is found to agree within 0.3 % in the mean and 6 % in the trend. </p><p> The different methods and parameter selections were then applied to the balloon borne SF<sub>6</sub> and CO<sub>2</sub> observations. We found the same systematic changes in mean AoA trend dependent on the specific selection. When applying a parameter choice that is suggested by the model results, the AoA trend is reduced from 0.15 years/decade to 0.07 years/decade. It illustrates that correctly constraining those parameters is crucial for correct mean AoA and trend estimates and still remains a challenge in the real atmosphere. </p><p> Engel, A., Möbius, T., Bönisch, H., Schmidt, U., Heinz, R., Levin, I., Atlas, E., Aoki, S., Nakazawa, T., Sugawara, S., et al.: Age of stratospheric air unchanged within uncertainties over the past 30 years, Nature Geoscience, 2, 28, 2009.</p><p> Engel, A., Bönisch, H., Ullrich, M., Sitals, R., Membrive, O., Danis, F., and Crevoisier, C.: Mean age of stratospheric air derived from AirCore observations, Atmospheric Chemistry and Physics, 17, 6825–6838, <a href="https://doi.org/10.5194/acp-17-6825-2017" target="_blank">https://doi.org/10.5194/acp-17-6825-2017</a>, 2017.

A convolution of observational and model data to estimate age of air spectra in the northern hemispheric lower stratosphere

“…In its presented form, all derived inverse age spectra assume the tropical tropopause to be a single source region into the stratosphere. Although this appears valid for the upper and middle stratosphere (Fueglistaler et al, 2009), studies have shown that for the lowermost extratropical stratosphere, quasi-isentropic transport across the local tropopause has critical influence and strongly affects trace gas burdens in that region (Hoor et al, 2005;Bönisch et al, 2009). Therefore, the assumption of single entry through the tropical tropopause layer is insufficient to estimate a precise age spectrum and must be modified to include air entrainment through the complete tropopause together with related seasonality.…”

“…Mechanical drivers of the BDC are planetary-and synoptic-scale atmospheric waves that get excited in the troposphere and propagate upward into the extratropical middle stratosphere where they finally break and transfer their momentum to induce a poleward motion (Haynes et al, 1991;Holton et al, 1995). The wave drag causes air to rise slowly in the tropics mainly through the tropical tropopause layer (TTL) to compensate for the poleward drift (Fueglistaler et al, 2009). Eventually, air descends at higher latitudes back into the troposphere.…”

The impact of SF<sub>6</sub> sinks on age of air climatologies and trends

“…The calculation uses a polynomial fit to the reference time series to approximate mean AoA. However, in our study we modified the parameters compared to those used in Engel et al (2009) to ensure that (passive) SF 6 -based AoA agrees with the ideal AoA derived from the linear tracer, following Fritsch et al (2019). Specifically, in our calculations we used a ratio of moments of 1.0 years and a fraction of input of 95 %.…”

“…The timeseries of the realistic SF 6 tracer emissions show a stronger increase in the 1980s (not shown). Fritsch et al (2019) showed that due to this increase the calculation of AoA based on SF 6 -like tracers is more sensitive to the chosen parameters in the AoA derivation for this time. The drop in AoA we see is caused by this limitation of the derivation from SF 6 -like tracers.…”

Impact of Lagrangian Transport on Lower-Stratospheric Transport Time Scales in a Climate Model

“…The model used in this work is EMAC, the MESSy (Modular Earth Submodel System) version of the ECHAM5 climate model (see Jöckel et al (2010) for details on EMAC and Roeckner et al (2006) for details on ECHAM5). EMAC is a modern chemistry-climate model which is commonly used for studies of the stratosphere and upper troposphere (Sinnhuber and Meul, 2015;Oberländer-Hayn et al, 2016;Fritsch et al, 2019), as well as studies of the troposphere. In this work, EMAC is operated at the T42L90MA spectral resolution, corresponding to a horizontal quadratic Gaussian grid of approximately 2.8 •…”