H. Bönisch

In this paper we present evidence that the observed increase in tropical upwelling after the year 2000 may be attributed to a change in the Brewer-Dobson circulation pattern. For this purpose, we use the concept of transit times derived from residual circulation trajectories and different in-situ measurements of ozone and nitrous dioxide. Observations from the Canadian midlatitude ozone profile record, probability density functions of in-situ N₂O observations and a shift of the N₂O-O₃ correlation slopes, taken together, indicate that the increased upwelling in the tropics after the year 2000 appears to have triggered an intensification of tracer transport from the tropics into the extratropics in the lower stratosphere below about 500 K. This finding is corroborated by the fact that transit times along the shallow branch of the residual circulation into the LMS have decreased for the same time period (1993–2003). On a longer time scale (1979–2009), the transit time of the shallow residual circulation branch show a steady decrease of about −1 month/decade over the last 30 yr, while the transit times of the deep branch remain unchanged. This highlights that changes in the upwelling across the tropical tropopause are not sufficient as an indicator for changes in the entire Brewer-Dobson circulation.

The seasonality of transport and mixing of air into the lowermost stratosphere (LMS) is studied using distributions of mean age of air and a mass balance approach, based on in-situ observations of SF₆ and CO₂ during the SPURT (Spurenstofftransport in der Tropopausenregion, trace gas transport in the tropopause region) aircraft campaigns. Combining the information of the mean age of air and the water vapour distributions we demonstrate that the tropospheric air transported into the LMS above the extratropical tropopause layer (ExTL) originates predominantly from the tropical tropopause layer (TTL). The concept of our mass balance is based on simultaneous measurements of the two passive tracers and the assumption that transport into the LMS can be described by age spectra which are superposition of two different modes. Based on this concept we conclude that the stratospheric influence on LMS composition is strongest in April with extreme values of the tropospheric fractions (α₁) below 20% and that the strongest tropospheric signatures are found in October with α₁ greater than 80%. Beyond the fractions, our mass balance concept allows us to calculate the associated transit times for transport of tropospheric air from the tropics into the LMS. The shortest transit times (<0.3 years) are derived for the summer, continuously increasing up to 0.8 years by the end of spring. These findings suggest that strong quasi-horizontal mixing across the weak subtropical jet from summer to mid of autumn and the considerably shorter residual transport time-scales within the lower branch of the Brewer-Dobson circulation in summer than in winter dominates the tropospheric influence in the LMS until the beginning of next year's summer.

We evaluate the transport of three‐dimensional chemical transport models in the upper troposphere and lower stratosphere applying observed distributions of CO₂ and SF₆. The data consist of high‐resolution in situ observations, obtained during all seasons at subtropical, middle and high latitudes over Western Europe within the SPURT (Spurenstofftransport in der Tropopausenregion) project (2001–2003). We show that the combination of the two passive tracers SF₆ and CO₂ with their different tropospheric characteristics and the propagation of the temporal trends of these two gases into the lower stratosphere is a powerful diagnostic for evaluation of model transport. The model evaluation shows that all models are able to capture the general features in the tracer distributions including the vertical and horizontal propagation of the CO₂ seasonal cycle. However, the modeled CO₂ cycles are a few months out of phase in the lowermost stratosphere due to tropospheric mixing. Two models show a too strong Brewer‐Dobson circulation causing an overestimation of the tracers in the lowermost stratosphere during winter and spring. One model displays a too strong tropical isolation leading to an underestimation of the tracers in the lowermost stratosphere during winter. All models suffer to some extend from diffusion and/or too strong mixing across the tropopause. In addition, the models show too weak vertical upward transport into the upper troposphere during the boreal summer. Sensitivity studies show that our initial conditions and boundary constraints are realistic and that a horizontal resolution higher than 2 degrees and an increase of the meteorology update frequency (from 6 to 3‐hourly) have negligible impact on the modeled CO₂ and SF₆ distributions.