As applied at the sub-tropical tropopause, our method shows that the mixing line can quantitatively be described in terms of stratospheric air proportion for turbulent, diffusive, transport. This Lagrangian approach hence supports the mixing interpretation and determines that, through mixing events, the tropospheric influence above the tropopause acts over a time scale of a week. Then, from further analyses of the back-trajectories and meteorological conditions, we identify the baroclinic activity as being responsible for the quasi-isentropic cross-tropopause motions involved in the mixing events.
Extending our method to the tropical region, we show that tracer profiles above the tropical tropopause can also be described as mixing lines resulting from the troposphere-stratosphere transport. In the tropics, the mixing between tropospheric air (from the TTL or the boundary layer) and purely stratospheric air is associated to a time-scale of a month, over which photochemical effects cannot be negleted. Neverheless, our approach emphasizes that diffusive processes can modelize the tropospheric influence on the vertical distribution of the chemical tracers in the tropical lower stratosphere, leading to the formation of mixing lines. We will also assess the vertical extension of this mecanism taking advantage from the data provided by the STRAT and CRAVE campaign.
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