Mixing processes at the tropical and sub-tropical tropopause layer

In the middle-latitudes, the lower stratosphere exhibits intermediate properties between the troposphere and the stratosphere, which are particulary obvious in a CO/O3 diagram. The observed relationships in the tracer-tracer space have been previously interpreted as resulting from mixing events between tropospheric and stratospheric air (Hoor et al, 2004 & Pan et al., 2006) . We use measurements from the Pre-AVE campaing and backtrajectories including a parametrisation of diffusive processes, to determine the mixing time-scales and to better describe the cross-tropopause pathways in the tropical and sub-tropical region.

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.