Final Activity Report Summary - DEMETRA (Determination and measure of transport and mixing at the tropopause) The tropopause is an atmospheric layer at 8-15 km of altitude. It separates from above the troposphere, where most of meteorological phenomena take place, and from below the stratosphere (a region characterised by mainly horizontal transport). By controlling the transport of air masses (and their chemical content) between the troposphere and the stratosphere, the tropopause is a key component of the climate system. Cross-tropopause transport affects critical phenomena including water vapour distribution, greenhouse gases vertical distribution, ozone depletion, and fate of long-lived pollutants like airplanes exhausts. These problems are important not only from a scientific viewpoint but they also condition our ability to predict the effect of natural and anthropogenic emissions and to plan environmental policies on firmer ground.The main contribution of this project to the understanding of tropopause dynamics has been the development of a theoretical tool able to pinpoint in space and time mixing events from winds datasets. Previously, mixing at the tropopause was detected by diagnostics like the effective diffusivity that depend on the latitude but have the main limitation of not resolving mixing variability along the longitude. Other methods, like the Lyapunov exponents, locate mixing events in both latitude and longitude, but quantify mixing in units that are not directly related to the diffusion of tracer gases. The method that has been developed in this project ("Lyapunov diffusivity") combines both the advantages, providing longitude and latitude variability of mixing as well as diffusion units. This method can measure mixing events on any two-dimensional layer and therefore can be extended to the entire stratosphere. As an example of application, I have been able to quantify cross-tropopause mixing events induced by El Nino that modulate the transport between the tropics and the extratropics. This project has also shown that mixing events detected in this way are qualitatively in good agreement with tracer dynamics, and notably observations of extratropical ozone intrusion.For the future and on the basis of this Marie-Curie project, I aim at a parameterisation of mixing events for reducing the errors in general circulation models and possibly to the development of a similar approach for studying mixing in the ocean.
