Extratropical case study of stratosphere–troposphere exchange using multivariate analyses from mozaic aircraft data

A multivariate analysis methodology, applied to ozone, water vapour and potential temperature data collected from MOZAIC aircraft allowed to identify and to quantify three types of air masses directly linked to stratosphere-troposphere exchanges (STE). These air masses occurred in February 1997 over the North Atlantic during the development of a Rossby wave, which is manifested in the form of four different structures, namely trough, ridge, streamer and cut-off low (COL). Here a study is conducted on 20 isobaric (i.e. at 230 hPa) flights crossing all these upper-level structures. It is shown that the first type of air mass corresponds to a mixed zone between the stratosphere and the troposphere when the tropopause is poorly defined and when there exist medium values and weak gradients of ozone, potential temperature and humidity. The second type of air mass reveals an irreversible transport from the troposphere to the stratosphere for all these structures leading to a “wet” stratosphere with high values of water vapour. The third type of air mass corresponds to the classical stratosphere with high values of ozone and low values of water vapour. The comparison with the model results of Kowol-Santen et al. (2000) shows that these first two air masses are directly linked to either diabatic processes or turbulent diffusion depending on the orientation and the type of the structure. The western side of the trough (with no mixing zone on the eastern side) and the ridge are dominated by clear air turbulence (CAT). On the other hand, the decay of the COL seems to be the result of strong convection on its northern and eastern flanks. Concerning the decay of the streamer, it is found to be due to both diabatic and turbulent processes. The above results suggest that the existence of a mixing zone during STE seems to be a more realistic concept instead of that where the tropopause is considered as a surface.