A 15-year climatology of stratosphere-troposphere exchange and its link to potential vorticity streamers and cutoffs

Dynamical features near the tropopause, such as filaments and isolated vortices of potential vorticity (PV), so called PV streamers and cut-offs, are often associated with extensive mass exchange between the troposphere and the stratosphere. Previous studies looked at individual events illustrating this link. Here, we use the 15-year ECMWF reanalysis (ERA) data to perform a detailed climatological study of (a) stratosphere-troposphere exchange, (b) PV streamers and cut-offs, and (c) the relationship between these two.

First, a Lagrangian approach is used to obtain a climatology of stratosphere-troposphere exchange (STE and TSE) on the northern hemisphere for the years 1979-1993. The adopted method, based upon the PV evolution along trajectories, allows (a) the identification of the regions where STE or TSE occurs, (b) a distinction between short and long lasting exchanges, and (c) a distinction of vertcally shallow and deep exchange events. Especially the long-lasting and vertically deep exchanges show a pronounced and robust seasonal variation with maximum for STE in spring and winter, and for TSE in autumn and winter. Whereas STE mainly occurs in the mid-latitudes, especially in the regions of the Atlantic and Pacific storm tracks, TSE is more predominant near the subtropics.

The second part presents a climatology of stratospheric and tropospheric PV streamers and cut-offs on isentropic surfaces. During the considered 15 years, there exists distinct seasonal cycle with, for instance, a maximum of stratospheric streamers and cut-offs in late summer and autumn, and a minimum in spring.

Finally, it is shown that streamers and cut-offs are often linked to STE and TSE. For a selected time period, the nearby surrounding in space and time of every exchange event is checked for the occurrence of streamers and cut-offs. This yields a simple count statistics, which shows for example that about 50 percent of STE exchange events are associated with stratospheric cut-offs. These findings strongly emphasize the importance of these prototype fluid dynamical structures for the mass and chemical tracer transport between the stratosphere and troposphere.