Extended Abstract (2.7M)P6.11
A 15-year climatology of stratosphere-troposphere exchange and its link to potential vorticity streamers and cutoffs
Michael Sprenger, ETH, Zuerich, Switzerland; and H. Wernli and M. Bourqui
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.
Poster Session 6, Storm Tracks/Strat-Trop interactions/Transport & Mixing—Eyeopener (Continental Breakfast)
Thursday, 7 June 2001, 8:30 AM-10:00 AM
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