MASS Seminar - Thomas Birner (Colorado State Univ)
Date Time Location
November 22nd, 2010 12:00pm-1:00pm 54-915
TITLE:

Stratospheric circulation impact on the structure and composition of the global tropopause region

ABSTRACT:

The stratospheric circulation (often referred to as Brewer-Dobson circulation) is a mechanically driven global scale circulation that transports mass and constituents (e.g. ozone and water vapor) from the cold tropical tropopause to the warmer extratropical regions. This circulation induces adiabatic cooling within its tropical upwelling branch and adiabatic warming within its extratropical downwelling branch.
In the first part of this talk it will be shown that these dynamical effects significantly modify the global tropopause structure. More than half of the equator-to-pole contrast in tropopause height is found to be due to stratospheric dynamics with the dominant changes taking place in the tropics. The existence of the region known as tropical tropopause layer is found to be almost entirely due to stratospheric dynamics.
In the second part of this talk transport within the stratospheric circulation is studied using residual circulation trajectories, i.e. trajectories driven by the residual mean meridional and vertical velocities. Diagnostics based on these residual circulation trajectories support the notion of two well-separated stratospheric circulation branches: a deep branch with upwelling over the inner tropics and downwelling over the polar regions, and a shallow branch with the main upwelling significantly displaced off the equator and downwelling over the mid-latitudes. Residual circulation transit times of air traveling from the tropical tropopause to the extratropical lowermost stratosphere are evaluated and their seasonal cycle is discussed in light of observed mean age of air estimates.

Host: Daniela Domeisen