Tuesday 22 Oct 2013: CliMathNet e-seminar: A Simple Conceptual Framework for Stratosphere Troposphere Coupling
Prof Mark Baldwin - University of Exeter
Harrison 209 16:00-17:00
At mid-to-high latitudes, the stratosphere contains >25 percent of the column of atmospheric mass. Large-scale vertically-propagating waves alter the strength of the stratospheric polar vortex and drive a synchronised meridional circulation. These stratospheric changes are associated with substantial effects on surface weather and climate, especially on the Northern and Southern Annular Modes (NAM, SAM) and associated long-lasting shifts in the jet streams, storm tracks, pre- cipitation, and likelihood of blocking events. Despite unambiguous observations of this phenomenon, as well as numerical simulations, the primary dynamics of this downward coupling are not understood. I will show that the meridional circulation extends below the tropopause, modulating the day-to-day thickness of the troposphere, especially at high northern latitudes. The vertical displacement of the polar air column acts like a plunger controlling the movement of water in a pipe. The return flow in the troposphere is also synchronised to the strength of the polar vortex. I will make the case that the altered meridional circulation acts as a trigger for tropospheric eddy feedbacks. This conceptual framework predicts surface pressure variations similar to the NAM, and linear variation of tropospheric phenomena with the strength of the polar vortex. The results point to the importance of the Arctic tropopause layer for assessing the fidelity of stratosphere– troposphere coupling in climate and weather forecast models.