19.07.2016: FB1-Seminar

14:00 h, Gr. Konferenzraum, Düsternbrooker Weg 20

Abstract:

The Brewer-Dobson Circulation is the slow overturning circulation of the stratosphere, which transports mass up into the tropics and poleward until it returns to the troposphere in the extratropics.  In concert with chemical processes, it sets the distribution of stratospheric ozone and water vapor, which have significant impacts on surface climate.   The circulation can be understood as a response to mechanical wave driving by planetary-scale Rossby waves and small-scale gravity waves.  Climate models almost uniformly project an increase in the Brewer-Dobson Circulation in response to anthropogenic forcing, but differ significantly in explaining how this change is effected, disagreeing whether resolved Rossby waves or parameterized gravity waves drive the change.  Given this uncertainty (and the fact that observations, if anything, hint at a recent weakening of the circulation), there has been justifiable concern about the model projections.

In this talk, I'll first show that understanding the response to global warming may actually be simpler than the question of what drives the climatological circulation.  The so-called increase of the Brewer-Dobson Circulation might better be described as an upward shift, associated with the general lifting of the entire atmospheric circulation in response to greenhouse gas forcing.  We relate the change in the circulation at a given pressure level to the change in the tropopause, which can in turn be linked to changes in surface temperature and tropospheric stability driven by greenhouse gases.  Second, an idealized atmospheric model allows us to explore the interaction between resolved Rossby waves and parameterized gravity waves, and thus explain how comprehensive models have come to such different conclusions as to what drives the response to global warming.  These interactions demand a more nuanced answer to the question: "What drives the Brewer-Dobson Circulation?”

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