Diagnosing Secondary Circulations

Secondary circulations, defined here as a dynamical response to a primary circulation, are fundamental to many phenomena in the atmosphere and ocean. Examples include frontal circulations near atmospheric extratropical cyclones and oceanic mesoscale eddies, the Hadley circulation, the Brewer-Dobson circulation, and the azimuthal-averaged overturning circulation in hurricanes. Diagnosing secondary circulations, including their relationship to the primary circulation, is essential for testing theories and, in particular, understanding why air rises in response to the action of the primary circulation.

In a landmark series of papers, Dan Keyser and collaborators established a rigorous framework for diagnosing secondary circulations in three-dimensions, and showed the utility of this approach for extratropical cyclones. The framework is notable for generalizing previous approaches, such that circulations may be analyzed through arbitrarily oriented orthogonal vertical planes. Among their findings using this method, Keyser et al. quantified the contributions to rising motion in the comma cloud of extratropical cyclones coming from the baroclinic wave and mesoscale frontal circulations. I will review these ideas, their application in the literature, and suggest potential future applications.