Zonally Symmetric Hough Modes and Meridional Circulations in the Middle Atmosphere

Abstract

The structure of the. meridional circulation driven by steady, zonally symmetric thermal or mechanical forcing is analyzed via the techniques of tidal theory. The relevant Hough functions are derived and, together with consideration of their associated vertical structure, these are used as the basis for discussion of forced meridional circulations in the middle atmosphere. Thus, the global extent of the complementary cooling associated with stratospheric warmings in the high-latitude winter hemisphere is explained as a manifestation of the global nature of the associated (residual) mean circulation. However, the response to equatorial forcing is more localized and it appears that the high-latitude manifestations of the quasi-biennial oscillation cannot be accounted for as a result of coupling, via a mean circulation, with the tropical stratosphere.