The Structure and Energetics of Transient Disturbances in the Northern Hemisphere Wintertime Circulation

Abstract

The hemispheric distributions of wintertime circulation statistics derived from the forecast fields of vertical motion are presented. The dominant features in the pattern for time-mean vertical velocity are consistent with the existence of thermally direct meridional circulations over the entrance regions of the principal jet streams, and thermally indirect circulations over the jet exit regions. Rising motions in the transient disturbances are seen to display positive temporal correlations with temperature and geopotential height over the major oceanic storm track regions. On the other hand, the western portion of the continents and the adjacent oceanic areas are characterized by downward eddy transports of geopotential energy at 850 and 500 mb, as well as much reduced temporal correlations between the vertical motion and temperature fields. The vertical phase structure of the transient disturbances at various geographical locations is studied by performing a cross-spectral analysis of the time series of geopotential height fields at 850, 500 and 250 mb. The local geopotential height fluctuations at different pressure levels are strongly coherent. Over the sites characterized by enhanced development of transient waves, geopotential height perturbations of synoptic temporal scales are seen to lag by about 60° (1/6 cycle) between the tropopause and 850 mb levels. The corresponding phase lag is reduced to about 25° over the western portion of the continents, and the disturbances acquire a barotropic character at these longitudes. The results of a detailed diagnosis of the local, time-averaged budgets of time-mean and transient eddy kinetic energy at 300 mb are discussed. The kinetic energy of the intensified time-mean flow at the jet stream cores is primarily maintained by the local, time-averaged ageostrophic circulations, which dominate over the effects due to eddy-mean flow interactions. The energy generated in these source regions is transported by the time-mean flow to the jet exit regions, where the thermally indirect circulations function as local sinks of mean kinetic energy. Analogously, eddy kinetic energy at the jet stream level is generated by ageostrophic motions in the transient disturbances over the western oceans, it is then advected to the western portion of the continents by the time-averaged flow, and is eventually dissipated by the super-geostrophic flow in the eddies at those longitudes. The regional character of the transient eddy statistics presented in this and earlier papers is interpreted in the light of the results from a recent modeling study by Simmons and Hoskins (1978) on the life cycle of nonlinear baroclinic waves.