Geographical Variations in the Vertical Structure of Geopotential Height Fluctuations1

Abstract

Temporal (but nonseasonal) fluctuations in the geopotential height field exhibit large regional contrasts in vertical structure, as manifested in the geographical distributions of the correlation between 1000 and 500 mb height, and the ratio of the amplitudes of the fluctuations at those levels. This geographical variability is investigated in order to ascertain its seasonal, frequency and zonal wavenumber dependence and its relation to other indicators of vertical structure: statistics involving the 1000–500 mb thickness, and the structure of the dominant mode in an eigenvector analysis expansion of geopotential height in the vertical. Results are based on operational analyses by the United States National Meteorological Center over a 15-year period. Particularly striking is the contrast between transient fluctuations over the eastern oceans, which exhibit a highly barotropic structure with strong vertical coherence in the geopotential height field and small temperature variability, and those over the interior of the continents, whose structure is much more baroclinic, with low or negative temporal correlations between 1000 and 500 mb height. Such contrasts show up clearly in station data; they are observed during both winter and summer, and for temporal frequencies ranging from synoptic to interannual time scales. They are largely a reflection of the vertical structure of planetary-scale fluctuations. There is also evidence of smaller scale regional contrasts in vertical structure, some of which appear to be associated with synoptic-scale disturbances. On the basis of 1000 and 500 mb height data alone it is possible to represent, with a high degree of accuracy, the geographical distribution of the shape of the dominant eigenvector in the expansion of the vertical profile of geopotential height in transient disturbances. The implications of these results on the design of observing networks and objective analysis procedures are discussed.