THE STRUCTURE OF THE AGEOSTROPHIC WIND-FIELD IN BAROCLINIC WAVES

A high-pass filtered time series of the 500-mb height at a grid point in the Pacific storm track is regressed against the ageostrophic wind field at all grid points on the NMC grids for nine winters in order to determine the characteristic relationship between the ageostrophic wind and the geopotential distribution in baroclinic waves. The resulting ageostrophic wind pattern does not consist simply of a zonally oriented divergent circulation as sometimes pictured in theoretical models. Rather, it is dominated by the zonal component along the axis of the baroclinic waves at the 250-mb level, and the meridional component along the poleward and equatorward flanks of the waves at the 850-mb level. Diagnosis of the disturbance fields indicates that the finite meridional width of the baroclinic jet and its associated wave disturbances is the key factor in determining the distribution of the ageostrophic wind. It is shown that the ageostrophic flow based on a variable Coriolis parameter should not be used alone to estimate the divergent circulation since the divergence of the geostrophic wind is of the same order of magnitude and partly compensates the divergence of the ageostrophic wind at upper levels and augments it at lower levels.