The 'scissors' effect: anisotropic and ageostrophic influences on wind correlation coefficients

An explanation is sought for systematic departures of observed wind component correlation coefficients from those expected for geostrophic flow in a horizontally homogeneous and isotropic atmosphere. These departures are such that the elongated contours of the zonal and meridional component coefficients are rotated towards one another like scissors. The effects of anisotropy, inhomogeneity and ageostrophy are considered from a theoretical standpoint. It is concluded that both anisotropy and ageostrophy could in principle contribute to the observed 'scissors'. Data from both North America and Australia suggest anisotropy is the main contributor, but the effect of ageostrophy is not negligible. The systematic contribution of anisotropy may be interpreted in terms of the poleward eddy transport of angular momentum.