Horizontal Divergence Associated with Zonally Isolated Jet Streams

Abstract

Horizontal divergence in the upper troposphere associated with zonally isolated jet streams in the climatological-mean fold for the Northern Hemisphere winter is examined by using the wind fields obtained from the NMC operational analyses in the 1980s. Divergence is dominant over the jet exit regions and convergence over the entrance regions, which is found to be consistent with the vertical-motion field in the ECMWF analyses. The divergence pattern cannot be fully explained in the framework of quasigeostrophic scaling. The vorticity advection by the ageostrophic flow across the tight vorticity gradient associated with the jet streams is found to be as strong as the advection by the geostrophic flow, and these two advective effects are in balance with the vortex-tube stretching associated with the observed divergence.