A Nonlinear Multiscale Theory of Atmospheric Blocking: Dynamical and Thermodynamic Effects of Meridional Potential Vorticity Gradient

Abstract

AbstractIn this paper, an extended nonlinear multiscale interaction model is proposed to examine nonlinear behavior of eddy-driven blocking as a Rossby wave packet in a three-dimensional background flow by dividing the background meridional potential vorticity gradient (PVy) into dynamical PVy related to the horizontal (mainly meridional) shear of background westerly wind (BWW) and thermodynamic PVy associated with the meridional temperature gradient (MTG). It is found that eddy-driven baroclinic blocking with large amplitude in the midtroposphere tends to have a longer lifetime (~20 days) in a baroclinic atmosphere with stratification than eddy-driven barotropic blocking without vertical variation (less than 15 days). It is shown that barotropic blocking shows a northwest–southeast orientation and has long lifetime, large retrogression, and slow decay only for weaker barotropic BWW and in higher latitudes. In a baroclinic atmosphere with stratification, baroclinic blocking shows long lifetime, strong eastward movement, slow decay, weak strength, and less local persistence for large barotropic BWW and under , but becomes less slow decay, weak retrogression, and large local persistence for small barotropic BWW and . Such a blocking with a north–south antisymmetric dipole, large amplitude, and long local persistence, characterized by a persistent large meander of westerly jet streams, is easily seen when baroclinic BWW and are small in the lower to midtroposphere. Comparatively, the magnitude of plays a larger role in the blocking change than that of , whereas the vertical variation of MTG is more important for the blocking change than the MTG itself for some cases.