Bifurcation properties of quasi-geostrophic, barotropic models and their relation to atmospheric blocking

Abstract

The bifurcation properties of a low-order, barotropic model on the sphere with orographic and a Newtonian type of vorticity forcing are reviewed. The low-order model results that multiple equilibria develop as a result of a sufficiently strong, long-wave, orographic forcing and that a suitably positioned wave vorticity forcing can enhance this bifurcation mechanism are verified with a high resolution, spectral model. The high-resolution model is integrated in time to find stable steady states. Bifurcations into multiple equilibria appear as sudden jumps in the amplitudes of the model components when the forcing is slowly changing in time.

A limited diagnostic study of some blocking events in the Northern Hemisphere during the months January–March 1979 is also presented. Calculations of the mountain torque and the eddy activity in certain regions partly support the blocking mechanism originally proposed by Charney and DeVore, and further developed by Källen.