A Numerical Investigation of the Dynamics of Blocking Waves in a Simple Two-Level Model

Abstract

The atmospheric blocking problem is discussed within the framework of a spectral two-level quasi-geostrophic model with β-plane geometry. By further developing the basic ideas of Egger (1978, 1979) for the formation of blocking centers, a baroclinic mechanism is proposed which is almost as simple as the barotropic concept of Egger. In both concepts nonlinear interactions of forced and free slowly moving waves are important for the accumulation of eddy energy in only a few quasi-stationary waves. In this paper the possibility is studied that long waves get energy baroclinically from the zonal mean flow rather than from external sources. The baroclinic instability of the longest waves is not of linear nature but purely nonlinear. From a series of model runs some statistics of the dynamics of model blocking were extracted. Indeed, blocking dynamics are significantly different from the dynamics prevailing in blocking-free periods. The “nuclei” (Egger, 1978) of the model-generated-blockings are the 500 mb Rossby stationary pressure waves (together with the corresponding temperature modes). It was found that the proposed baroclinic type of block is well represented in the model simulations. One of the numerical experiments is studied in more detail in order to give more information on the spectral energetics of the flow.