Dynamics of Blocking Highs

It is proposed that the nonlinear interaction of forced waves and slowly moving free waves May lead to the development of blocking highs under favorable conditions. This idea is tested numerically for three types of quasi-geostrophic inviscid channel flow with low spectral resolution in the zonal direction where the forcing is provided by orography. First, barotropic channel flow without wave-mean flow interaction and a constant mean velocity u¯ is considered. The forcing induces standing waves, whereas the phase speed c of the free waves is given by the Rossby formula. A booking high develops when u¯ is chosen to make c≈0 for some of those waves which can interact with the forced standing waves. Blocking does not occur without forcing. Next, the model is extended to include wave-mean flow interaction. Blocking highs with realistic lifetimes develop when u¯ is appropriately chosen initially. The meridional profile of u¯ shows a double let when the block is fully developed. As a third step baroclinicity is admitted (two-level model). Blocking highs develop even under these more general conditions and show a strong preference for certain positions with respect to the orography. It is concluded that the proposed mechanism is capable of producing flow patterns in the channel which show good similarity to observed cases of blocking. However, the model used so far is so simple that it remains open as to whether the blocking mechanism tested in the model may be held responsible for blocking activity in the atmosphere.