A survey of blocking mecfhanisms, with application to the Australian region

A variety of dynamical mechanisms has been proposed in the literature to explain the phenomenon of blocking. There are two major phenomena which require explanation, namely (i) what process initiates the blocked state, and (ii) what is the mechanism which maintains the blocked state over a considerable period of time? The proposed mechanisms are reviewed in the light of the characteristics of blocking in the Australian region, particularly the Tasman Sea. On the basis of the existing synoptic data it is not possible to positively identify any particular mechanism as the dominant process, but it is possible to eliminate some contenders and identify those which show the greatest promise. Two salient features emerge from the observations, namely that Australian region blocking is apparently a local phenomenon (rather than a hemispheric one) in many cases, and normally has a north-south dipole (low-high) structure. The most promising mechanisms which are consistent with these features appear to be as follows: (i) the baroclinic instability mechanism (Frederiksen 1982) of a flow which is influenced S, by local topography and thermal forcing, producing the dipole structure, and (ii) the ‘modon’ mechanism (McWilliams 1980) aided by thermal forcing associated with SST patterns, which can account for the persistence; the latter may be regarded as a localised form of multiple equilibria. The inclusion of thermal forcing is a new contribution of this paper, and it enhances the potential significance of modons for blocking.