Dynamics of an atmospheric blocking as deduced from its local energetics

This paper reports an analysis of the roles of temporal scale interactions (seasonal, intraseasonal and high‐frequency components) in the generation and maintenance of a pronounced block over the North Atlantic that lasted for three weeks in February 1983. This blocking disturbance had a distinct dipole structure initially oriented in an east‐west direction downstream from a strongly diffluent south‐west‐north‐east oriented seasonal jet. Although it has an equivalent barotropic structure, there is a well‐defined vertical velocity field with ascending (descending) motion on its western (eastern) flank. As it develops, matures and decays, it rotates systematically in a clockwise direction. The contributions from the various temporal scale interactions to the episodal average local energetics of this block are evaluated. There are five comparably important processes controlling the intensity, configuration, and evolution of the block. The synoptic eddy‐straining mechanism proposed by Shutts is manifested in three energetics terms, of which one is found to be particularly large. The blocking disturbance also barotropically extracts kinetic energy at a significant rate from the seasonal diffluent jet under the influence of the latter's strong deformation field. The pressure work process, the baroclinic conversion process, and above all, the nonlinear dynamics of the blocking disturbance itself are quantitatively important in redistributing the energy within the blocking region. The effects of the diabatic and subgridscale processes are found, as residues, to be substantially dissipative on the block.