Origin of dynamical features preceding the onset of blocking

Detailed investigation of a blocking event during January 1985 reveals that the planetary-scale stretching deformation associated with this event developed several days before block onset due to the interaction between high-frequency flow components and a low-frequency reservoir of anticyclonic potential vorticity. The high-frequency flow components in turn were associated with pulses of synoptic-scale, anticyclonic potential vorticity advected toward the block-onset region by the planetary-scale flow. The origin of each of these key dynamical features is being investigated. In particular, the importance of tropical and midlatitude diabatic processes in the origin of the synoptic-scale anticyclonic pulses and the low-frequency anticyclonic potential vorticity reservoir is being examined. The relevance of these features to other blocking cases is also being investigated and will be reported.