Planetary-scale preconditioning for the onset of blocking

The local preconditioning of the midtropospheric planetary-scale flow prior to the onset of a blocking episode during January 1985 is investigated. The preconditioned flow is anomalously diffluent, or characterized by anomalously negative planetary-scale, geostrophic stretching deformation. This deformation increases in magnitude with time during the transition to blocking; this tendency in turn is quasigeostrophically forced by the shape of the planetary-scale component of potential vorticity transports. In particular, the planetary-scale component of potential vorticity advection that became increasingly anticyclonic with eastward distance at a rate that increased northward near the block-onset region forced the local planetary-scale flow to become more diffluent prior to blocking. Self-interactions among the synoptic-scale waves and synoptic-to- planetary-scale interactions contributed more importantly than self-interactions among planetary-scale waves to this preconditioning. In the frequency domain, the preconditioning is primarily attributable to the interactions between low- and high-frequency components of the flow, notably to the advection of slowly varying, low potential vorticity by the high-frequency flow.