Planetary-Scale Preconditioning for the Onset of Blocking

Abstract

Abstract 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 ...