The onset of blocking flow (persistent easterly geostrophic flow over midlatitudes) in the Northern Hemisphere middle troposphere is typically preceded, in analyses of the planetary-scale geostrophic wind field, by anomalously weakened westerly flow (u' < 0) and anomalously strengthened southerly flow (v' . 0) in the middle troposphere just west of the incipient block. This precondition for the transition to blocking contributes to anomalously negative planetary-scale geostrophic stretching deformation, D1' ( = du'/dx - dv'/dy), in the middle troposphere near the incipient block. The transition to blocking can be described by increasingly negative D1' near the developing block; these negative tendencies begin several days before the onset of blocked flow. The negative D1' tendencies were forced quasigeostrophically, in one blocking case, by midtropospheric potential vorticity transports which became increasingly anticyclonic with eastward and northward distance (while projecting onto the planetary scale) near the incipient block. Quantitatively, the quasigeostophically diagnosed deformation tendencies did not fully account for the analyzed deformation tendencies, suggesting that nonquasigeostrophic processes were contributing to preconditioning of the flow during this blocking case.
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12th Conference on Atmospheric and Oceanic Fluid Dynamics