The propagation of eddies in diffluent jetstreams: Eddy vorticity forcing of ‘blocking’ flow fields

Abstract

Numerical experiments are performed with barotropic models to test a hypothesized mechanism by which barotropic eddies can, through Reynolds stresses, reinforce blocking flow patterns. Eddies propagating into a split Jetstream suffer an east‐west compression and north‐south extension of their vorticity fields and this enhanced, local enstrophy cascade is associated with energy transmission to the straining flow (i.e. the blocking flow field) and a characteristic pattern of vorticity forcing by transient motion. As a first step in demonstrating this mechanism, the barotropic vorticity equation is integrated in a form linearized about a state chosen to represent a split Jetstream with a prescribed eddy forcing function upstream. After thirty days of model integration, the time‐mean eddy kinetic energy, enstrophy and vorticity flux divergence are calculated, together with mean eddy vorticity flux vectors. The resulting eddy vorticity forcing field can be used to determine the second‐order induced flow. Experiments with the nonlinear barotropic vorticity equation show that dipole blocking patterns can be created simply by the introduction of an eddy generator into sufficiently weak, uniform westerly flow. Eddies alone are responsible for the block since the time‐mean vorticity equation has no external forcing functions such as an orographic term. Monthly mean statistics are again calculated and terms in the time‐mean vorticity equation are compared. Anticyclonic eddy vorticity forcing appears just upstream of the ‘blocking high’ in agreement with data analysed by Illari (1982) for the anomalous circulation over Europe in July 1976. Model flow fields fluctuate in a manner highly reminiscent of blocking episodes as observed in 500mb contour maps with strong bursts of southerly winds on the western flank of blocking highs during the approach and subsequent stagnation of each depression. The proposed straining mechanism and model simulations are in agreement with early observational studies of blocking.