Abstract/Summary

Conceptual models of blocking structures are constructed by reducing the two-dimensional atmospheric vorticity ®eld to a few point vortices. The ¯ow is assumed to be barotropic and divergence-free, and a blocking event is represented by a point vortex dipole. The focus is here on the motion of the blocking dipole under the in¯uence of the zonal mean ¯ow. This is modelled in three different ways: A dipole embedded in a latitude-dependent zonal mean ¯ow exhibits neutrally stable oscillations; their period is estimated analytically. A cyclonic point vortex approaching from upstream can either pass the dipole or break it up, so that an -shaped pattern of three vortices emerges. The stationarity of a blocking between two troughs is modelled by four point vortices. These low-order point vortex models are compared with the dynamics of real blockings in case studies. Despite their high degree of simpli®cation, those models reproduce the kinematics of blocking events properly. This results from the discretization of the ¯ow to its actual physical states, the vortices, in contrast to the common, purely mathematical discretization to grid points. Thus, point vortex dynamics are proposed to be a powerful completion of continuous ¯uid dynamics in explaining blocking events.