A Barotropic Envelope Rossby Soliton Model for Block–Eddy Interaction. Part I: Effect of Topography

Abstract

A new forced envelope Rossby soliton model in an equivalent barotropic beta-plane channel is proposed to describe the interaction between an incipient block (planetary scale) and short synoptic-scale eddies. This model is based on two assumptions, motivated by observations that (i) there exists a zonal scale separation between the planetary-scale and synoptic-scale waves and (ii) that the range of synoptic-scale zonal wavenumber is comparable to the planetary-scale zonal wavenumber. These assumptions allow an analytical treatment. The evolution of the planetary-scale block under the influence of synoptic-scale eddies is described by a forced nonlinear Schrödinger equation that is solved numerically, while the feedback of block development on the preexisting synoptic-scale eddies is derived analytically. It is shown that the planetary-scale projection of the nonlinear interaction between synoptic-scale eddies is the most important contributor to the amplification and decay of the planetary-scale blocking dipole or anticyclone, while the synoptic?planetary-scale interaction contributes significantly to the downstream development of preexisting synoptic-scale eddies. Large-scale topography plays a secondary role compared to the synoptic-scale eddies in exciting the block. However, it plays a role in inducing a standing planetary-scale ridge prior to block onset, which fixes the geographical location of the block and induces meridional asymmetry in the flow. In particular, the topographically induced planetary-scale ridge that is almost in phase with a dipole component of blocking flow is found to be a controlling factor for the northward deflection of storm tracks associated with blocking anticyclones.