Baroclinic Modons as Prototypes for Atmospheric Blocking

Abstract

A two-layer quasi-geostrophic channel model on a ?-plane is used to investigate the properties of dipole eddies which may be relevant models for atmospheric blocking. It is shown that quasi-stationary equivalent barotropic dipole eddies, similar to the 1½-layer(reduced gravity) ?modons? of Stern, can be resonantly excited in ?realistic? westerly zonal wind conditions, including vertical shear, and that these eddies are persistent on lifetimes which are comparable with those of blocking. These eddies cannot persist indefinitely due to interactions with stationary Rossby wave modes. The decay of these eddies into Rossby waves is studied under varying zonal flow conditions and it is found that the decay rate increases as the group velocity of the stationary Rossby waves increase. The decay rate is also found to be sensitive to the closeness of the channel walls which can restrict the development of the radiation field. Meanwhile the loss of fluid (and hence potential vorticity) from the vortices is achieved in narrow tongues which emerge from the stagnation point on the downstream side of the dipole. The decay due to Rossby-wave radiation does not appear to alter the propagation speed of the dipoles with respect to the zonal flow allowing them to remain as quasi-stationary anomalies despite large changes in circulation amplitude. This decay mechanism is briefly compared with spindown induced by Ekman friction and it is concluded that Rossby wave radiation is probably the more efficient process as it affects all vertical levels simultaneously.