Planetary-scale baroclinic envelope Rossby solitons in a two-layer model and their interaction with synoptic-scale eddies

Abstract

In this paper, planetary-scale baroclinic envelope Rossby solitons for zonal wavenumber 2 in a two-layer model are first investigated. It is found that when the shear of basic state westerly winds between the upper and lower layers is weak, both the upper- and lower-layer envelope Rossby solitons are almost in phase and exhibit vortex pair block structures which have a weak baroclinicity. The possibility of the application of planetary-scale envelope Rossby soliton to observed dipole blocks is discussed. Second, a highly idealized model having weak vertical shear is proposed to investigate the interaction between baroclinic planetary-scale dipole soliton (weak incipient dipole block) and a train of synoptic-scale waves (eddies) upstream. It is shown that in the interaction process, both the planetary-scale dipole soliton and the synoptic-scale eddies are deformed simultaneously. The amplitude of the dipole soliton block can be amplified through the near-resonant forcing of synoptic-scale eddies. In the intensification process of the dipole soliton block, its zonal scale is prolonged, and its phase velocity gradually tends to be equal to its group velocity so that the block envelope and carrier wave can be phase-locked at a certain time. This process reflects a transfer of dipole blocking system from dispersion to nondispersion. This may explain why the synoptic-scale eddies can reinforce and maintain blocking dipole. Conversely, in the decay process, the zonal scale of the dipole soliton block is shortened, and the discrepancy between its phase velocity and group velocity gradually increases. This process may describe a transfer of dipole blocking system from nondispersion to dispersion, which leads to the break-down of the dipole block. Due to the feedback of the intensified dipole block on the synoptic-scale eddies, the upper-layer eddies are split into two branches, and the lower-layer eddies are split into three branches. Moreover, the instantaneous total streamfunction fields (the streamfunction field of planetary-scale envelope Rossby soliton plus the streamfunction field of the deformed synoptic-scale eddies) that describe the coupling between baroclinic dipole envelope soliton and synoptic-scale eddies are found to bear a remarkable resemblance to the synoptic maps observed during a blocking episode. It appears that the eddy-forced envelope soliton model established here could describe the initiation, maintenance and decay of dipole blocking by the baroclinic synoptic-scale eddies. Therefore, this theoretical model seems to provide a theoretical framework for further studying the interaction between synoptic-scale eddies and dipole block.