Interaction between a slowly moving planetary—scale dipole envelope rossby soliton and a wavenumber— two topography in a forced higher order nonlinear Schrödinger equation

Abstract

A parametrically excited higher—order nonlinear Schrödinger (NLS) equation is derived to describe the interaction of a .slowly moving planetary—scale envelope Rossby soliton for zonal wavenumber-two with a wavenumber—two topography under the LG—type dipole near-resonant conditioa The numerical solution of this equation is made. It is found that in a weak background westerly wind satisfying the LG—type dipole near—resonance condition, when an incipient envelope Rossby soliton is located in the topographic trough and propagates slowly, it can be amplified through the near—resonant forcing of wavenumber-two topography and can exhibit an oscillatioa However, this soliton can break up after a long time and excite a train of small amplitude waves that propagate westward. In addition, it is observed that in the soliton—topography interaction the topographically near—resonantly forced planetary—scale soliton has a slowly westward propagation, but a slowly eastward propagation after a certain time. The instantaneous total streamfunction fields of the topographically forced planetary—scale soliton are found to bear remarkable resemblance to the initiation, maintenance and decay of observed omega—type blocking high and dipole blocking. The soliton perturbation theory is used to examine the role of a wavenumber—two topography in near—resonantly forcing omega—type blocking high and dipole blocking. It can be shown that in the amplifying process of forced planetary—scale soliton, due to the inclusion of the higher order terms its group velocity gradually tends to be equal to its phase velocity so that the block envelope and carrier wave can be phase—locked at a certain time. This shows that the initiation of blocking is a transfer of amplified envelope soliton system from dispersion to nondispersion. However, there exists a reverse process during the decay of blocking. It appears that in the higher latitude regions, the planetary—scale envelope soliton—topography interaction could be regarded as a possible mechanism of the establishment of blocking.