Middle latitude forcing of a model tropical atmosphere with horizontally nonuniform basic states

A model of the tropical atmosphere, based on the primitive equations with beta-plane geometry and a latitudinally and longitudinally varying basic state is used to simulate the response of the tropics to middle latitude forcing. The equations of motion are linearized about a seasonally dependent basic state flow corresponding to the observed wind field in the upper troposphere. The dynamical steady state responses of the model atmosphere to planetary wave modes, each with a prescribed azimuthal wavenumber, meridional wavenumber, equivalent depth, and period at a middle latitude boundary are determined. The quasi-linear interactions between a forcing mode and the longitudinally varying part of the basic state flow give rise to a family of tropical modes with azimuthal wavenumbers different from that of the forcing. The behavior of each mode is governed by its refractive index which depends on the parameters of the wave mode and the basic state. It is shown that under certain parametric conditions the refractive index associated with a non-forcing mode can be such that a quasi-resonant response is possible. The horizontal structure of these quasi-resonances resemble that of the observed mixed Rossby-gravity waves with meridional velocity maxima and zonal velocity minima near the equator. The quasi-resonances of significant amplitude occur only with azimuthal wavenumbers four, five, and six corresponding to observations. In the winter and summer seasons, the amplitudes of the responses are stronger for forcings from the boundary with weaker basic state zonal flow. For a given azimuthal wavenumber, the quasi-resonant period is smallest in summer and largest in winter. One of the major limitations of the model is the height independence of the basic state flow which requires non-forcing modes to have the same height dependence as the forcing mode. The only quasi-resonances that can be excited by meridionally propagating middle latitude waves are modes with a large vertical length scale and small horizontal divergence. Thus the periods of these modes are somewhat smaller than those of the observed mixed Rossby-gravity modes.

Name thesis:degree_name

Ph.D.

Level thesis:degree_level

Dissertation

Discipline thesis:degree_discipline

Physics

Year dc:date

2011

Author dc:creator

• Wilson, Jeffrey David

Contributors dc:contributor

• Mak, Mankin

• middle latitude forcing
• model tropical atmosphere
• horizontally nonuniform basic states

Statement dc:rights

• 1983 Jeffrey David Wilson

Language dc:language

en

Identifier

591453

OAI identifier oai:identifier

oai:www.ideals.illinois.edu:2142/25378