The Non-Linear Response of a Two-Layer, Baroclinic Ocean to a Stationary, Axially-Symmetric Hurricane: Part I. Upwelling Induced by Momentum Transfer

James J. O'Brien National Center for Atmospheric Research, Boulder, Colo.

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Robert O. Reid Texas A&M University, College Station

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Abstract

This study is concerned with the theoretical description of upwelling induced in a stratified, rotating, two-layer ocean by momentum transfer from an intense stationary, axially-symmetric atmospheric vortex. The dynamic internal response of the ocean is assumed to be axially-symmetric which permits consideration of the solution in two independent variables, radial distance and time. Numerical integration via the method of characteristics is utilized to obtain values of radial velocity, tangential velocity, and depth of the upper layer for a period of two days. Transfer of momentum between the air and the sea and between the upper and lower layers are allowed. Transfer of heat and moisture with the atmosphere is not considered.

A general model is derived which leads to a hierarchy of models of increasing complexity. The detailed solution of the first of these is illustrated.

Results agree qualitatively with observations taken in the Gulf of Mexico following hurricane Hilda, 1964. Intense upwelling is confined to within twice the radius of maximum winds. The displaced warm central waters produce some downwelling adjacent to the upwelled region. The degree of upwelling is time-dependent and the hurricane-force winds must act on the ocean for several hours before significant upwelling occurs. The model indicates a strong coupling of the radially propagating internal wave mode and the vortex mode of the system. This coupling confines the significant internal disturbances to within the wind-forced region.

Abstract

This study is concerned with the theoretical description of upwelling induced in a stratified, rotating, two-layer ocean by momentum transfer from an intense stationary, axially-symmetric atmospheric vortex. The dynamic internal response of the ocean is assumed to be axially-symmetric which permits consideration of the solution in two independent variables, radial distance and time. Numerical integration via the method of characteristics is utilized to obtain values of radial velocity, tangential velocity, and depth of the upper layer for a period of two days. Transfer of momentum between the air and the sea and between the upper and lower layers are allowed. Transfer of heat and moisture with the atmosphere is not considered.

A general model is derived which leads to a hierarchy of models of increasing complexity. The detailed solution of the first of these is illustrated.

Results agree qualitatively with observations taken in the Gulf of Mexico following hurricane Hilda, 1964. Intense upwelling is confined to within twice the radius of maximum winds. The displaced warm central waters produce some downwelling adjacent to the upwelled region. The degree of upwelling is time-dependent and the hurricane-force winds must act on the ocean for several hours before significant upwelling occurs. The model indicates a strong coupling of the radially propagating internal wave mode and the vortex mode of the system. This coupling confines the significant internal disturbances to within the wind-forced region.

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