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Stationary Rossby Wave Propagation through Easterly Layers

Edwin K. SchneiderCenter for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, MA 02139

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Ian G. WattersonCenter for Meteorology and Physical Oceanography, Massachusetts Institute of Technology, Cambridge, MA 02139

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Abstract

The linearized shallow water equations on a sphere are solved numerically to examine the sensitivity of the steady response to midlatitude mountain forcing to the zonal mean basic state. The zonal mean basic state consists of meridionally varying zonal winds ū(y) and meridional winds v̄(y). Cases are considered where ū is westerly everywhere, outside a tropical region where it is easterly. A zonal wavenumber three mountain confined to the Northern Hemisphere midlatitudes, where ū>0, provides the forcing.

When v̄≡0 the usual result of negligible Southern Hemisphere response to the mountain forcing is found. However, a modest mean meridional velocity [0(3 m s−1)] that is directed from north to south through the easterly layer leads to significant Southern Hemisphere response. An argument based on the local dispersion relation is offered to explain this effect. It is concluded that critical latitude effects on wave propagation are sensitive to the structure of the mean meridional circulation in the critical latitude region of the model. The result of the simplified model suggests that a more relevant model with a zonally symmetric basic state consisting of zonal winds and meridional circulation varying with height as well as latitude should be investigated.

Abstract

The linearized shallow water equations on a sphere are solved numerically to examine the sensitivity of the steady response to midlatitude mountain forcing to the zonal mean basic state. The zonal mean basic state consists of meridionally varying zonal winds ū(y) and meridional winds v̄(y). Cases are considered where ū is westerly everywhere, outside a tropical region where it is easterly. A zonal wavenumber three mountain confined to the Northern Hemisphere midlatitudes, where ū>0, provides the forcing.

When v̄≡0 the usual result of negligible Southern Hemisphere response to the mountain forcing is found. However, a modest mean meridional velocity [0(3 m s−1)] that is directed from north to south through the easterly layer leads to significant Southern Hemisphere response. An argument based on the local dispersion relation is offered to explain this effect. It is concluded that critical latitude effects on wave propagation are sensitive to the structure of the mean meridional circulation in the critical latitude region of the model. The result of the simplified model suggests that a more relevant model with a zonally symmetric basic state consisting of zonal winds and meridional circulation varying with height as well as latitude should be investigated.

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