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A Modeling Study of the Effect of the Andes on the Summertime Circulation of Tropical South America

Richard KleemanBureau of Meteorology Research Centre, Melbourne, Victoria, Australia

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Abstract

A simple linear two level model of the atmosphere is developed which has a reasonable representation of the external and tropically important baroclinic modes. By blocking the lower layer of the model with a meridional wall, the interaction between diabatic forcing centered in the Amazon basin and the Andes is studied. This forcing can be considered a source of Rossby waves that scatter from the above partial barrier. The scattering process is examined analytically by making the long-wave approximation, with the conclusion that transmission of external Rossby waves, reflection of baroclinic Kelvin waves, and the creation of topographic jets are likely to be important.

Numerical solutions without the long-wave approximation are then considered and the effects of the above scattering process are examined. The upper level circulation is shown to be qualitatively similar to that obtained without a barrier. The low-level circulation west of the barrier is weak in the winds and consists of a positive geopotential response centered at approximately 30°S. To the east the circulation near the barrier shows similarity to models of the Somali jet. The model produces quite strong trade winds and places a low-pressure center in approximately the position where one is actually observed.

Abstract

A simple linear two level model of the atmosphere is developed which has a reasonable representation of the external and tropically important baroclinic modes. By blocking the lower layer of the model with a meridional wall, the interaction between diabatic forcing centered in the Amazon basin and the Andes is studied. This forcing can be considered a source of Rossby waves that scatter from the above partial barrier. The scattering process is examined analytically by making the long-wave approximation, with the conclusion that transmission of external Rossby waves, reflection of baroclinic Kelvin waves, and the creation of topographic jets are likely to be important.

Numerical solutions without the long-wave approximation are then considered and the effects of the above scattering process are examined. The upper level circulation is shown to be qualitatively similar to that obtained without a barrier. The low-level circulation west of the barrier is weak in the winds and consists of a positive geopotential response centered at approximately 30°S. To the east the circulation near the barrier shows similarity to models of the Somali jet. The model produces quite strong trade winds and places a low-pressure center in approximately the position where one is actually observed.

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