Dynamics of the South American Coastal Desert

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  • 1 Centro de Previsão de Tempo e Estudos Clim´ticos—CPTEC, Instituto Nacional de Pesquisas Espaciais, S´o Paulo, Brazil
  • | 2 Department of Meteorology, The Pennsylvania State University, University Park, Pennsylvania
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Abstract

The world's driest coastal desert is in South America along the coasts of Peru and Chile. The desert's maintenance is investigated by studying the local dynamics of the low-level southerly flow along the coast. A linear boundary-layer model is used in which a Boussinesq atmosphere is driven by a surface thermal contrast on a β plane. The resting basic state is stably stratified. Constant mechanical and thermal diffusivities are assumed in the momentum and heat equations, respectively. The dynamics of the buoyancy field is governed by a three-dimensional eighth-order differential equation in which the meridional dependence enters parametrically. Results are shown for different values of the constants involved as well as for solutions on an f plane and a semigeostrophic β plane.

The results indicate that the effect of nonuniform rotation is responsible for the presence of subsidence along the coast and inland. This coastal subsidence helps maintain the desert by increasing the static stability and suppressing deep convection. The predicted vertical wind profiles agree well with the observations for Lima, Peru. Sensitivity tests indicate that the flow depends on the interplay between stratification, friction, and the Coriolis parameter and its variation (β). The mechanical frictional effects are mainly constrained to a shallow Ekman layer, whereas the thermal effects are manifested in deeper layers controlled by the β effect.

Abstract

The world's driest coastal desert is in South America along the coasts of Peru and Chile. The desert's maintenance is investigated by studying the local dynamics of the low-level southerly flow along the coast. A linear boundary-layer model is used in which a Boussinesq atmosphere is driven by a surface thermal contrast on a β plane. The resting basic state is stably stratified. Constant mechanical and thermal diffusivities are assumed in the momentum and heat equations, respectively. The dynamics of the buoyancy field is governed by a three-dimensional eighth-order differential equation in which the meridional dependence enters parametrically. Results are shown for different values of the constants involved as well as for solutions on an f plane and a semigeostrophic β plane.

The results indicate that the effect of nonuniform rotation is responsible for the presence of subsidence along the coast and inland. This coastal subsidence helps maintain the desert by increasing the static stability and suppressing deep convection. The predicted vertical wind profiles agree well with the observations for Lima, Peru. Sensitivity tests indicate that the flow depends on the interplay between stratification, friction, and the Coriolis parameter and its variation (β). The mechanical frictional effects are mainly constrained to a shallow Ekman layer, whereas the thermal effects are manifested in deeper layers controlled by the β effect.

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