Thermally driven surface winds in the tropics

Thesis (Ph. D.)--University of Washington, 1998

An integrated study is performed to examine the response of the atmosphere to a largescale, elevated heat source in the tropics. Special emphasis is placed on the surface winds in response to the heating.The vertical structure of the low-level atmospheric response to an elevated large-scale, low-frequency heat source in the tropics is first explored using the tidal approach. The projections of heating onto the meridional eigenfunctions on an equatorial beta-plane show that, at low frequencies, the forced response is mainly supported by the continuous modes with moderate large negative equivalent depth. The total response is vertically uniform below the heating in an atmosphere with only Newtonian cooling.To further investigate the thermally forced response in the presence of both thermal and momentum damping, the linearized equations are decomposed into a set of shallow water equations using the vertical eigenfunctions in a vertically semi-infinite domain. The results show that the three-dimensional structure of the forced response is closely tied to the vertical structure of the heating, and the forced signals driven by deep heating propagate faster in the horizontal than those driven by shallow heating.The zonal decay scale in a shallow water system is proportional to the inverse of the square root of the product of the Rayleigh friction rate and the Newtonian cooling rate. Hence, in both the Rayleigh-friction-dominant case and the Newtonian-cooling-dominant case, the solutions are essentially zonally uniform in a zonally cyclic domain. However, since the characteristic meridional scale, which is the equatorial radius of deformation multiplied by the fourth root of the Prandtl number, is very large for the Rayleigh-friction-dominant case, the forced response can extend far outside the heating latitudes. Contrast to that, in the Newtonian-cooling-dominant case, the characteristic meridional scale is very small and the forced response is confined to the heating latitudes.The vertical structure of thermally driven circulation is significantly affected by the nature of dissipation. Newtonian cooling homogenizes the atmospheric motion in the vertical, and a strong vertically uniform wind is found below the heating. When Rayleigh friction dominates, the circulation driven by the heat source is confined to the layer where the heat source is located.Finally, the numerical solutions from a primitive-equation model confirm the previous analytical results.