NOTES AND CORRESPONDENCE Thermally Forced Surface Winds on an Equatorial Beta Plane*

The vertical structure of the low-level atmospheric response to an elevated large-scale, low-frequency heat source in the Tropics is explored using linear tidal theory on an equatorial beta plane. Through the calculation of the projection of a large-scale, low-frequency thermal source onto the meridional eigenfunctions, the contributions from a set of discrete meridional eigenfunctions with positive equivalent depths, and a continuous spectrum of meridional eigenfunctions with negative equivalent depth, are examined. The positive equivalent depth eigenfunctions have been discussed in some literature while the continuous spectrum of the negative equivalent depth eigenfunctions is new. The authors find that, at lower frequencies, the forced response is mainly supported by those continuous modes for which the absolute values of the negative equivalent depths are neither very small nor very large. The implications of these results for thermally driven surface winds are discussed and summarized by Eqs. (4.2) and (4.6). In the inviscid case, since the solution associated with the continuous modes with negative equivalent depth is vertically evanescent, it is expected that the vertical energy transfer from the elevated thermal source to the surface is limited. However, in the presence of Newtonian cooling, the continuous modes that contribute significantly to accounting for the large-scale heat source are those modes with moderate values of negative equivalent depth as frequencies goes to zero so that the forced horizontal winds become vertically uniform below the heating. Hence, surface winds can be driven by the elevated heat source in the presence of only linear thermal damping.