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 li near tidal theory on an equatorial beta plane. Through the calculation of t he projection of a large-scale, low-frequency thermal source onto the merid ional eigenfunctions, the contributions from a set of discrete meridional e igenfunctions with positive equivalent depths, and a continuous spectrum of meridional eigenfunctions with negative equivalent depth, are examined. Th e positive equivalent depth eigenfunctions have been discussed in some lite rature while the continuous spectrum of the negative equivalent depth eigen functions is new. The authors find that, at lower frequencies: the forced r esponse is mainly supported by those continuous modes for which the absolut e values of rile negative equivalent depths are neither very small nor very large. The implications of these results for thermally driven surface finds are di scussed 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 tr ansfer 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 mode s with moderate values of negative equivalent depth as frequencies goes to zero so that the forced horizontal winds become vertically uniform below th e heating. Hence, surface winds can be driven by the elevated heat source i n the presence of only linear thermal damping.