A MECHANISM OF SCALE SELECTION IN TROPICAL CIRCULATION AT OBSERVED INTRASEASONAL FREQUENCIES

It is shown in this work that two prominent intraseasonal oscillations of the tropical atmosphere-namely, the 3-4-day westward propagating w ave observed over the equatorial Atlantic and Pacific Oceans and the q uasi-biweekly (or 10-20 day) oscillation observed over the Indian Ocea n summer monsoon region-can be understood as arising from selective ex citation of the tropical normal modes in the presence of moist feedbac ks and a moisture relaxation timescale. Unlike in earlier studies of a similar nature, the central theme of the present work is that the dyn amics of the moisture variable is governed by a moisture relaxation ti mescale tau. In other words, the large-scale flow is not in exact quas i equilibrium with the precipitational heating. The implications of th is hypothesis are investigated by using a shallow-water model of the t ropical atmosphere on an equatorial beta plane, with a fixed vertical structure. The two major findings of the present work are: 1) both the 3-4-day wave and the 10-20-day wave can be understood as intrinsic mo des of the tropical atmosphere, excited by the same basic mechanism-na mely, moist feedbacks in the presence of a moisture relaxation timesca le, and 2) while the 3-4-day wave is represented by a maximally growin g mixed Rossby gravity wave, driven selectively unstable by moist feed backs, the 10-20-day wave represents a new mode of the tropical atmosp here that is excited by the moist feedbacks in the presence of mean we sterlies. For both of these waves, the agreement between observed stru cture and theoretical predictions is excellent. Thus, the present work presents a mechanism that explains two major low-frequency tropical o scillations as intrinsic modes of the tropical atmosphere.