Nonlinear Theory of the Formation and Structure of the Intertropical Convergence Zone

Abstract

The formation of the intertropical convergence zone is attributed to the self-excitation of a large-scale, zonally symmetric disturbance in the conditionally unstable and convectively activated tropical atmosphere, and the nature of the disturbance is represented by a nonlinear quasigeostrophic model. The solutions are obtained by representing the space variations by Fourier series in the ascending region and exponential series in the descending region, and the coefficients are represented by power series of a small parameter defined in terms of the CISK parameter B. These solutions show that the circulation created is mainly a zonal wind system endowed with large horizontal shear in the ascending region, the major portion of which is independent of height when B is greater than twice the critical value of B and, consequently, the total wind is concentrated in the lower levels. Both the vertical and the horizontal profiles given by the theory resemble the available observed profiles closely. The vertical diffusion and radiative cooling coefficients needed by the theory are about 2-5 m2sec?1 and 2?10?6sec?1 respectively. The time development of the disturbance was analyzed by a second-order approximation. It is shown that the disturbance approaches its equilibrium amplitude asymptotically through damped oscillations.