Origin of Low-Frequency (Intraseasonal) Oscillations in the Tropical Atmosphere. Part I: Basic Theory

Abstract

Abstract A theory of the origin of intraseasonal oscillations of the tropical atmosphere is presented and tested by simple model experiments. This study forces on the validation of the basic theory against key features of the observed 40–50 day oscillation. It is shown that the observed eastward propagation of intraseasonal oscillation in the tropical atmosphere arises as an intrinsic mode of oscillation resulting from an interaction of convection and dynamics via the so-called “mobile” wave-CISK mechanism. Through this mechanism, the heat source feeds on the east-west asymmetry of forced equatorial waves. As a result, Kelvin waves are selectively amplified, which in turn causes the heat source to propagate eastward. This mechanism also prevents small-scale waves from immediate destabilization, contrary to the results of traditional wave-CISK theory. The “mobile” wave-CISK establishes a new dynamics equilibrium state between convection and the wind field to form a wave packet or collective motion with rel...