A mechanism of the Madden-Julian Oscillation based on interactions in the frequency domain

The surface and boundary-layer fluxes of moisture exhibit a large amplification as the waves in the Madden-Julian Oscillation (MJO) time-scales interact with synoptic time-scales of 2 to 7 days. This amplification is clearly seen when the datasets are cast in the frequency domain for computations of the respective fluxes. Those flux relations carry triple-product nonlinearities, and the fluxes on the time-scale of the MJO are evaluated using co-spectra of triadal frequency interactions. The trigonometric selection rules on interactions among these frequencies are largely satisfied by the time-scales of the MJO and two others that reside in the synoptic time-scales. Tropical instabilities provide a rich family of tropical disturbances that appear to be ready and waiting to interact with the MJO time-scales (since these satisfy the selection rules for non-vanishing interactions). A consequence of these nonlinear interactions in the frequency domain is a two- to three-fold amplification of the surface fluxes. Although this analysis does not address how a small signal in the sea surface temperature Oil the time-scale of the MJO arises in a coupled atmosphere-ocean model, we are able to show that its presence enables a large amplification of this time-scale vertically across the planetary boundary layer. Given a low-frequency ocean with many time-scales, this process amplifies the fluxes on the time-scale of the MJO this amplification eventually feeds back to the ocean via amplified surface stresses, and an equilibrium state with a robust MJO in the coupled system is realized. The datasets for this study were derived from a coupled ocean-atmosphere model that was able to resolve a robust MJO in its Simulations. This Study also examines the character of sensible-heat fluxes and momentum within the same framework.