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

Abstract

SUMMARYThe surface and boundary-layer uxes of moisture exhibit a large ampli cation as the waves in the Madden-Julian Oscillation (MJO) time-scales interact with synoptic time-scales of 2 to 7 days. This ampli cation is clearly seen when the datasets are cast in the frequency domain for computations of the respective uxes. Those ux relations carry triple-product nonlinearities, and the uxes 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 satis ed by the time-scales of the MJO and two others that reside in the synoptic timescales. 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 ampli cation of the surface uxes. Although this analysis does not address how a small signal in the sea surface temperature on the time-scale of the MJO arises in a coupled atmosphere-ocean model, we are able to show that its presence enables a large ampli cation of this time-scale vertically across the planetary boundary layer. Given a low-frequency ocean with many time-scales, this process ampli es the uxes on the time-scale of the MJO; this ampli cation eventually feeds back to the ocean via ampli ed 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 uxes and momentum within the same framework.

Empirical orthogonal functions and related techniques in atmospheric science: A review

“…It has been identified from various fields such as zonal and divergent wind, SLP, and OLR in the tropics (Madden and Julian, 1972;Kiladis and Weickmann, 1992), and here we choose to use OLR. For details on the mechanisms involved in MJO the reader is referred, for example, to Hendon and Salby (1994), Matthews (2000), and Krishnamurthi et al (2003). The OLR data used here come from NCEP/NCAR reanalyses over the tropical region from 30°S to 30°N.…”

Investigating origin of the inadequate medium range predictability of the lower tropospheric ultra-long waves in tropics

“…Thus the effect of one scale to other scale may also be known. The device was extensively used by Krishnamurti et al (2003); Krishnamurti and Chakraborty (2006) to delineate the nonlinear aspects of the intraseasonal oscillation. The formalism of the triad interaction entails the Fourier series of nonlinearities associated with the triple product term.…”

Synoptic-scale disturbances over the northern South China Sea and their responses to El Niño

“…The previous studies have shown the close relation between the synoptic-scale disturbances and the MJO, and suggested that the synoptic-scale disturbances could be one important factor to trigger the MJO. Krishnamurti et al (2003) have shown that the interaction of MJO time-scales with synoptic time-scales in the constant flux and boundary layers leads to a large amplification at the MJO time-scales. Matthews and Kiladis (1999) found that the high-frequency waves propagating into the Indian Ocean region at the beginning of the MJO cycle, which may be important in the initiation of intraseasonal convective anomalies there.…”