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

• Published in: Quarterly journal of the Royal Meteorological Society Vol. 129; no. 593; pp. 2559 - 2590
• Main Authors: Krishnamurti, T. N., Chakraborty, D. R., Cubukcu, Nihat, Stefanova, Lydia, Vijaya Kumar, T. S. V.
• Format: Journal Article
• Language: English
• Published: Chichester, UK John Wiley & Sons, Ltd 01.07.2003; Wiley
• Subjects: Climate modelling; Convection, turbulence, diffusion. Boundary layer structure and dynamics; Earth, ocean, space; Exact sciences and technology; External geophysics; Intraseasonal oscillations; Meteorology; Nonlinear dynamics of waves; Physics of the oceans; Sea-air exchange processes; Sensible heat; Intraseasonal variation; Humidity transfer; Surface layer; Atmospheric instability; Feedback regulation; Climate modelling Intraseasonal oscillations Nonlinear dynamics of waves; Time scale; Climate models; Amplification; Momentum transfer; Synoptic scale; Atmospheric boundary layer; Nonlinearity; Frequency spectrum; Madden Julian oscillation; Heat transfer; Ocean atmosphere interaction
• ISSN: 0035-9009; 1477-870X
• DOI: 10.1256/qj.02.151

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 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 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. Copyright © 2003 Royal Meteorological Society