Characterizing the higher-frequency variability within the MJO convective envelope

Some of the most recent theories on the initiation and dynamics of the Madden Julian Oscillation (MJO) have been building on its multi-scale nature (e.g. Majda and Stechmann, 2009; Solodoch et al., 2011; Yang and Ingersoll, 2011). This multi-scale nature has been pointed out in many observational studies (e.g. Nakazawa, 1988; Hendon and Salby, 1994; Straub and Kiladis, 2003), but we still lack detailed information on the composition of the observed MJO envelope in terms of higher-frequency variability. Questions of interest include whether the MJO favors higher-frequency waves of particular scales and propagation direction, and how this variability changes in time and from one event to another. We investigate here the composition of the observed MJO convective envelope and its convectively-suppressed counterpart following different observational approaches. MJO anomalies of enhanced and suppressed convection are detected: (1) in the MJO space-time filtered CLAUS brightness temperature field following Wheeler & Kiladis (1999; 30-96 day, zonal wavenumbers 1-9), (2) through an object-based method recently developed by Dias et al (2011) which consists in detecting 3-dimensional contiguous regions of brightness temperature below/above a range of thresholds, in the latitude-longitude-time domain. Higher-frequency variability (in the 1.25 - 30 day range) is also diagnosed and characterized through both the spectral filtering and object approaches. Space-time filtering is applied to brightness temperature to isolate various modes (westward or eastward only, Kelvin, Mixed-Rossby-Gravity, Inertio-Gravity waves following Kiladis 2009) and grid-point wavelet analysis is then performed as a measure of the higher-frequency variability and its variation in time. Mesoscale convective systems are isolated following Dias et al 2011. Overall, the results confirm that the composition of the convective envelope of the MJO in terms of higher-frequency variability can vary significantly from one MJO event to another, for a given season and geographical basin. Consistently, the average composition is relatively close to the climatology. We however find a slight increase in eastward (higher-frequency) power in the convectively active phase of the MJO, and conversely, increase in westward power in the suppressed phase.