Climate Diagnostics Center, University of Colorado, Boulder
We propose a mechanism that may contribute to observed near-2 day variability in cloudiness observed over the equatorial Pacific Ocean during active phases of the Madden-Julian oscillation (MJO). Our hypothesis is motivated by the following already-established results: 1) Embedded within the MJO are eastward propagating 'superclusters' (Nakazawa 1988), which are made up of a collection of cloud clusters. 2) Emanating from the superclusters are westward propagating patches of enhanced cloudiness (Nakazawa 1988), which were identified as inertial-gravity waves with a spectral peak near two days by Takayabu (1994a). 3) Sub-synoptic scale variability is enhanced on diurnal (and other) time scales during active periods of the MJO (Hendon and Liebmann 1994). We presume that inertial-gravity waves are initiated by convective activity within envelope of the supercluster. Thus, since the eastward movement of the supercluster is at about the same speed as the phase speed of the westward moving inertial gravity wave, diurnal forcing moving eastward with the supercluster would project onto two-day westward propagating inertial-gravity waves. The essential dynamics of our argument are illustrated with a linear shallow water model forced by stationary and eastward propagating mass sources which are diurnally modulated. High-resolution satellite imagery is used to argue our hypothesis. Although observational results are less than conclusive, we believe that the data do suggest that the origin of observed near-2 day variability may at least partly lie in the mechanism we propose.