ON THE GENERATION OF 2-DAY CONVECTIVE DISTURBANCES ACROSS THE WESTERNEQUATORIAL PACIFIC

We propose a mechanism that may contribute to observed near-2-day vari ability in cloudiness observed over the equatorial Pacific Ocean durin g active phases of the Madden-Julian oscillation (MJO). Our hypothesis is motivated by the following already-established results: 1) Embedde d within the MJO are eastward-propagating 'superclusters' (Nakazawa, 1 988), which are made up of a collection of cloud clusters. 2) Emanatin g 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 sca les during active periods of the MJO (Hendon and Liebmann, 1994). We p resume that inertial-gravity waves are initiated by convective activit y within the envelope of the supercluster. Thus, since the eastward mo vement of the supercluster is at about the same speed as the phase spe ed of the westward-moving inertial gravity wave, diurnal forcing movin g eastward with the supercluster would project onto two-day westward-p ropagating inertial-gravity waves. The essential dynamics of our argum ent are illustrated with a linear shallow-water model forced by statio nary and eastward-propagating mass sources which are diurnally modulat ed. High-resolution satellite imagery is used to argue our hypothesis. Although observational results are less than conclusive, we believe t hat the data do suggest that the origin of observed near-2-day variabi lity may at least partly lie in the mechanism we propose.