Intraseasonal Modulation of Synoptic-Scale Disturbances and Tropical Cyclone Genesis in the Eastern North Pacific

Abstract

he influence of the Madden?Julian oscillation (MJO) on synoptic-scale waves, important precursors to tropical cyclones and tropical cyclogenesis, is investigated using a regional model of the eastern North Pacific basin. Cyclogenesis frequency is evaluated with respect to the MJO using a combination of the regional model and the archive best-track data. The statistics of the regional model compare well to the data and suggest that the MJO predominantly deters cyclogenesis during the suppressed phase rather than enhances it during the active phase when compared to neutral conditions. Synoptic-scale variability of cloudiness and eddy kinetic energy and analysis of vortex track statistics show a consistent enhancement of wave strength during the active phases of the MJO supported by both barotropic energetics and tropospheric moisture availability. Vertical wind shear, of critical importance to cyclone development, also exhibits strong variability associated with the phase of the MJO but, contrary to synoptic activity and moisture, results in unfavorable conditions for cyclogenesis during the active phase of the MJO in the region. The result is for the MJO to enhance cyclogenesis frequency relative to neutral conditions during the early active phase but otherwise to result in suppressed activity. In addition to the basinwide longitudinal structure, the local MJO exhibits a distinct north?south structure not apparent in the global pattern but in agreement with recent studies of tropical eastern North Pacific subseasonal variability.