Development of synoptic-scale disturbances over the summertime tropical northwest Pacific

This study addresses the origin of the synoptic-scale disturbances that occ ur in the tropical western North Pacific ocean (WP) region in Northern Hemi sphere summer. These have been called "easterly waves" and "tropical depres sion-type" (TD) disturbances. This analysis uses the National Center for En vironmental Prediction-National Center for Atmospheric Research reanalysis dataset. By performing a regression analysis on several terms in the vortic ity equation at 850 hPa, it is shown that the TD disturbances propagate app roximately as barotropic Rossby waves at 850 hPa. Given this, ray-tracing c alculations and the wave activity diagnostic introduced by Plumb are used t o show that wave accumulation is a promising candidate for the initial deve lopment mechanism of the TD disturbances. The expected local "growth rate" from this mechanism is simply the convergence of the group velocity, which reaches values corresponding to a growth timescale of 3 days. This converge nce is dominated by, but somewhat larger than, the convergence in the time- mean flow. The wave accumulation mechanism can operate either on waves comi ng from outside the WP region or on those generated in situ: in particular, mature tropical cyclones are probably a climatologically important source of waves. While: the results presented here provide no direct information o n the nature of the feedbacks between diabatic processes and large-scale wa ve dynamics, they do indicate that no linens instability mechanism involvin g any diabatic process need be invoked to explain the initial development o f TD disturbances. It is possible, rather, that diabatic processes do not p rovide a positive feedback until the disturbances reach finite amplitude, w hether at the stage of true tropical cyclogenesis or some prior intermediat e stage.