This study addresses the origin of the synoptic-scale disturbances which occur in the tropical western North Pacific ocean (WP) region in northern hemisphere summer. These have been called ``easterly waves'' and ``tropical depression type'' (TD) disturbances. Our analysis uses the National Center for Environmental Prediction - National Center for Atmospheric Research Reanalysis data set. By performing a regression analysis on several terms in the vorticity equation at 850 hPa, we show that the TD disturbances propagate approximately as barotropic Rossby waves at 850 hPa. Given this, we use ray tracing calculations, and the wave activity diagnostic introduced by Plumb, to show that wave accumulation is a promising candidate for the initial development mechanism of the TD disturbances. The expected ``growth rate'' from this mechanism is simply the convergence of the group velocity, which reaches values corresponding to a growth time scale of 3 days. This convergence is dominated by, but somewhat larger than, the convergence in the time-mean flow. The wave accumulation mechanism can operate either on waves coming 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 our results provide no direct information on the nature of the feedbacks between diabatic processes and large-scale wave dynamics, they do indicate that no linear instability mechanism involving any diabatic process need be invoked to explain the initial development of TD disturbances. It is possible, rather, that diabatic processes do not provide a positive feedback until the disturbances reach finite amplitude, whether at the stage of true tropical cyclogenesis or some prior intermediate stage.