The impact of diurnally varying thermal forcing on the tropical circulation is studied with a linearized shallow water model on an equatorial beta plane. The mass and flow fields are interpreted as an internal mode with equivalent depth of the order of 250 m. This is the preferred vertical mode excited by typical cumulus heating profiles in the tropics. The diurnal forcing with the horizontal scale of the Amazon source partitions approximately 40% of the input energy to slow modes. The fast modes are trapped by the critical latitudes contributing significantly to oscillations in the divergence field, which emanate out of the source region. The divergence field at the source is approximately in phase with the forcing; the vorticity field lags the forcing by approximately 10 hours. A comparison of the model results with the 500 mb level vertical motion, derived from the National Meteorological Center analysis, shows a similar pattern of propagating oscillations. It is suggested that the diurnally forced convection over the tropical sector of South America controls the timing of heavy precipitation over the adjoining regions.