TROPICAL WAVES IN A GCM WITH ZONAL SYMMETRY

Tropical wave phenomena have been examined in the last 520 days of two 15-year runs of a low-resolution general circulation model (CCM0). Th e model boundary conditions were simplified to all-land, perpetual equ inox, and no topography. The two runs were for fixed soil moisture at 75% and 0%, the so-called ''wet'' and ''dry'' models. Both models deve lop well-defined ITCZs with low-level convergence erratically concentr ated along the equator. Highly organized eastward-propagating waves ar e detectable in both models with different wave speeds depending on th e presence of moisture. The wave amplitudes (in, e.g., vertical veloci ty) are many orders of magnitude stronger in the wet model. The waves have a definite transverse nature as precipitation (low-level converge nce) patches tend to move systematically north and south across the eq uator. In the wet model the waves are distinctly nondispersive and the transit time for passage around the earth is about 50 days, consisten t with the Madden-Julian frequency. The authors are also able to see m ost of the expected linear wave modes in spectral density plots in the frequency-wavenumber plane and compare them for the wet and dry cases .