Convectively Coupled Global Rossby Modes in an Idealized Moist GCM

Abstract The westward-propagating convectively coupled equatorial wave (CCEW) variability produced by an idealized general circulation model (GCM) is investigated. The model is a zonally symmetric aquaplanet with a slab ocean. Water vapor in the model may condense and produce latent heating, but there is no parameterization of cloud processes, only a quasi-equilibrium convection scheme. The CCEWs produced by the model are found to be sensitive to the heat capacity of the slab and the strength of surface friction. In spectral space, the westward-propagating precipitation variability in the model is dominated by sharp peaks in spectral power at zonal wavenumbers 5 and 6. These precipitation peaks are situated along the dispersion curve of the Rossby–Haurwitz waves, suggesting a connection between the global Rossby modes and precipitation variability. Composites of these disturbances reveal global circulation patterns that extend into the midlatitudes. The moisture variance budget of these disturbances shows that moisture advection by the global Rossby modes maintains the accompanying moisture signal. This is interpreted as downgradient advection of the background moisture gradient of the intertropical convergence zone. The locations of the precipitation peaks are sensitive to Doppler shifting by the zonal winds; when this Doppler shift becomes too weak, the frequencies of the global Rossby modes become too high to effectively couple to convection. A linearized primitive equation model shows that the presence of vertical shear in the background zonal winds is vital for producing a forced response that resembles the modes produced by the GCM. The forced response of the linear model is optimally located to enhance the original circulation of the global mode.