The Influence of Surface Friction on the Madden Julian Oscillation in an Idealized Moist General Circulation Model

The sensitivity of the Madden-Julian Oscillation (MJO) to surface friction is explored in an idealized moist general circulation model (GCM). The model has a comprehensive radiative transfer scheme and prognostic water vapor which can condense to heat the atmosphere, but no parameterization of cloud processes, only a simple quasi-equilibrium convection scheme. The MJO is produced in the model by the prescription of a strong, zonally asymmetric convergence of energy into the model’s slab ocean, which creates a wavenumber-1 pattern of surface temperature in the tropics. The resulting warm pool portion of the tropics supports strong MJO-like variability. The model’s MJO is shown to be very sensitive to surface friction, in the form of the roughness length for momentum within the model’s parameterization of surface fluxes and diffusivity in the planetary boundary layer. With sufficiently weak surface friction (small roughness length), the MJO is effectively eliminated and the model cannot generate coherent eastward propagation of equatorial disturbances. In a series of experiments with variable surface roughness, it is shown that the surface friction modulates the generation of available potential energy: as surface roughness is decreased, MJO anomalies in convective heating and temperature become less correlated to one another, with this effect being most pronounced on the western flank of the model’s warm pool, where MJO events initiate. Surface friction also modulates the strength of the low-level convergence ahead of the MJO, and connections to previous theories which focus on this frictional convergence as a destabilizing influence are explored. This low-level convergence also has a corresponding shallow meridional circulation in the lower free troposphere which drives the moistening of the free troposphere prior to the onset of the active phase of the MJO; the physical mechanisms for this shallow circulation are also investigated within the model.