Idealized MJO modeling: ocean coupling and interaction with the mean state

I will present results from ongoing development of a highly idealized model designed to capture the essential dynamics of the Madden-Julian oscillation (MJO). This model represents the column-integrated moist static energy field along a constant-latitude line near the equator, based on assumptions that vertical and meridional structure are known. The thermodynamics are simplified using the weak temperature gradient approximation and very simple physical parameterizations, and the dynamics are simplified by an assumption that the wind response to a given effective heat source can be represented by the quasi-static linear, damped response on an equatorial beta plane (Matsuno/Webster/Gill).

I will focus on development of a nonlinear version of the model in which two coupling to a linear slab ocean mixed layer is included, and the import of dry air from subtropical latitudes is represented by a weak Newtonian damping on the moist static energy field. These features allow not just the transient disturbances but also some aspects of the mean state to be represented. The ocean coupling and moist static energy damping allow cloud-radiative feedback to be strong enough to generate strong transients without causing the mean state to behave pathologically (as happens otherwise). Varying a specified ocean heat transport with respect to longitude produces a warm pool, and implicit Hadley and Walker circulations. This allows for more organic generation of eastward-propagating, MJO-like disturbances, even when the ocean mixed layer depth is large so that the direct role of coupling in the disturbances is small.