Planetary scale selection of the Madden–Julian Oscillation in an air-sea coupled dynamic moisture model

Abstract

The authors present an air–sea coupled dynamic moisture (ASDM) model to explore the mechanism of preferred planetary scale of the Madden–Julian Oscillation (MJO). This ASDM model, extended from the original frictionally coupled dynamic moisture model and a mixed layer model, can present a good simulation of planetary-scale characteristics of the MJO, including a slow eastward propagation of 5 m s−1, coupled Rossby–Kelvin wave-like structure, and phase leading of convective center by warm sea surface temperature (SST). It is interesting that the planetary-scale selection in terms of instability is only found in the nonlinear ASDM model with the assumption of positive-only heating. Such a scale selection, however, cannot be found in the ASDM model with linear heating or in an uncoupled atmosphere model. The essential mechanism for the scale selection is nonlinear positive-only heating, and the air-sea interaction primarily provides an instability source to support this scale selection. This scale selection is attributed to different phase speeds of coupled moist Rossby–Kelvin waves and dry Kelvin waves. From short-wave initial perturbations, the easterly wind anomalies of eastward-propagating dry Kelvin waves from a stronger wave component will catch up with other weaker moist components and suppress their westerly wind anomalies, resulting in weak SST gradient as well as reduced moisture convergence and precipitation for these suppressed components. As a result, the wavenumber-one structure is selected. The Warm Pool-like mean state will select the stronger component among the initial short-wave perturbations and accelerate this scale selection process. These results provide new insight into MJO mechanism in terms of air-sea interaction.