Asymmetric intraseasonal events in the stochastic skeleton MJO model with seasonal cycle

The stochastic skeleton model is a simplified model for the Madden–Julian oscillation (MJO) and intraseasonal-planetary variability in general involving coupling of planetary-scale dry dynamics, moisture, and a stochastic parametrization for the unresolved details of synoptic-scale activity. The model captures the fundamental features of the MJO such as the intermittent growth and demise of MJO wave trains, the MJO propagation speed, peculiar dispersion relation, quadrupole vortex structure, etc. We analyze here the solutions of a stochastic skeleton model with an idealized seasonal cycle, namely a background warm pool state of heating/moistening displacing meridionally during the year. The present model considers both equatorial and off-equatorial components of the envelope of synoptic scale convective activity, which allows for a large diversity of meridionally symmetric and asymmetric intraseasonal events found in nature. These include examples of symmetric events with MJO quadrupole vortex structure, half-quadrupole events with off-equatorial convective heating structure, as well as tilted events with convective heating structure oriented north-westward and associated northward propagation that is reminiscent of the summer monsoon intraseasonal oscillation. The model also reproduces qualitatively the meridional migration of intraseasonal variability during the year, that approximatively follows the meridional migration of the background warm pool.