Further Evidence of Two Different MJO Flavors in High-Resolution Coupled Model Simulations

The atmospheric, hydrostatic Flow-following, finite volume Icosahedral Model (FIM) has been coupled with an icosahedral-grid version of the three-dimensional Hybrid Coordinate Ocean Model (iHYCOM) to simulate weather phenomena on subseasonal timescales including the Madden-Julian Oscillation (MJO). Here, two MJO events are simulated for 32-day periods at very high resolution (approximately 30 km and 15 km horizontal grid spacing) with both the coupled FIM-iHYCOM and the atmosphere-only FIM. Most importantly, these simulations confirm a prior study that suggested the existence of two MJO “flavors”: one that is strongly coupled to the upper ocean, and another that is governed primarily by internal atmospheric dynamics. For the coupled-flavor event, FIM-iHYCOM runs simulate an MJO that agreed well with the observed phase and amplitude; in contrast, the FIM-only runs fail to generate an MJO signal. For the internal-atmospheric-flavor event, both coupled and uncoupled runs generate similar MJO signals, albeit in poor agreement with observations. Additional tests reveal that a scale-aware parameterization for deep convection allows for precipitation anomalies to cluster into larger-scale features than when deep convection is explicitly resolved, although the MJO signal (amplitude and propagation) are somewhat similar. Changing horizontal resolution has a relatively minor impact on MJO characteristics, in line with earlier studies. Overall, these results provide further evidence that both the dynamics and the predictability of the MJO may be highly case-dependent.