The role of ocean coupling on MJO simulation in the CMIP6 models

Ocean coupling in general circulation models (GCMs) may have a large impact on the mean-state moisture distribution, precipitation coherence, and simulation of the Madden-Julian Oscillation (MJO). Initialised-forecast runs have previously shown that coupling to a mixed-layer ocean improves MJO predictions, as does the initial intraseasonal SST variability. Here, the effect of full airsea coupling on the simulation of the MJO and wider tropical mean state moisture patterns and variability is investigated in climate-length experiments from all GCMs participating in the Sixth Coupled Model Intercomparison Project (CMIP6). We also seek to understand whether process-level biases affect the MJO more through their direct interaction with the disturbance, or more through their indirect effect on the background state. We compliment this with investigating how tropical air-sea feedbacks modify the precipitation organization and intensity. We apply a set of process-oriented diagnostics to analyse scales of spatial and temporal variability in precipitation. We focus on investigating differences between the coupled experiment (CMIP6 historical) with the uncoupled experiment (CMIP6 AMIP). Preliminary results show greater MJO skill in the coupled configuration for all models at the Maritime Continent propagation metric and almost all models at the East/West power ratio (see Figure). The spatio-temporal coherence of tropical Indian Ocean precipitation is also increased in the coupled configuration. Diagnosing the processes responsible for related biases, including SST-surface-flux-convection phase relationships, column moistening, surface pressure east of MJO convection, SST gradients and low-level convergence, shall help to pinpoint the improvements most likely to advance MJO simulation.