Understanding Ocean Feedbacks to the MJO through the Lens of Convection-Moisture Coupling

The ability of ocean-atmosphere coupled feedbacks to improve MJO simulation has been noted in dozens of modeling studies over the past two decades. Recent experiments revealed that sub-monthly SST perturbations in coupled simulations produce sharper near-equatorial meridional moisture gradients and improved MJO eastward propagation compared to atmosphere-only simulations forced with monthly mean SSTs from the coupled model. DeMott et al. (2019) linked coupled vs uncoupled differences in mean state moisture patterns to the response of parameterized convection to SST-driven changes in boundary layer available potential energy. The flux-buoyancy-convection-moistening process chain offers a novel perspective for understanding the role of surface fluxes for tropical convection compared to that of column moisture budgets.

In this talk, we consider the influence of surface buoyancy flux feedbacks to the evolution of rainfall and mid-level moisture about the mean rainfall-column saturation fraction curve (i.e., the precipitation "uptick"). Newly developed diagnostics are applied to the coupled and uncoupled model output of DeMott et al. (2019) and distilled into a process metric that succinctly characterizes tropical rainfall-moisture coupling strength. We then apply this diagnostic to a subset of CMIP5 and CMIP6 model output to reveal models where ocean coupling may act as a crutch for MJO simulation performance.