Direct and Indirect Ocean Feedbacks to the MJO in Four GCM Experiments

A suite of air-sea interaction diagnostics tailored to intraseasonal timescales is applied to coupled and atmosphere-only simulations with four different general circulation models (CGCMs and AGCMs, respectively). Mean state differences between coupled and uncoupled simulations for each model are minimized by forcing the AGCM with 31-day smoothed SSTs from the CGCM. With this framework, differences in MJO behavior for each model pair arise from the removal of ocean feedbacks.

Direct effects of intraseasonal SST fluctuations are the surface flux-driven enhancement of MJO rainfall anomalies. Compared to observations, the surface flux feedback is exaggerated is some models. This exaggeration is rooted in a dry bias in boundary layer humidity, which erroneously elevates the contributions of surface fluxes to the MJO moist static energy budget.

The more robust convection in the coupled simulations drives indirect feedbacks to ocean coupling, such as enhanced circulation anomalies that foster increased moistening east of MJO convection and improved MJO propagation. Despite identical SST climatologies in each CGCM-ACGM model pair, other indirect effects of coupling arise, such as mean state differences in column-integrated water vapor and low-level atmospheric stability, both of which can influence moistening processes and MJO propagation.

In all four models, the indirect effect of ocean coupling is larger than the direct (i.e., surface flux) effect, but the magnitude of the direct effect varies considerably from model to model.