Ocean Feedbacks to the MJO in CMIP5 models

Recent studies have documented the role of the tropical column water vapor (CWV) distribution for MJO propagation. Other studies demonstrate improved MJO simulation when atmosphere-only general circulation models (GCMs) are coupled to an ocean model.

In this study, we explore the role of surface fluxes and their SST modulation for MJO maintenance and propagation in a subset of CMIP5 models. When averaged over the MJO lifecycle, surface fluxes are a sink of column integrated moist static energy (MSE), and oppose its tendency. Surface fluxes, and their regulation by intraseasonal SST variations, are uncorrelated with MJO simulation fidelity—a finding that supports the paradigm that the MJO is primarily regulated by atmospheric processes.

However, MJO simulation skill is positively correlated with the specific humidity (q) near-surface vertical gradient (delta-q) when SST is anomalously warm and rainfall is undergoing a transition from suppressed to active conditions. Under these conditions, delta-q positively contributes to shallow moistening in advance of deep MJO convection via surface flux feedbacks. We find that during these conditions, delta-q can be primarily driven by saturation q at SST, the 2m q, or both in roughly equal proportions, depending on the model. In other words, the sensitivity of delta-q to oceanic or atmospheric conditions is model dependent.

Our findings highlight the need for improved understanding of the processes that regulate the upper ocean heat content and the atmospheric boundary layer moisture content, as both jointly set the delta-q that contributes to MJO propagation.