Enhanced MJO-like Variability at High SST in a Super-Parameterized GCM

The Madden-Julian Oscillation (MJO) is the dominant mode of tropical intraseasonal variability, characterized by an eastward propagating envelope of convective anomalies with a 30-70 day timescale. In this study, we report a significant increase in MJO-like variability in a super-parameterized version of the NCAR Community Atmosphere Model run with high sea surface temperatures (SST). A series of zonally symmetric aquaplanet simulations exhibit a tripling of variance in intraseasonal outgoing longwave radiation as equatorial SST is increased from 26C to 35C. The simulated intraseasonal variability also transitions from an episodic phenomenon to one with a semi-regular period of 25 days. Moist static energy (MSE) budgets of composite MJO events are used to diagnose the physical processes responsible for the relationship with SST. This analysis points to an increasingly positive contribution from vertical advection, associated in part with a steepening of the mean vertical MSE profile in the lower troposphere. This work has implications for organized tropical variability in past warm climates as well as anthropogenic global warming scenarios.