Global-scale convective aggregation: Implications for the Madden-Julian Oscillation

Abstract

Previous work has shown that convection will self-organize in cloud-system-resolving model simulations of radiative-convective equilibrium, and it has been suggested that the convective envelope of the Madden-Julian oscillation (MJO) may be organized by similar processes on a much larger scale. Here we present support for that hypothesis based on simulations with SP-CAM with globally uniform SST. Without rotation, convection self-organizes into large (-4000 km) clusters surrounded by dry regions, while with Earth-like rotation the model produces a robust MJO. The nonrotating aggregation and MJO are found to have similar budgets of moist static energy, both being supported by diabatic feedbacks, particularly cloud-longwave interaction. Mechanism denial experiments show that longwave heating anomalies associated with high clouds are essential to the nonrotating aggregation, and amplify the MJO. Simulations using the conventional CAM show a weaker MJO and a much weaker tendency for nonrotating aggregation, and both MJO activity and aggregation intensity are found to increase with the entrainment rate in the deep convection parameterization.