Non-rotating convective aggregation and the Madden-Julian oscillation

The Madden-Julian oscillation (MJO) is the dominant mode of tropical intraseasonal variability, but its dynamics remain poorly understood. It has been suggested that the MJO is a form of convective self-aggregation, similar to that seen in non-rotating cloud-resolving simulations but on a larger spatial scale.  Here we present support for that hypothesis based on SP-CAM simulations with globally uniform SST. Under non-rotating conditions, convection self-organizes into large (~4000km) clusters surrounded by dry regions, while with Earth-like rotation the model produces a robust MJO.  The non-rotating 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 confirm that longwave heating is essential to both phenomena. Non-rotating simulations using the conventional CAM with a weak MJO show a much weaker tendency to aggregate, and both MJO activity and aggregation intensity are found to depend strongly on the parameterized convective entrainment rate.