Convective Self-Aggregation and the MJO in the Super-Parameterized CAM

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Monday, 5 January 2015
Nathan P. Arnold, Colorado State University, Fort Collins, CO

Mesoscale aggregation of convection has been reported in cloud-resolving model (CRM) simulations with relatively small (100-1000km), non-rotating domains. This aggregation is thought to be driven by feedbacks between convection, radiation and tropospheric moisture. It has been suggested that the observed Madden-Julian Oscillation (MJO) is a “moisture mode” driven by physics similar to that of the simulated small-scale aggregation, but alternative theories of the MJO remain viable. Here we show that a super-parameterized global model run with uniform sea surface temperature and no rotation also produces convective aggregation, with a characteristic scale of 2000-5000km. Moisture and moist static energy (MSE) budgets indicate that the mechanisms driving this large-scale aggregation are similar to those driving aggregation in CRMs. When rotation is added, the model simulates robust MJO activity. A set of mechanism-denial experiments and runs with intermediate rotation are used to link the non-rotating aggregation to the MJO. Implications for MJO theories will be discussed.