MJO Initiation in an Aquaplanet GCM and Comparison to DYNAMO/CINDY Observations

An aquaplanet general circulation model that produces an extremely strong and regular intraseasonal oscillation is used to study the MJO initiation process. Very few primary MJO events occur in the model that are not preceded by significant precursor MJO activity, which makes the MJO activity more regular than observed. The initiation of MJO events on the western side of the tropical warm pool is preceded by the arrival of moisture anomalies associated with circumnavigation of a Kelvin wave front and off-equatorial moisture anomalies imprinted by Rossby gyres produced by the previous cycle of MJO convection. Both vertical and horizontal moisture advection associated with Rossby gyres contribute to moistening. Frictional convergence appears responsible for the anomalous Kelvin wave moistening.

A sensitivity experiment is conducted in which Kelvin wave circumnavigation into the warm pool is removed through application of damping to assess the impact on MJO initiation and regularity in the model. It is shown that suppressing circumnavigation substantially impacts the formation location and spatial distribution of MJO activity, although does not substantially impact the regularity of events. Results from other aquaplanet configurations are also discussed. Findings are discussed in the context of observational studies on MJO initiation, including the DYNAMO/CINDY field experiment, where horizontal advection appeared to be a key moistening process in advance of the October 2011 initiation event.