Was there a December MJO during DYNAMO? (Invited Presentation)

The December 2011 convective event over the Indian Ocean during the DYNAMO field campaign is used to illustrate the complexities in describing a particular event as “an MJO” or “not an MJO.” Depending on the index used (wavenumber-frequency filtering, RMM, OMI, VPM, etc.), the threshold in that index for qualifying as an MJO, and the particular methodology by which the long-term variability is removed from the raw fields, this event and others alternate between “being” and “not being” MJOs. Composites of MJO-like convective events similar those seen during October through December 2011 during DYNAMO, which have longitudinally constrained convective envelopes that dissipate over the Maritime Continent and 20-30 day time scales, illustrate that these smaller-scale MJOs do not project strongly onto the RMM index and thus may be missed in composites using the commonly applied threshold of RMM=1. These results also explain why using a threshold of RMM=1 for MJO initiation (termination) composites may lead to MJO-like behavior prior to (after) day 0.