Toward Understanding MJO Onset: The Sensitivity of Organized Convection to Equatorial Wave Dynamics

The Madden-Julian Oscillation (MJO) has been documented to influence extreme weather events, such as mid-latitude precipitation events and tropical cyclones, around the world. It is also known to interact with other long-term modes of climate variability, such as the NAO and ENSO. Increased predictability of the MJO may also lead to increased predictability of the other phenomena that it impacts. To successfully predict MJO occurrence and behavior, a thorough understanding of the dynamics controlling onset and propagation of the MJO are necessary. The Dynamics of the Madden-Julian Oscillation (DYNAMO) and ARM Madden-Julian Oscillation Experiment (AMIE) field campaigns were carried out in the central equatorial Indian Ocean during late 2011-early 2012 and were designed to investigate potential physical mechanisms responsible for driving the MJO.

Early studies using DYNAMO/AMIE data have shown that precipitation associated with the MJO active period is characterized by mesoscale convective systems containing embedded convection and large stratiform precipitation regions. The stratiform regions have a heating maximum in the upper troposphere, which when combined with low-level heating, induces a combined Kelvin-Rossby response in the anomalous circulation. Production of large stratiform regions and their "top-heavy" heating profiles is thus critical for MJO development. This study investigates the role of upper-tropospheric dynamics in favoring the formation of mesoscale systems with very large stratiform precipitating regions. ERA reanalysis clearly indicates anomalies of zonal wind propagating eastward into the DYNAMO domain at the onset of each of three observed MJO events. Temperature anomalies propagating into the region from the west are also detected. The wind and temperature anomalies are associated with the eastward propagation of a low wavenumber Kelvin mode. Additionally, short-term convective events linked to higher-frequency equatorial wave dynamics were also detected during DYNAMO. A long-term regional modeling study will show the sensitivity of the development of stratiform precipitation and top heavy heating to changes in the tropospheric profiles of zonal and meridional wind (and thus divergence), temperature, and humidity associated with the Kelvin and higher frequency wave modes.