The Relationship between the Boreal Summertime Madden–Julian Oscillation and Tropical Deep Convection

The Madden-Julian Oscillation (MJO) is the predominant mode of variability in tropical deep convection and exhibits substantial seasonality in its convective characteristics. From a regional perspective, the MJO exhibits off-equatorial propagation during boreal summertime and is characterized by a convective dipole that propagates north and east across the Indo-West Pacific. While the MJO itself is regional in scale, individual convective elements constitute the convective envelope within the convectively active portion of the dipole. We therefore ask the question: What insights can be made by equivalently considering both the mesoscale convective elements and the anomalies in regional convection, rather than only considering the MJO as a predominantly downscale-driven convective anomaly?

We simulate 40 days of a canonical boreal summertime MJO event that occurred during July/August 2016 using the Regional Atmospheric Modeling System (RAMS), a cloud-resolving model with an integrated bin-emulating double-moment microphysics scheme. The lateral boundaries were forced with ERA-Interim reanalysis, while the interior of the domain was allowed to freely evolve according to the model physics. Within the simulation, the convective dipole propagates across the Maritime Continent, within which a plethora of convective elements exist, including isolated continental convection, maritime mesoscale convective systems, and even a typhoon. Simulated precipitation anomalies align well with those from GPM observations, particularly for the spatio-temporal extent of intraseasonal anomalies.

The convective elements were tracked, and convective characteristics such as cell longevity, spatial expanse, and cloud-top height were logged. The evolution of convective characteristics was then analyzed relative to the phase of the MJO. We find that convective evolution during the boreal summertime MJO proceeds as follows: cell longevity is increased during the initial phases, followed by an increase in cell number in the intermediate phases, progressing into an increase in cell expanse in the terminal phases. We offer a hypothesis regarding the physical mechanisms driving this relationship that considers both the mesoscale and regional character of the MJO.