Upscale Impact of Mesoscale Convective Systems on MJO analog above the Equator and Its Parameterization in An Idealized GCM

The Madden-Julian oscillation (MJO) is observed to be organized in a hierarchical structure that the eastward-moving planetary-scale envelope usually contains multiple synoptic-scale superclusters with numerous embedded mesoscale convective systems (MCSs). Present-day GCMs fail to explicitly resolve MCSs due to their coarse resolutions. It is hypothesized that such inadequate treatment of MCSs and their upscale impact leads to the poorly simulated MJOs in the GCMs.

Here we used a simple 2D multicloud model to mimic typical behaviors of GCMs with clear deficiencies. Explicit expressions of eddy transfer of momentum and temperature have been obtained based on the Mesoscale Synoptic Equatorial Dynamics (MESD) model, originally derived by Majda (2007). The results show that upscale impact of westward-moving MCSs at a slow speed provides favorable conditions for eastward propagation of the MJO analog, consistent to theoretical predictions by the MESD model. When modulated by deep heating excess, this upscale impact favors well-organized planetary-scale envelopes. When modulated by vertical shear strength, the upscale impact of upshear-propagating MCSs induces significant westerly wind burst, another realistic feature of the observed MJO. Under these two modulation effects, an interesting scenario is obtained with planetary-scale envelopes switching their directions in climatological time scales, mainly due to the interactions among upscale impact of MCSs, MJO analog, and background state. Such zonal symmetry is broken after introducing modulation effects of deep heating excess on westward-moving MCSs. Instead, persistent eastward-moving MJO analog is simulated.

The merits of this study lie in (1) providing an idealized testbed to understand how upscale impact of MCSs influences the propagation, variability and vertical structure of the MJO above the equator, (2) providing prototype strategies for parameterizing upscale impact of MCSs in GCMs.