Role of stratiform heating on the organization of convection over the monsoon trough

Tuesday, 19 April 2016: 8:45 AM
Ponce de Leon B (The Condado Hilton Plaza)
Ajayamohan Ravindran, New York University, Abu Dhabi, United Arab Emirates; and B. Khouider, A. J. Majda, and Q. Deng

It has been recently demonstrated that stratiform heating (SH) plays a critical role in the scale-selection of organized tropical convection, in an aquaplanet version of a coarse-resolution Atmospheric General Circulation Model coupled to a stochastic multicloud cumulus parameterization scheme. It has been established that, in the case of an equatorially centered warm pool sea-surface forcing, when the model is tuned to produce stronger and space and time extended stratiform anvils, it promotes planetary and intraseasonal Madden-Julian oscillation (MJO)-like organization while in the cases of weaker and short lived stratiform clouds, it leads to synoptic-scale convectively coupled Kelvin-like waves. This is partly due to the extent and strength of stratiform downdrafts that trigger cold pools in the model's atmospheric boundary layer and partly to the important role of tilted heating in the MJO dynamics. The study is extended here to the case of an asymmetric forcing by placing the warm pool forcing north of the equator mimicking the migration of the Intertropical Convergence Zone (ITCZ) during summer to understand the impact of changes in SH on the monsoon dynamics. Six sensitivity experiments were carried out to investigate the response of convection over the monsoon trough (MT) to changes in SH and the latitude of the warm pool center (10oN vs 15oN). It is shown here that the mean monsoon circulation and convection over the MT is sensitive to SH, in the same fashion as in the case of an equatorial warm pool; while strong SH drives planetary- and intraseasonal-scale organization of convection over the MT, weaker SH promotes synoptic-scale waves. More precisely, northeastward propagating monsoon intraseasonal oscillations (MISO) prevail when SH is strong while low pressure systems (LPS)-like disturbances characterize the MT variability when SH is weaker, especially when the warm pool is at 15oN. While the strength of the MT increases with the SH, its westward extent is inversely proportional to the SH, which is consistent with the prevalence of westward moving LPS in this regime. Only in the purely LPS regime do the background vorticity and zonal wind profiles over the MT are consistent with observations. This further demonstrates the importance for the global climate models to produce realistic climatology in order to better simulate synoptic disturbances such as LPS.
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