Abstract
This study employs a two-layer Quasi-Geostrophic (QG) channel model with a linear, empirical parameterization of diabatic heating to investigate the impact of the U.S. Mesoscale Convective Systems (MCSs) on the northern extratropical synoptic variability in boreal spring. Major modes of synoptic variability in the model are obtained from fast-growing “dry” and “moist” optimal disturbances developing in a climatological spring background flow. Compared to the no-heating scenario, the introduction of domain-wide diabatic heating generally reduces the zonal scale and increases the amplitude of optimal disturbances, leading to two better-defined maxima of synoptic-scale variance corresponding to the North Pacific and North Atlantic storm tracks. Dominated by MCS heating, diabatic heating over the central U.S. not only significantly enhances synoptic variability over the eastern U.S. and North Atlantic but also promotes the circumglobal zonal propagation of synoptic-scale disturbances, extending its impact to planetary and hemispheric scales. The transient (synoptic) eddy forcing of the background zonal flow is notably amplified by MCS heating, manifested as more prominent westerly accelerations that strengthen and extend downstream the Atlantic jet. The implications of the findings from this highly idealized analysis are further discussed in the context of the upscale feedback of mesoscale disturbances to extratropical low-frequency variability and the coupling between long-term trends of the U.S. MCS activity and regional and hemispheric circulation changes.
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