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Abstract:
The baroclinic annular mode (BAM) is the leading mode of variability in extratropical atmospheric eddy activity representing its hemispheric-scale pulsing. Focusing mainly on sub-weekly disturbances, this study shows impacts of a midlatitude oceanic frontal zone on the BAM signature and its dynamics through “aqua-planet” atmospheric general circulation model experiments with zonally uniform SST profiles prescribed. Though idealized, one experiment with a distinct frontal SST gradient as observed along Antarctic Polar Frontal Zone in the South Indian Ocean, reasonably reproduces BAM signature in the Southern Hemisphere, as captured in atmospheric reanalysis data. In the positive phase of the BAM characterized by enhanced activity of sub-weekly disturbances, they tend to exhibit more distinct baroclinic structures in the lower troposphere and more meridionally-elongated structures in both the upper and lower troposphere. These structural modulations favor more efficient baroclinic development of the disturbances via their poleward heat transport and downstream development, featuring the positive phase of the BAM. Comparison with another sensitivity experiment where the frontal SST gradient is artificially relaxed suggests that, while the BAM is essentially a manifestation of atmospheric internal dynamics, BAM-associated variability in lower-tropospheric poleward heat flux and upper-tropospheric kinetic energy of the disturbances are substantially amplified and the BAM-associated modulation in the baroclinic structure becomes more significant, both by an oceanic front, due to more organized and coherent baroclinic wave packets.
