Significant Modulations in Storm-Track Activity and Transient Eddy Structure in Association with the Southern Hemisphere Baroclinic Annular Mode

The baroclinic annular mode (BAM) in the Southern Hemisphere (SH) is the leading mode of variability in extratropical eddy activity characterized by its hemispheric-scale pulsing, but its mechanisms have not been fully understood. Our investigation of atmospheric reanalysis data reveals BAM-associated systematic modulations not only in fluxes associated with sub-weekly transient disturbances but also in their spatial structure both involved in the dynamics of the BAM. Specifically, in the positive phase of the SH BAM characterized by enhanced activity of transient disturbances, their lower-tropospheric baroclinic structure becomes more distinct with significantly higher correlation between sub-weekly fluctuations in temperature and poleward wind, while they tend to be more elongated meridionally in both the upper and lower troposphere and thus yield stronger eastward wave-activity flux. Occurring under the enhanced lower-tropospheric background baroclinicity, these structural modulations of the disturbances favor more efficient baroclinic development via enhanced poleward heat transport and their downstream development, which can contribute to hemispheric-scale enhancement of eddy kinetic energy. In addition, a tendency of the disturbances to exhibit horizontally tilting structure becomes more evident in the positive phase with higher correlation between the zonal and meridional wind fluctuations. This is favorable for increasing transport of westerly momentum from the subtropics toward a midlatitude eddy-driven jet, and the resultant anomalous zonal wind acceleration has a significant projection on the Southern Annular Mode. This modulation contributes to the decay of the BAM signature. The decay is also contributed to by the reduced background baroclinicity due to the enhanced eddy heat flux.

A set of aqua-planet AGCM simulations with zonally uniform sea-surface temperatures suggests that the BAM-associated pulsing in storm-track activity and structural modulations are likely manifestations of atmospheric internal dynamics, which can be substantially more pronounced in the presence of a midlatitude oceanic frontal zone through the formation of more organized baroclinic disturbances under the impact of sharp SST gradients. The climatological-mean storm-track activity tends to be sensitive to the intensity and latitude of the background SST fronts, which is found to lead to inter-basin differences in BAM-associated modulations of storm-track activity in the SH.