[A-CG30] Extratropical oceans and atmosphere

[ACG30-P07] Impacts of a Midlatitude Oceanic Frontal Zone on the Southern Baroclinic Annular Mode

*Morio Nakayama¹, Hisashi Nakamura^1,2, Fumiaki Ogawa³ (1.Research Center for Advanced Science and Technology, The University of Tokyo, Tokyo, Japan, 2.Japan Agency for Marine-Science and Technology, Yokohama, Japan, 3.Department of Natural History Sciences, Graduate School of Science, Hokkaido University, Sapporo, Japan)

Keywords:midlatitude oceanic frontal zone, storm track, climate variability

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 through “aqua-planet” atmospheric general circulation model experiments with zonally uniform SST profiles prescribed. Though idealized, one experiment with a distinct frontal SST gradient 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 by an oceanic front, due to more organized and coherent baroclinic wave packets.