9:45 AM - 10:00 AM
[ACG29-04] Influence of a Midlatitude Oceanic Frontal Zone on the Baroclinic Annular Mode in the Southern Hemisphere
Keywords:annular mode, midlatitude oceanic frontal zone, stormtrack
As a major mode of annular variability in the Southern Hemisphere, the baroclinic annular mode (BAM) represents the pulsing of extratropical eddy activity. This study assesses the influence of a midlatitude oceanic frontal zone on the BAM and its dynamics, through a set of “aqua-planet” atmospheric general circulation model experiments with zonally uniform sea-surface temperature (SST) profiles prescribed. Though idealized, one experiment with realistic frontal SST gradient well reproduces observed BAM signatures and time evolution as a manifestation of a typical lifecycle of migratory baroclinic disturbances. In the other experiment, elimination of the frontal SST gradient leads to marked weakening and equatorward shift of the BAM-associated variability, in association with the corresponding modifications in the climatological-mean stormtrack activity. The midlatitude oceanic frontal zone enhances and thereby anchors the BAM variability by energizing sub-weekly disturbances through maintaining near-surface baroclinicity and moisture supply to cyclones. The former is through anomalous heat supply from the ocean that acts to restore the near-surface baroclinicity under the anomalous meridional heat flux associated with transient disturbances. Those experiments and observations further indicate that the BAM modulates momentum flux associated with transient disturbances to induce modest but robust meridional shift of the polar-front jet, suggesting that the BAM can help maintain the Southern Annular Mode. They also suggest that the quasi-periodic behavior of the BAM is likely to reflect atmospheric dynamics involved in not only sub-weekly disturbances.