[A-CG36] Extratropical oceans and atmosphere

[ACG36-11] Impacts of a Midlatitude Oceanic Frontal Zone on the Baroclinic Annular Mode Signature: Its inter-basin differences in the Southern Hemisphere

*Morio Nakayama¹, Hisashi Nakamura¹, Satoru Okajima^1,2, Fumiaki Ogawa^1,3, Yu Kosaka¹, Takafumi Miyasaka¹, Ryo Satoh¹ (1.Research Center for Advanced Science and Technology, The University of Tokyo , 2.Institute of Life and Environmental Sciences, University of Tsukuba, 3.Faculty of Bioresources, Mie University)

Keywords:baroclinic annular mode, midlatitude oceanic frontal zone

Recent studies have highlighted the significance of a midlatitude oceanic frontal zone, characterized by strong meridional gradients of sea-surface temperature (SST), in energizing baroclinic disturbances and thereby influencing extratropical atmospheric circulation. This study explores the influence of the oceanic frontal zone on the baroclinic annular mode (BAM), which represents hemispheric pulsing in extratropical eddy activity. Focusing on the pulsing of sub-weekly disturbances which effectively captures BAM-associated modulations of baroclinic processes, this study shows that the oceanic frontal zone significantly amplifies and thereby anchors the BAM signature by comparing aqua-planet atmospheric general circulation model experiments with zonally-uniform SST profiles with and without the frontal SST gradient. The enhancement of sub-weekly disturbances in the positive BAM is associated with those exhibiting more distinct baroclinic structure, which can be detected more clearly in the experiment with the frontal SST gradient. Furthermore, by analyzing the JRA-55 reanalysis data, this study shows that the BAM observed in the Southern Hemisphere (SH) exhibits a zonally-asymmetric signature with greater amplitude in storm-track cores over the South Atlantic and Indian Ocean. It is found that many of the positive BAM events are associated with zonally-confined baroclinic wave packets which develop in various longitudes and affect broader longitudinal sectors via their downstream development. Still, the BAM signature strongly reflects those originating in the Southeast Pacific and intensifying over the distinct oceanic frontal zone in the South Atlantic and Indian Ocean. These results suggest the importance of strong baroclinic development of sub-weekly disturbances over the distinct oceanic frontal zone on the BAM activity in the SH.