[A-CG47] Dynamics of Oceanic and Atmospheric Waves, Vortices, and Circulations

[ACG47-P09] Enhanced eddy activity along the Subantarctic Front under intensified
westerly winds

*Takuro Matsuta¹, Humio Mitsudera², Yukio Masumoto^3,4, Hideharu Sasaki⁴, Ryo Furue⁴, Tomomichi Ogata⁴ (1.Faculty of Env.Earth Science, Hokkaido University, 2.Institute of Low Temperature Science Hokkaido University , 3.Department of Earth and Planetary Science, The University of Tokyo, 4.Japan Agency for Marine-Earth Science and Technology)

Keywords:Antarctic Circumpolar Current, Lorenz energy cycle, Stability analysis, Subantarctic Front

The westerlies in the Southern Hemisphere have intensified and shifted southward since the mid-20th century. Previous studies have suggested that the expected increase in isopycnal slopes and the acceleration of the Antarctic Circumpolar Current (ACC) are significantly weakened by baroclinic instability. This “eddy saturation” primarily occurs downstream of major bottom topographic features, such as the Kerguelen Plateau. As a result, these eddy “hotspots” are thought to regulate the ACC's response to changes in westerly winds. To improve our understanding of the ACC’s response to the intensified westerlies, we conduct a sensitivity study using an eddy-resolving, quasi-global ocean general circulation model, OFES. The reference run is driven by climatological atmospheric forcing, while the sensitivity run uses artificially intensified climatological westerlies. Using the Lorenz energy cycle, we find that the baroclinic energy pathway is enhanced over the Subantarctic Front (SAF) as well as the hotspots identified by previous studies. A linear stability analysis reveals that the intensification of subtropical gyres north of the SAF, along with enhanced Ekman upwelling south of the SAF due to intensified wind stress curl, increases the vertical shear of zonal velocity along the SAF, thereby enhancing baroclinic instability. We also performed the same stability analysis on the 1985–2018 and 1955–1984 periods of a hindcast run of OFES, confirming the results from the climatological sensitivity study. These findings suggest that the SAF becomes another eddy hotspot as the wind stress curl continues to increase.