[A-CG47] 海洋と大気の波動・渦・循環の力学

[ACG47-P09] 偏西風の強化に伴う亜南極前線上の渦活動の強化

*松田 拓朗¹、三寺 史夫²、升本 順夫^3,4、佐々木 英治⁴、古恵 亮⁴、尾形 友道⁴ (1.北海道大学地球環境科学研究院、2.北海道大学低温科学研究所、3.東京大学大学院理学系研究科地球惑星科学専攻、4.海洋研究開発機構)

キーワード：南極周極流、ローレンツエネルギーサイクル、安定性解析、亜南極前線

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.