[A-CG47] 海洋と大気の波動・渦・循環の力学

[ACG47-17] 風応力の変化に対する南極周極流の応答

*吉田 槙之助¹、松田 拓朗¹ (1.北海道大学大学院環境科学院)

キーワード：南大洋、南極周極流、中規模渦

The Antarctic Circumpolar Current (ACC) is primarily maintained by strong westerly winds in the Southern Hemisphere. The Southern Annular Mode (SAM) index represents the meridional shift and intensity of the westerly winds, and it has been reported that the SAM index exhibits a long-term positive trend, implying a southward shift and strengthening of the westerly winds. An important question is whether coarse-resolution models, which rely on parameterizations of mesoscale eddies, can accurately capture the response of the ACC to changes in wind stress. To explore this, we compare the response to wind stress changes between coarse and eddy-resolving models. We use a zonally reentrant channel model with the Massachusetts Institute of Technology general circulation model. The domain extends 6000 km zonally, 2000 km meridionally, and 3982 meters deep. The horizontal resolutions for coarse and eddy-resolving models are 50 km and 5 km, respectively. In the coarse model, mesoscale eddies are represented by the Gent and McWilliams scheme with fixed GM coefficients. Salinity is neglected, and only temperature is considered. A Gaussian ridge with 300-km width is inserted. A zonal wind stress is applied to the surface as a sine function, and we vary its maximum value. In each experiment, the model is run for 30 years. We first compare the results of the equilibrium states. As wind stress increases, the sensitivity of circumpolar transport declines in both the coarse and eddy-resolving models, while the coarse model only slightly overestimates the sensitivity of circumpolar transport. Horizontal structures of barotropic streamlines indicate that enhanced curvature in the standing meander downstream of the ridge intensified in both models. This leads to stronger asymmetry in bottom pressure difference across the ridge, which contributes to increasing topographic form stress. However, the horizontal distributions of eddy kinetic energy (EKE) differ notably: in the eddy-resolving model, EKE grows linearly with increasing wind stress, while it does not develop significantly in the coarse model. Previous studies indicated that mesoscale eddies are responsible for the reduced sensitivity of circumpolar transport. However, despite the different responses of EKE in the coarse and eddy-resolving models, the reduced sensitivity is established in both models. Additionally, recent studies have indicated that topographic form stress responds to changes in wind stress within a few days, a timescale significantly shorter than that typically associated with baroclinic instability. These findings suggest that other processes, beyond mesoscale eddies, may play critical roles in determining the sensitivity of ACC transport. To fully understand the differences in behaviour between the coarse and eddy-resolving models, we need to examine the adjustment process to wind stress changes.