1. Rhythms of the Agulhas Current Within the Framework of Energetic Anisotropy.
- Author
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Guo, Xin, Yang, Xiao‐Yi, Deng, Qiang, Lin, Hongyang, and Wang, Dongxiao
- Subjects
AGULHAS Current ,ACCELERATION (Mechanics) ,KINETIC energy ,ANTICYCLONES ,STRAIN rate ,EDDIES - Abstract
This study investigates the mechanisms driving the acceleration and meandering of the Agulhas Current (AC), focusing on the role of eddy‐mean flow interactions. The analysis revealed that anticyclones originating from the Mozambique Channel and south of Madagascar played pivotal roles in accelerating the AC. Simultaneously, when anticyclones collide with the AC, they undergo processes of rotating and elongating into ellipses. In addition to the previously suggested barotropic instability induced by anticyclones, this study revealed that the merging of cyclones with the AC plays a role in the generation of meanders. Upstream cyclones reduce the horizontal potential vorticity gradient, facilitating eddies to traverse the current. The AC envelops these cyclones and flows in meandering patterns. The places where these meanders form are not exclusive to the Natal Bight. In addition, we further diagnose the kinetic energy conversion to reveal the interaction between eddy anisotropy (i.e., eddy deformation and orientation) and mean flow strain (i.e., stretching and shearing). The results suggest that the anisotropy of anticyclonic and cyclonic eddies prompts downscale KE transfer and the growth of meanders, establishing a positive feedback loop. Contrary to the findings of previous hypotheses, the acceleration of AC in turn leads to a decrease in the mean flow strain rate, exerting negative feedback on energy conversion and inhibiting the development of meanders. These two feedback mechanisms work together to determine the fate of AC meandering. The energetic anisotropy diagnosis holds potential applicability to other western boundary current systems. Plain Language Summary: The Agulhas Current (AC) is the strongest western boundary current in the Southern Hemisphere. The variabilities in its strength and meandering behavior influence the local marine environment and meridional heat transport. This research is devoted to investigating the mechanisms behind the acceleration and formation of meandering. We find that the acceleration in AC is attributed to upstream anticyclones because the velocities of these anticyclones align with the mean flow in their interaction area. Additionally, this study first demonstrates that nonlocal cyclones can reduce the barrier of the horizontal potential vorticity gradient. These cyclones thus cross the current, causing the AC to form a pulse‐like meandering flow. Meandering also occurs locally due to the barotropic instability of the AC, indicating a downscaled conversion of kinetic energy. We employ a diagnosis of the kinetic energy budget, highlighting the significant influence of eddy anisotropy, including the squeezing of anticyclones and the growth of cyclones. However, the acceleration of the AC would reduce its strain, which abates the original barotropic instability. Key Points: Rhythms of the Agulhas Current, acceleration, and meanders on the synoptic scale were studied from a new energetic anisotropy perspectiveAgulhas Current meanders can be generated either by local anticyclone‐mean flow interactions or upstream cyclone mergingBalance between eddy anisotropy and mean flow strain decides meander growth, with broader applications to other western current systems [ABSTRACT FROM AUTHOR]
- Published
- 2024
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