Multi‐Stage Development Within Anisotropy Insight of an Anticyclone Eddy in Northwestern South China Sea in 2021

• Published in: Geophysical research letters 2023-10, Vol.50 (19), p.n/a
• Main Authors: Qiao, J., Qiu, C., Wang, D., Huang, Y., Zhang, X.
• Format: Article
• Language: English
• Subjects: Anisotropy; anisotropy analysis; Anticyclones; Autonomous underwater vehicles; AUV observation; Barotropic instability; Barotropic mode; Control stability; Deformation; Developmental stages; Eddies; Eddy kinetic energy; eddy‐mean flow interaction; Energy; Energy budget; Energy transfer; Energy transition; Flow stability; Instability; Kinetic energy; Mesoscale eddies; mesoscale eddy structure; Mesoscale phenomena; Ocean circulation; Ocean currents; Oceans; Shape; Shear; Structural stability; Thermocline; Underwater vehicles; Vortices
• ISSN: 0094-8276; 1944-8007
• DOI: 10.1029/2023GL104736

Mesoscale eddies help regulate ocean energy cascades. Eddies deformation influences barotropic instability, which represents kinetic energy transfer between scales; however, the barotropic instability structure has not been well studied. We investigated an intra‐thermocline eddy (ITE) and developed a novel anisotropic method to examine the horizontal barotropic instability. The development of the ITE was monitored using a state‐of‐the‐art autonomous underwater vehicle from May to July. The ITE became trapped in June and moved eastward in July. Based on anisotropic theory, the barotropic instability was separated into isotropic and anisotropic productions. The anisotropy contained information regarding shape and mean flow feedback of the eddy. Barotropic instability was the main source for ITE eastward propagation and was dominated by anisotropic production. Following a shape and anisotropy change, the ITE gained the mean‐flow kinetic energy by the anisotropy shear production in June and by the anisotropy stretch production when moving eastward in July. Plain Language Summary Mesoscale eddies play vital roles in ocean circulation and are important in energy cascades between large‐scale ocean circulation and dissipate scales. The barotropic instability could induce kinetic energy transition between scales, however, the underlying mechanism has not been well studied. We developed a novel method to decompose the horizontal barotropic instability into isotropic production and anisotropic production. The development of a mesoscale eddy was monitored continuously using a state‐of‐the‐art autonomous underwater vehicle. This mesoscale eddy became stationary in June and began to move eastward in July. In the observation case, the barotropic instability mainly controlled the eddy kinetic energy budget and is dominated by anisotropic production, indicating the contribution of the mean‐flow strain. We found that, the mesoscale eddy gained the mean‐flow kinetic energy via the anisotropy shear production when it was stationary, while via anisotropy stretch production when it moved eastward. Key Points For 3 months, a state‐of‐the‐art AUV monitored the development of an intra‐thermocline eddy (ITE) in terms of vertical transects Novel anisotropy analysis was developed and revealed interactions between eddy anisotropy and mean‐flow strain in barotropic instability The anisotropic partitioning of the ITE at different stages helped gain kinetic energy from the mean flow