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A study on an interaction between an upper-layer eddy and a bottom topography using a quasi-geostrophic model
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| Title: | A study on an interaction between an upper-layer eddy and a bottom topography using a quasi-geostrophic model |
| Other Titles: | 準地衡流モデルを用いた上層の渦と海底地形の相互作用に関する研究 |
| Authors: | 竹内, 猛晶 Browse this author |
| Issue Date: | 23-Mar-2023 |
| Publisher: | Hokkaido University |
| Abstract: | This study aims to understand the fundamental aspects of the interactions between mesoscale eddies and bottom topography using the quasi-geostrophic approximation.
First, the interaction between an upper-layer vortex and a bottom topography is investigated using an f-plane two-layer quasi-geostrophic model with a point vortex and step-like topography. The contour dynamics method is used to formulate the model. A steadily propagating linear solution along the topography, known as the pseudo-image solution, is derived analytically for a weak point vortex, and the nonlinear solution is obtained numerically. The numerical experiments show that the nonlinear pseudo-image solution collapses with time, with saddle-node points in the velocity field playing a critical role in this collapse. Even after the collapse, the point vortex propagates along the topography similarly to that in the steadily propagating solution. Numerical experiments with various initial conditions show that the point vortex has two types of motion in this system: motion along the topography and motion away from the topography. In the latter case, the point vortex and lower-layer potential vorticity anomaly form a heton-like dipole structure. Moreover, the motion classification results show that an anticyclonic (cyclonic) point vortex on the deeper (shallower) side is more likely to form a dipole structure than a cyclonic (anticyclonic) vortex on the deeper (shallower) side when its initial distance from the topography is the same.
Second, we examine the influence of the finite width of the topography and the β-effect on the interactions between an upper-layer eddy and a bottom topography using the two-layer quasi-geostrophic model with a Gaussian eddy in the upper layer and sloping topography. In the f-plane, anticyclonic (cyclonic) eddies initially move along the topography in the opposite (same) direction of the topographic Rossby wave. As time elapses, the anticyclonic eddies move away from the topography even when their strength is small, whereas the cyclonic eddies continue to move along the topography even when their strength is large.
These results suggest that in the f-plane, the formation of a heton-like dipole is a common phenomenon in the case of anticyclonic eddies, while an isolated eddy in the lower layer cannot strongly interact with the upper-layer eddies in the case of cyclonic eddies. In the β-plane, anticyclonic eddies move westward onto the sloping topography when their strength is small or the β-effect is large, whereas when their strength is large or the β-effect is small, they move eastward, forming the heton-like dipole. In contrast to the case of the f-plane, the dipole can move eastward long distances in the β-plane. Meanwhile, the cyclonic eddies move westward
onto the topography independent of their strength. These results show that the presence of the β-effect prevents the upper-layer eddy from interacting with the bottom topography and enables the dipole moving eastward to move a long distance. |
| Conffering University: | 北海道大学 |
| Degree Report Number: | 甲第15263号 |
| Degree Level: | 博士 |
| Degree Discipline: | 環境科学 |
| Examination Committee Members: | (主査) 特任教授 久保川 厚, 教授 三寺 史夫, 教授 堀之内 武, 助教 水田 元太, 教授 安田 一郎 （東京大学大気海洋研究所） |
| Degree Affiliation: | 環境科学院（地球圏科学専攻） |
| Type: | theses (doctoral) |
| URI: | http://hdl.handle.net/2115/89546 |
| Appears in Collections: | 課程博士 (Doctorate by way of Advanced Course) > 環境科学院(Graduate School of Environmental Science)
学位論文 (Theses) > 博士 （環境科学）
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