Internal Hydraulic Transition and Turbulent Mixing Observed in the Kuroshio over the I-Lan Ridge off Northeastern Taiwan
Journal
Journal of Physical Oceanography
Series/Report No.
Journal of Physical Oceanography
Journal Volume
52
Journal Issue
12
Start Page
3179
End Page
3198
ISSN
0022-3670
1520-0485
Date Issued
2022-12
Author(s)
Chang, Ming-Huei
Cheng, Yu-Hsin
Yeh, Yu-Yu
Yang, Yiing Jang
Matsuno, Takeshi
Jan, Sen
Endoh, Takahiro
Tsutsumi, Eisuke
Chen, Jia-Lin
Guo, Xinyu
Abstract
Complex small-scale processes and energetic turbulence are observed at a sill located on the I-Lan Ridge that spans across the strong Kuroshio off Taiwan. The current speed above the sill is strong (1.5 m s21) and unsteady (60.5 m s21) due to the Kuroshio being modulated by the semidiurnal tide. Above the sill crest, isothermal domes, with vertical scales of;20 and;50 m during the low and high tides, respectively, are generated by turbulent mixing as a result of shear instability in the bottom boundary layer. Tidally modulated hydraulic character modifies the small-scale processes occurring on the leeward side of the sill. Criticality analysis, performed by solving the Taylor–Goldstein equation, suggests that the observed lee waves and intermediate layer sandwiched by two free shear layers are related to the mode-1 and mode-2 critical control between the sill crest and immediate lee, respectively. Around high tide, lee waves are advected fur-ther downstream, and only mode-1 critical control can occur, leading to a warm water depression. The shear instabilities ensuing from the hydraulic transition processes continuously mediate flow kinetic energy to turbulence such that the status of marginal instability where the Richardson number converges at approximately 0.25 is reached. The resultant eddy diffu-sivity Kr is concentrated from O(1024) to O(1023) m2 s21 and has a maximum value of 10 m2 s21. The sill on the western flank of the Kuroshio is a hotspot for energetic mixing of Kuroshio waters and South China Sea waters.
Subjects
Internal waves
Kelvin-Helmholtz instabilities
Small scale processes
Topographic effects
Turbulence
Publisher
American Meteorological Society
Type
journal article