Numerical Studies of Submesoscale Island Wakes in the Kuroshio

Abstract

Submesoscale wake formation at Green Island (∼7 km) in the Kuroshio is examined by the three‐dimensional numerical simulations, which are validated by field observations. On the basis of geophysical (rotating and stratified) flow, the wake exhibits sequentially detached recirculation, containing upwelling of cold water, propagates downstream via advection, forming an along‐stream oscillating wake, resembling to the von K árm án vortex streets (VKVS). Evidence includes (1) the shedding frequency as a function of the horizontal eddy viscosity shows a trend analogous with classical wakes; (2) the wake behaviors depend on the Reynolds number (Re), where the turbulent transition regime is determined; and (3) the aspect ratio of the island wakes is similar to the ratio of the VKVS. Unlike classical wakes, the vortex street features are adapted by inertial and barotropic instabilities. The inertial instability has large growth rate and tends to slightly destabilize the anticyclonic recirculation. The barotropic instability could be a secondary process to generate eddy kinetic energy at downstream. Finally, our model suggests the hotspot of the turbulent mixing in the wake is located at the plane free shear layer as a result of the vertical shear instability, which is induced by the island‐shelf effect and the tilting of the vertical vorticity.

Temporal Variation and Spatial Structure of the Kuroshio-Induced Submesoscale Island Vortices Observed from GCOM-C and Himawari-8 Data

“…Therefore, it is appropriate to choose L = 5-6 km for research on Green Island. However, some researchers have instead chosen L = 7 km, which indicates a width of 100 m for the island in the water [5,14,15]. It is worth noting that the choice of L will significantly affect the values of St and Re.…”

“…Take the case of this study as an example. If the ν h value was chosen to be 15 m 2 /s [14] or even 0.2-7 m 2 /s [15], the points of the case could be far from the fitted curve equation. Table 1.…”

“…The minimum U 0 in 38 cases is 0.85 m/s. To make St > 0.1095, when U 0 = 1 m/s, T < 13.95 h, when U 0 = 1.2 m/s, T < 11.62 h, when U 0 = 1.4 m/s, T < 9.96 h. According to the field observation data [14], the vortex shedding period is 12 h. However, there are many cases in this study that showed a longer period for vortex shedding, which results in a lower St. According to our observations, a low SST vortex formed over a long period of time, but it did not detach for a long time.…”

“…Research on underwater island wakes suggests that a depth range with a high inverted Reynolds number is supposed to have density overturns and a high Thorpe scale with a turbulent kinetic energy dissipation rate of O(10 -6 -10 -5 )W kg ⁄ , which corresponds to an eddy diffusivity of O(10 -2 -10 -1 )m 2 s ⁄ [9]. Numerical model research has suggested that the vortex street features are adapted by inertial and barotropic instabilities [14], and the shedding period of the vortex is synchronized to a tidal period [15].…”

Multi-Sensor Observations of Submesoscale Eddies in Coastal Regions

“…It illustrates that the average vorticity of anticyclonic eddies is larger than that of cyclonic eddies; moreover, the instability of anticyclonic eddies is also larger, according to the values of total deformation. Moreover, since surface current fields in this region are influenced by tides and topography, the vorticity of the surface current field is not large compared with western boundary region currents [45]. In addition, no significant difference is found for the radii of anticyclonic and cyclonic eddies.…”