Abstract
Although previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. Here, we conducted a series of intensive in-situ turbulence observations using a state-of-the-art tow-yo microstructure profiler in the Kuroshio flowing over the seamounts of the Tokara Strait, south of Kyusyu Japan, in November 2017, June 2018, and November 2019, and employed a high-resolution numerical model to elucidate the turbulence generation mechanisms. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. The observed turbulence could supply nutrients to sunlit layers, promoting phytoplankton primary production and CO2 uptake.
Highlights
Previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream
Our observations and the numerical simulations cannot resolve all the scales from larger scales to the submesoscale instability and the associated microscale turbulence at once, the correspondence between the negative potential vorticity (PV) at submesoscales and the large turbulent dissipation rates at microscales suggests that the negative PV generated on the steep slopes of the seamounts and associated inertial–symmetric instability followed by the K-H instability are the most plausible sources of the observed 100-km-wide 100–1000-fold enhancement in turbulent dissipation rates along the Kuroshio across the Tokara Strait (Fig. 10)
The strong turbulence near the seamount associated with the lee waves and negative PV from the bottom boundary layer of ~100 m thickness is, the likely source of observed high-vertical wavenumber near-inertial internal waves
Summary
Previous studies reported that currents over topographic features, such as seamounts and ridges, cause strong turbulence in close proximity, it has been elusive how far intense turbulence spreads toward the downstream. We find that the Kuroshio flowing over seamounts generates streaks of negative potential vorticity and near-inertial waves. With these long-persisting mechanisms in addition to other near-field mixing processes, intense mixing hotspots are formed over a 100-km scale with the elevated energy dissipation by 100- to 1000-fold. A geostrophic current over the topography can radiate lee waves extracting energy from the general circulation to turbulent dissipation, which is estimated to account for the global energy sink as large as 0.15–0.23 TW3. Recent in situ observations in the Southern Ocean reported the lee waves propagating in the Drake Passage over the rough topography, consistent with the linear internal-wave theory[3], the depth-integrated energy dissipation rates are two orders of magnitude smaller than the vertical internalwave energy flux[15], implying that a large fraction of lee wave energy is not dissipated locally. Recent high-resolution numerical studies have revealed that the Gulf Stream[6] and the California Undercurrent[5,7] flowing over the continental slope can alter the sign of the Ertel’s potential vorticity (PV) on their anticyclonic side, leading to a submesoscale inertial–symmetric instability and associated inevitable forward energy cascade and dissipation[16]
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