Numerical study on tidal mixing along the shelf break in the Green Belt in the southeastern Bering Sea

Abstract

1] Tidal mixing and its associated iron and nutrients flux from a subsurface layer along the shelf break has been considered as one of the key processes to maintain the high summertime biological productivity in the Green Belt in the southeastern Bering Sea. In the present study, tidal mixing near the shelf break is examined with a three-dimensional highresolution numerical model to quantify the enhanced mixing and to reveal the underlying physical mechanisms based on the different characteristics of diurnal and semidiurnal internal waves. Strong turbulent mixing with energy dissipation rates of over 1 3 10 28 W/kg is reproduced along the Green Belt in the numerical model forced by barotropic diurnal and/or semidiurnal tides at open boundaries. The energy dissipation off the shelf break near the sea surface is enhanced due to the semidiurnal internal waves, whereas the dissipation near the bottom of the shelf break is enhanced by the diurnal topographically trapped waves. An additional experiment with stratification representative of winter conditions shows a significant influence of stratification on the energy dissipation off the shelf break due to the change in features of both diurnal and semidiurnal internal waves. Low-mode semidiurnal baroclinic energy flux from the Aleutian Passes is shown to increase the energy dissipation along the shelf slope between Pribilof and Zhemchug Canyons. These results suggest that the strong vertical mixing along the shelf break induced by the diurnal and semidiurnal tides both play important roles in maintaining the iron and nutrients supply along the Green Belt.

Climate Process Team on Internal Wave–Driven Ocean Mixing

“…The percentage of local dissipation may be systematically higher in marginal seas or areas where lower modes are not free to escape (St. Laurent 2008;Nagai and Hibiya 2015). Similarly, near-field tidal dissipation can be increased by topographically trapped internal waves generated by subinertial tidal constituents (Tanaka et al 2013), that is, the diurnal constituents at latitudes >30° and the semidiurnal constituents at latitudes >74.5°. The energy density in such trapped motions increases with latitude and is all dissipated locally (Musgrave et al 2016).…”

Observations of Diurnal Coastal-Trapped Waves with a Thermocline-Intensified Velocity Field

“…On the continental shelf, CTWs are typically bottom intensified at the shelf break, except in the limiting case of weak stratification (Crawford and Thomson 1984). Tanaka et al (2013) found enhanced bottom dissipation at the shelf break of the Bering Sea due to D1 trapped waves with bottom-intensified velocities, which contributed to the nutrient supply and productivity of the shelf. Irreversible mixing from shear instabilities can occur when the gradient Richardson number, Ri 5 N 2 /k›u/›zk 2 , is less than a critical threshold of 0.25 (e.g., Ivey et al 2018).…”

“…f . Both subinertial and superinertial waves may have a combined effect on mixing near topography (Musgrave et al 2016(Musgrave et al , 2017Stashchuk and Vlasenko 2017;Waterhouse et al 2017), with potential implications for the biological productivity of the shelf via enhanced fluxes of nutrients (e.g., Tanaka et al 2013). However, as trapped waves are highly dependent on the characteristics of the topography and stratification (Huthnance 1978), the role of these waves in modifying mixing will vary by region and seasonally.…”

“…Huthnance (1978) explored the effect of stratification, as well as numerous other factors, on the velocity structure and propagation speeds of CTWs. He found that a strong and continuous stratification (i.e., no pycnocline) led to the formation of bottom-intensified velocities at the shelf edge, and Tanaka et al (2013) found this resulted in enhanced fluxes of nutrients at the seabed in the southeastern Bering Sea. Observations from southeast Australia (Freeland 1988) and the Adriatic Sea (Mihanović et al 2009) have captured diurnal CTWs with intensified velocity and temperature perturbations, respectively, within the pycnocline.…”

Energy Fluxes in Coastal Trapped Waves

“…Nevertheless, recent work has sought to examine the energetics of these trapped waves as an important component of ocean processes near topography and at the basin boundaries (Tanaka et al 2010;Niwa and Hibiya 2011;Müller 2013;Tanaka et al 2013;Fer et al 2015;Klymak et al 2016;Musgrave et al 2017;Li et al 2017;Masunaga et al 2017;Hughes and Klymak 2019). Many authors (including this author) have calculated energy fluxes or constructed energy budgets for subinertial trapped internal tides by undertaking the same analysis that has been successfully applied to superinertial tides over a flat bottom (e.g., Kang and Fringer 2012).…”