The Modification of Bottom Boundary Layer Turbulence and Mixing by Internal Waves Shoaling on a Barrier Reef

Abstract

Results are presented from an observational study of stratified, turbulent flow in the bottom boundary layer on the outer southeast Florida shelf. Measurements of momentum and heat fluxes were made using an array of acoustic Doppler velocimeters and fast-response temperature sensors in the bottom 3 m over a rough reef slope. Direct estimates of flux Richardson number R f confirm previous laboratory, numerical, and observational work, which find mixing efficiency not to be a constant but rather to vary with Fr t , Re b , and Ri g . These results depart from previous observations in that the highest levels of mixing efficiency occur for Fr t , 1, suggesting that efficient mixing can also happen in regions of buoyancy-controlled turbulence. Generally, the authors find that turbulence in the reef bottom boundary layer is highly variable in time and modified by nearbed flow, shear, and stratification driven by shoaling internal waves.

High‐resolution carbon budgets on a Palau back‐reef modulated by interactions between hydrodynamics and reef metabolism

“…These values are larger than one might expect over a reef with weak currents, and are attributed to the high turbulence dissipation over the reef due to large coral roughness. Bottom roughness, or in this case coral morphology and rugosity, can create turbulent mixing above the coral canopy (Davis and Monismith 2011). The vertical scale of such fluxes is likely related to coral canopy height (Reidenbach et al 2006; J. Koseff pers.…”

“…Therefore, it is necessary to carefully account for horizontal advection of reef water masses on different spatial and temporal scales, the carbon signature of which can significantly influence reef metabolism (Monismith 2007;Anthony et al 2011;Falter et al 2013). Additionally, turbulent mixing near the reef bottom can attenuate or amplify the biogeochemical signal expression of reef metabolism in seawater , and it varies with flow rates near the corals, water column shear, and stratification (Davis and Monismith 2011). Monismith et al (2010) suggest that higher benthic community roughness or rugosity could relate to greater reef biomass and by extension reef metabolism, compared to situations of low relief reefs in slow waters.…”

Observations of Small-Scale Secondary Instabilities during the Shoaling of Internal Bores on a Deep-Ocean Slope

“…It is worth noting that the present buoyancy Reynolds numbers [Re b 5 «/(nN (Shih et al 2005;Davis and Monismith 2011;Dunckley et al 2012;Bouffard et al 2013;Walter et al 2014). Our observations may thus highlight a specific case of efficient mixing that occurs during internal wave breaking on deep sloping topography, when the breaking is associated with high available potential energy.…”

“…These difficulties lie in the fact that turbulence is generally intermittent (the stationary hypothesis needed to compute G does not hold) and that separating reversible and nonreversible density fluctuations is complex, making any estimation of J b difficult (Bouffard et al 2013). This question was addressed in numerical (Slinn andRiley 1996, 1998;Umlauf and Burchard 2011;Mashayek and Peltier 2013) and laboratory studies (Ivey and Nokes 1989) but also from in situ measurements (van Haren et al 1994;Davis and Monismith 2011;Dunckley et al 2012;Walter et al 2014).…”

The Impact of a Variable Mixing Efficiency on the Abyssal Overturning

“…Thus, as stratification 118 decreases to very low levels, it is required that R f also approaches zero. Consistent with these theoretical expectations, the past two decades have seen a growing number of studies, based on 120 direct numerical simulations, laboratory experiments or field work, documenting reduced mixing efficiencies in actively mixing or weakly stratified waters (Gloor et al 2000, Barry et al 2001, 122 Jackson and Rehmann 2003, Rehmann and Koseff 2004, Shih et al 2005, Inall 2009, Davis and Monismith 2011, Hult et al 2011, Dunckley et al 2012, Bouffard and Boegman 2013, Bluteau et 124 al. 2013).…”