Bottom boundary layer mixing processes across internal seiche cycles: Dominance of downslope flows

Abstract

In a stratified lake, where an internal seiche maintained an oscillating bottom boundary layer (BBL), processes including strain-induced periodic stratification (SIPS), a downslope mean flow, and upslope-propagating internal waves controlled fluctuating BBL mixing rates. To investigate these processes, high-resolution temperature and velocity profiles were measured within 2 m of the sloping bed, through more than 100 cycles of upslope and downslope flows. The associated seiche-induced velocity reversals coincided with the arrival of upslope-propagating cold fronts. Although fronts are sometimes associated with internal wave breaking, in this lake the measured frontal slopes were small and breaking did not occur. When fronts were not propagating past instruments, near-bed stratification was consistent with SIPS, being intensified by downslope flows and reduced by upslope flows. Owing to a downslope near-bed mean flow, which was previously shown to cancel an upslope internal-wave Stokes drift, peak downslope velocities regularly exceeded peak upslope velocities. Given these strong downslope velocities, turbulent dissipation rates were most intense during downslope phases. Therefore, during downslope flow, creation of stratification by SIPS coincided with elevated dissipation, creating conditions for effective mixing. Correspondingly, two simple parameterizations for mixing efficiency suggest that most of the total near-bed buoyancy flux (≥ 82%), and most of the irreversible flux (87%), occurred during the downslope-flow phase of internal seiche cycles. The observed mechanism for dominant mixing during downslope flow has potential to act in other environments where internal waves propagate up sloping beds.

In situ flux estimates reveal large variations in methane flux across the bottom boundary layer of a eutrophic lake

“…Temporal seiche-induced temperature fluctuations are small compared to top-to-bottom temperature fluctuations during mid-and late-summer, indicating that strong stratification prevents overturning (Henderson and Deemer 2012). Correspondingly, previous work (Nielson and Henderson 2022) determined the seiche in Lacamas to be in the damped, linear regime during the summer (Horn et al 2001). The seiche is responsible for velocity fluctuations with a period of approximately 10-24 h (Henderson and Deemer 2012;Henderson 2016a).…”

“…0) limit. As explained by Nielson and Henderson (2022), Eq. 10 is derived from a parameterization by Venayagamoorthy and Stretch (2010).…”

“…Equation 10 closely resembles Eq. 6 of Nielson and Henderson (2022), written in a form that remains finite in the unstratified () limit. As explained by Nielson and Henderson (2022), Eq.…”

“…6 of Nielson and Henderson (2022), written in a form that remains finite in the unstratified () limit. As explained by Nielson and Henderson (2022), Eq. 10 is derived from a parameterization by Venayagamoorthy and Stretch (2010).…”

Periodic boundary layer separation and lateral intrusions observed above a sloping lakebed

“…Through multiple tidal cycles in the costal ocean, periodic offshore jets can generate intrusions extending kilometers from slopes (Cheriton et al 2014;Masunaga et al 2017). In lakes, similar patterns of periodic boundary layer separation and lateral transport have been described, either near the stationary nodes of a vertically standing internal seiche (hereinafter "seiche") (e.g., Boegman et al 2003;Marti and Imberger 2008;Boegman 2009), or propagating up a slope with a vertically propagating seiche (Nielson and Henderson 2022). Vertically propagating seiches, which were examined previously using mid-lake observations (Henderson and Deemer 2012;Henderson 2016a), are contrasted with vertically standing seiches in Fig.…”