Turbulent production in an internal wave bottom boundary layer maintained by a vertically propagating seiche

Abstract

Internal seiches, which supply the energy responsible for mixing many lakes, are often modeled as vertically standing waves. However, recent observations of vertical seiche propagation in a small lake are inconsistent with the standard, vertically standing model. To examine the processes responsible for such propagation, drag and turbulent production in the bottom boundary layer of a small lake are related to the energy supplied by a propagating seiche (period 10–24 h). Despite complex and fluctuating stratification, which often inhibited mixing within 0.4 m of the bed, bottom stress was well represented by a simple drag coefficient model (drag coefficient 1.5 × 10−3). The net supply of seiche energy to the boundary layer was estimated by fitting a model for internal wave vertical propagation to velocity profiles measured above the boundary layer (1–4.5 m above lakebed). Fitted reflection coefficients ranged from 0.3 at 1 cycle/d frequency to 0.7 at 2.4 cycles/d (cf. near‐unity coefficients of classical seiche theories). The net supply of seiche energy approximately balanced boundary layer turbulent production. Three of four peaks in production and energy flux occurred 0.8–2.2 days after strong oscillating winds, a delay comparable to the time required for seiche energy to propagate to the lakebed. A model based on the estimated drag coefficient predicted the observed frequency dependence of the seiche reflection coefficient. For flat‐bed regions in narrow lakes, the model predicts that reflection is controlled by the ratio of water velocity to vertical wave propagation speed, with sufficiently large ratios leading to weak reflection, and clear vertical seiche propagation.

Evolution and dynamics of the vertical temperature profile in an oligotrophic lake

“…Klaić et al: Evolution and dynamics of the vertical temperature profile in an oligotrophic lake fluences on surface air temperatures (e.g., Klaić and Kvakić, 2014), wintertime snowstorms produced by lakes (e.g., Kristovich et al, 2017), and lake breezes (e.g., Potes et al, 2017). The role of lakes in the regional climate has also been investigated (e.g., Bryan et al, 2015) as well as lake responses to climate and land use change (e.g., Huziy and Shushama, 2017;Roberts et al, 2017;Hipsey et al, 2019).…”

Hydrodynamics of a periodically wind-forced small and narrow stratified basin: a large-eddy simulation experiment

“…Wind stress acting on the water surface drives baroclinic responses in ens closed stratified basins [12,39], which can lead to a variety of hydrodynamically-controlled phenomena. These phenomena can span from basin-scale internal waves [42,2,61,56] and upwelling/downwelling motions [63,60], to localized and intermittent turbulence and mixing episodes in the interior of the stratified waterbody [53,9], including the near-sediment bottom [19,24,33,31,34,21]. Numerical models able to capture the aforementioned hydrodynamic processes and their interactions are needed to explore the physics of the aquatic ecosystems.…”

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

“…Despite the importance of BBLs, significant questions remain concerning the processes controlling BBL mixing rates. In many stratified lakes, BBLs are energized by oscillating near‐bottom currents, forced by basin‐scale internal waves called internal seiches (Wüest and Lorke 2003; Boegman 2009; Henderson 2016a). Through seiche cycles, BBL mixing displays order‐of‐magnitude temporal variability (MacIntyre et al 1999; Bryant et al 2010; Deemer et al 2015), yet the causes of this variability are not fully understood.…”