The Interaction of Large Amplitude Internal Seiches with a Shallow Sloping Lakebed: Observations of Benthic Turbulence in Lake Simcoe, Ontario, Canada

Abstract

Observations of the interactions of large amplitude internal seiches with the sloping boundary of Lake Simcoe, Canada show a pronounced asymmetry between up- and downwelling. Data were obtained during a 42-day period in late summer with an ADCP and an array of four thermistor chains located in a 5 km line at the depths where the thermocline intersects the shallow slope of the lakebed. The thermocline is located at depths of 12–14 m during the strongly stratified period of late summer. During periods of strong westerly winds the thermocline is deflected as much as 8 m vertically and interacts directly with the lakebed at depth between 14–18 m. When the thermocline was rising at the boundary, the stratification resembles a turbulent bore that propagates up the sloping lakebed with a speed of 0.05–0.15 m s−1 and a Froude number close to unity. There were strong temperature overturns associated with the abrupt changes in temperature across the bore. Based on the size of overturns in the near bed stratification, we show that the inferred turbulent diffusivity varies by up to two orders of magnitude between up- and downwellings. When the thermocline was rising, estimates of turbulent diffusivity were high with KZ ∼10−4 m2s−1, whereas during downwelling events the near-bed stratification was greatly increased and the turbulence was reduced. This asymmetry is consistent with previous field observations and underlines the importance of shear-induced convection in benthic bottom boundary layers of stratified lakes.

Logarithmic velocity structure in the deep hypolimnetic waters of Lake Michigan

“…[, Lake Baikal, see their Table and Figure ]; both the Baltic Sea and Lake Baikal observations are similar to the present field site as they have hypolimnetic currents dominated by near‐ and subinertial processes. Our inferred LOW turbulent diffusivities substantially higher than those reported in smaller lakes with stronger currents [e.g., Lorke , ; Cossu and Wells , ]; this is because of the near‐bottom stratification present for those studies which, when parameterized, yields lower estimates for the turbulent diffusivity than the unstratified LOW estimate. The same arguments hold true for Lake Erie's central basin, which yield lower near‐bottom dissipation and turbulent diffusivity values than our measurements, in spite of stronger near‐bottom currents [ Bouffard et al ., ] ; while the lateral extent of Lake Erie's central basin is comparable to Lake Michigan, it is a shallow basin for which the bottom boundary layer is strongly impacted by the overlying stratification.…”

Species‐specific effects of subdaily temperature fluctuations on consumption, growth and stress responses in two physiologically similar fish species

“…Subdaily fluctuations can occur naturally in nearshore areas in large lakes (Wells & Parker 2010;Troy et al 2012;Cossu & Wells 2013) or via anthropogenic activities including near industrial thermal discharges (Cooke & Schreer 2003;Coulter et al 2014). Subdaily fluctuations can occur naturally in nearshore areas in large lakes (Wells & Parker 2010;Troy et al 2012;Cossu & Wells 2013) or via anthropogenic activities including near industrial thermal discharges (Cooke & Schreer 2003;Coulter et al 2014).…”

Movements of the thermocline lead to high variability in benthic mixing in the nearshore of a large lake

“…Since density in Lake Ontario is predominately determined by the temperature, our analysis was based on temperature inversions calculated from the benthic thermistor chain measurements. These benthic chains had a sensor separation of 0.1 m in 2012 and 0.14 m in 2013, which is comparable to the 0.042 m resolution used by van Haren [] and 0.15 m separation used by Cossu and Wells []. We identified temperature inversions when the temperature difference between two vertically separated thermistors at heights z 1 and z 2 exceeded .…”

“…During warmer down‐slope flow, differential advection of lighter fluid leads to enhanced stratification and reduced boundary mixing. Shear‐induced convection can cause a strong asymmetry in the level of bottom boundary layer turbulence and stratification between up‐ and down‐slope flow phases [ Lorke et al ., ; Cossu and Wells , ]. Shear‐induced convection has been studied in a number of oceanographic studies [see Simpson et al ., ; Rippeth et al ., ; Moum et al ., ; van Haren et al ., ], and has also been described in theoretical studies [ Becherer and Umlauf , ; Umlauf and Burchard , ].…”