Preconditioning of an underflow during ice-breakup in a subarctic lake

Abstract

The fate of inflows into lakes has been extensively studied during summer stratification but has seen relatively little focus during the weak winter stratification, with or without ice-cover. Field observations are presented of groundwater inflow into a shallow bay of a subarctic lake. Atmospheric forcing of the bay during the study period was extremely variable and coincided with spring ice-cover break-up. Two dominant wind regimes were identified; (1) weak wind-forcing (wind speed \5 m s -1 or land-fast ice-cover), and (2) strong wind-forcing (wind speed [5 m s -1 and open water). At a relatively constant temperature of *3.3°C, the groundwater inflow was closer to the temperature of maximum density than the water in the main body of the lake, which during the observed winter stratification is *1.2°C. During weak wind-forcing, the stratification within Silfra Bay approximated two-layers as this denser groundwater formed a negatively buoyant underflow. A calculated underflow entrainment rate of 2.8 9 10 -3 agrees well with other underflow studies. During strong wind-forcing, the water column out to the mouth of the bay became weakly stratified as the underflow was entrained vertically by wind-stirring. Observed periods of mixing can be predicted to occur when turbulent kinetic energy (TKE) production by wind stirring integrated over the underflow hydraulic residence time in the bay exceeds the potential energy associated with the stratification. A decrease of ice cover, as observed in the studied subarctic lake over the last decade, will result in the underflow being more frequently exposed to the strong wind-forcing regime during winter, thereby altering the winter distribution of groundwater inflow within the lake.

Axisymmetric circulation driven by marginal heating in ice‐covered lakes

“…Here Δ b = 0.5 × 10 ‐4 m s −2 is the buoyancy difference between the density flow and the overlying lake waters (Figure a). The in situ entrainment rate, E = (d h down )(d x ) −1 , can be directly estimated from relative slopes of the lake bottom and the upper boundary of the downslope flow detected from isolines of constant buoyancy (temperature) [e.g., Forrest et al ., ] as E ≈ 2–5 m km −1 ≈ 3.5 × 10 −3 . The good agreement between both estimates of E validates the assumption of steady state.…”

Winter Limnology: How do Hydrodynamics and Biogeochemistry Shape Ecosystems Under Ice?

“…Through‐flow velocities of river and stream water are of order 10 −4 –10 −2 m s −1 and depend in part on the surface roughness of the ice‐water interface (Bengtsson et al., 1996; Hamblin & Carmack, 1990), which in turn is modified by ice‐trapped gas bubbles emitted from the sediment (Engram et al., 2020). The depth of intrusion and extent of mixing are also a function of temperature, as warming can result in negatively buoyant plumes (Forrest et al., 2012), and of the volume of snowmelt in relation to the volume of the lake (Cortés et al., 2017; Jansen et al., 2019; Melack et al., 2021).…”