Mixing and internal dynamics of a medium‐size and deep lake near the Arctic Circle

Abstract

The goal of the research was to identify the mixing features that distinguish wind forced, medium‐size fjord‐type lakes near the Arctic Circle from systems of similar geometry, but in a temperate climate. Experimental data and the results of 3D numerical hydrodynamic simulations were analyzed for Lake Lagarfljót (27 km long; 2 km wide; 110 m maximum depth; 65°N) during the 5‐month ice‐free period in 2009. The results showed that probably the most distinctive feature of arctic lakes is their low water column stability, one order of magnitude lower than that of mid‐latitude lakes, even in mid‐summer. The second characteristic is the high rate of energy imparted by wind in the Arctic, estimated as one order of magnitude larger than in a temperate climate. Frequent wind events with Lake and Wedderburn numbers below 1 occurred during the ice‐free period, leading to a strong shear at the base of the surface mixed‐layer and the upwelling of deep metalimnetic layers. As a result, nearly continuous density stratification developed in the shallowest 70 m of the water column, and longitudinal temperature gradients may therefore be very significant. On average, 15% of the wind forcing drove large‐scale internal motions. This energy was rapidly dissipated, partly as a result of stronger bottom velocities, nonlinear surges, and hydraulic jumps that repeatedly formed in the lake. The initial V1H1 setup rapidly evolved into spatially complex oscillatory modes, which, given the high latitude, are affected by the Earth's rotation, even in narrow basins.

Inflow Dynamics in Weakly Stratified Lakes Subject to Large Isopycnal Displacements

“…Particle settling was assumed negligible following Striberger et al (2011), who argued that most of the sediments or rock flour brought by Jökulsá í Fljótsdal stays in suspension in the water in the summer and deposit during the winter. These assumptions were further supported by model results at the lake outflows which reproduce the evolution of the C SS signal measured at this site (RMSE = 0.011 kg m −3 and Figure A1 in Priet‐Mahéo et al, 2019). By neglecting particle settling in the simulations, convective sedimentation (e.g., Jazi & Wells, 2020) is not accounted for.…”

“…The scaling function f ( We ) links the thermocline displacement (or wind set up) Δ H max and the time‐filtered Wedderburn number We (Appendix and see Shintani et al, 2010 for finite amplitude displacements). In the specific case of Lake Lagarfljót, the internal oscillations fade out quickly after the wind ceases (Priet‐Mahéo et al, 2019), so that the isopycnal displacements are largely associated with the magnitude and direction of the wind forcing. Hence, in these systems, one could also assume that the relevant time scale Δ t Ri , to check the quasi‐stationary assumption, is that of the wind forcing.…”

“…It will be assumed that the stratification can be described as consisting of two layers. The epilimnion in the density profiles was identified following Priet‐Mahéo et al (2019), as the layer where densities differ at most 0.03 kg m −3 from the surface density. Similarly, the hypolimnion was taken as the layer where densities differ at most 0.03 kg m −3 from bottom densities.…”

“…High‐latitude lakes, located near or above the Arctic Circle, are an example of such dynamic systems. Subject to low solar insolation, cold and wet weather, and strong wind forcing, these lakes tend to exhibit weak water column stability and experience frequent and long‐lasting upwelling events (see Priet‐Mahéo et al, 2019). Given the large amplitude of the isopycnal displacements occurring in (sub) arctic systems, we hypothesize that the initial fate of negatively buoyant river plumes, whether they form interflows or underflows, could vary depending on the amplitude and direction (downwelling‐upwelling) of those displacements near the inlet at the time of the inflow (see conceptual model in Figure 1).…”

Internal waves pump waters in and out of a deep coastal embayment of a large lake

“…With a Burger number on the order of unity, internal wave dynamics in Kempenfelt Bay are expected to be influenced by the Earth's rotation, which would manifest in the internal wave field. Similarly, Priet‐Mahéo et al () observed internal Kelvin waves in the long (27 km) and narrow (2 km) arctic Lake Lagarfljót (65°N, S i = O(10 −1 ) or O(1)) resulting in counterclockwise propagating temperature disturbances that decayed offshore. The influence of the Coriolis force upon the secondary thermocline tilt can be seen by looking at successive panels of Fig.…”