Wind‐shear‐generated high‐frequency internal waves as precursors to mixing in a stratified lake

Abstract

Field data were used to estimate the wavelength, phase speed, direction of propagation, frequency, and the vertical structure of high-frequency internal waves observed on the crests of basin-scale waves of Lake Kinneret during periods of strong wind. Shear stability analysis indicates that these waves were generated by shear in the surface mixing layer. The characteristics of the high-frequency internal waves changed within a wind event as the result of the evolution of the background flow conditions following the deepening of the surface layer and the propagation of the basin-scale internal waves. When the background conditions were appropriate, the vertical structure of the unstable mode was such that the perturbations generated visible sinuous internal waves that in turn modified the density profile in the metalimnion in such a way that secondary shear instabilities were triggered. The high-frequency internal waves were observed over larger distances, but poor coherence in temperature records from stations 200 m apart indicated that individual high-frequency internal waves were dissipated locally; these waves are thus a local mechanism allowing energy to be drawn from the energized surface layer and transported to the metalimnion, where it sustains turbulence. Part of the energy extracted from the surface layer was also returned to the mean flow in the metalimnion; high-frequency internal waves are therefore also a source of momentum for the metalimnetic currents. The vertical excursions of the waves also indicate that they could potentially play a role in phytoplankton growth by significantly altering the light regime at relatively high frequencies.

Poincaré wave–induced mixing in a large lake

“…We believe that this upwelling is uncommon in the western part of the central basin because this process requires a relatively long calm period after a wind event. Such conditions are unlikely in the Laurentian Great Lakes region during summer (Hamblin 1987) because the synoptic storm systems return every 5-10 d and are capable of resetting the thermocline gradient (Gó mez- Giraldo et al 2006). The role of this upwelling event on the biogeochemistry of the western part of Lake Erie requires investigation, because the upwelling initially brings cold oxygenated water to the shallow western basin, but the subsequent return current will carry warm nutrient-laden epilimnetic water from the onshore basin eastward to the hypoxic central basin.…”

“…Details on these methods can be found in similar studies on medium-sized lakes (Boegman et al 2003;Gó mez-Giraldo et al 2008) and the ocean (Sun et al 1998;Smyth et al 2011). The Taylor-Goldstein equation was solved using a modified Matlab code provided by W. Smyth (Oregon State University, http://roach.coas.oregonstate.…”

Role of gas ebullition in the methane budget of a deep subtropical lake: What can we learn from process‐based modeling?

“…Previous modeling studies have addressed various physical and biogeochemical processes in Lake Kinneret (Pan et al 2002;Gal et al 2003Gal et al , 2009Bruce et al 2006;Rimmer et al 2006a;Vernieres et al 2006;Shilo et al 2007;G omez-Giraldo et al 2008;Li et al 2013). However, none of these studies assessed the sources and sinks of CH 4 in the lake.…”

Improving the estimation of Lake Kinneret's heat balance and surface fluxes using the Kalman Filter algorithm

“…These data record the temperature changes at each depth in time and integrates heat storage along the vertical profile. In a case of high/low frequency internal waves that occur within the upper 10 m water profile (Gomez‐Giraldo , Imberger and Marti ), it may add a significant horizontal heat flux component, which was previously eliminated by the assumptions of uniform temperature with depth. However, in this study the changes in the water temperature associated with local heat waves or currents of different temperature are treated as measurement errors and we focus on “smoothed” changes of the heat storage.…”

“…Once again, we tested two independent methods for determining . The first method was to calculate the standard deviation of the measured based on three independent thermistor chains that measured simultaneously several tens of meters horizontally apart from each other in Lake Kinneret (Gomez‐Giraldo ). In the second method, we used a MA procedure, similar to the one we used to smooth the meteorological variables ( see above).…”