Poincaré wave–induced mixing in a large lake

Abstract

A 10,000-km 2 hypoxic 'dead zone' forms, during most years, in the central basin of Lake Erie. To investigate the processes driving the hypoxia, we conducted a 2-yr field campaign where the mixing in the lake interior during the stratification period was examined using current meters and temperature-loggers data, as well as . 600 temperature microstructure profiles, from which turbulent mixing was computed. Near-inertial Poincaré waves drive shear instability, generating , 1-m amplitude and 10-m wavelength high-frequency internal waves with , 1-m density overturns that lead to an increase in turbulent dissipation by one order of magnitude. The instabilities are associated with enhanced vertical shear at the crests and troughs of the Poincaré waves and may be correlated with the local gradient Richardson number. Poincaré wave-induced mixing should be an important factor when the Burger number , 0.25. The strong diapycnal mixing induced by the Poincaré wave activity will also significantly modify the energy-flux paths. For example, we estimate that, in Lake Erie, 0.85% of the wind energy is transferred to the lake interior (below the surface layer); of this, 40% is dissipated in the interior metalimnion and 60% is dissipated at the bottom boundary. In smaller lakes, 0.42% of wind energy is transferred to the deeper water, with 90% dissipated in the boundary and 10% in the interior metalimnion.

Behavior and mixing of a cold intermediate layer near a sloping boundary

“…8 seems to correspond to bands of lower Richardson number, suggesting that shear instabilities are at work in the interior of the water column. These may be induced by internal shear or high-frequency internal waves locally-generated by the Poincaré internal wave (Bouffard et al, 2012). To examine a possible systematic relation between stratification, shear and dissipation, we plotted 4-m resolution bin of ǫ as function of N 2 and S 2 in a manner similar to MacKinnon and Gregg (2003), Carter et al (2005), Palmer et al (2008), Schafstall et al (2010), van der Lee and Umlauf (2011) and others (Fig.…”

“…Forrester, 1970Forrester, , 1974Ingram, 1979;Wang et al, 1991;Galbraith, 1992), a second objective is to quantify to what extent the CIL behavior and the mixing are controlled by internal tides. This last point is also relevant to other coastal basins since recent studies suggest that internal tides (or near-inertial waves in the absence of tides) can partly control the dynamics (stratification, shear and dissipation) and mixing of coastal basins and lakes (e.g., MacKinnon and Gregg, 2003;Carter et al, 2005;van der Lee and Umlauf, 2011;Bouffard et al, 2012).…”

Dynamic hypoxic zones in Lake Erie compress fish habitat, altering vulnerability to fishing gears

“…2). Temperature changes indicated rapid movement of the thermocline consistent with internal waves (Bouffard et al 2012), and in most cases these episodes advected low oxygen waters across the sensors. Corresponding switches from normoxia to hypoxia and back were always greater than 2.0 mg·L −1 ·day −1 and often exceeded 7.5 mg·L −1 ·day −1 (75% of occurrences).…”

Near‐inertial waves in Lake Erie

“…We also computed wind energy input rate E wind 5 s 3 W (Wu 1980;Bouffard et al 2012) and the net surface heat flux (short and long wave radiation and sensible and latent heat fluxes, positive into the lake; Fischer et al 1979). We calculated the bottom stress…”

“…where V i is the observed (low-pass filtered > 1 h) ADCP velocity, index i denotes the directional component (east-west ,2,3,4,5,6,7,8.5,9,9.5] 1228 TC 2008 May 01st-October 15th 10 [1,2,3,5,6,7,8,11.5,12,12.2,13,13.5] 1228 TC 2009 May 01st-October 15th 10 [1,2,3,4,6,7,8,9,10,10.5,11,11.5,12] ,2,3,4,5,6,7,8,9,10,11,12,12.5, 13,13.5,14,14.5,15.25] 341 TC 2008 July 31st-October 15th 10 [1,2,3.5,4,5,6,7,8,9,12.75,13.5,14,14.5,15, 15.25,15.5,15.9,16.5,16.75,17] ,2,3,4,5,6,7,8,10,11.5,12.5,13, 14,15,15.5,16,16.5] 1231 TC 2009 May 01st-October 15th 10 [1,2,3,4,6,7,8,9,12,13,14,15,16,17,18,18.5,19] i 5 EW or north-south i 5 NS), t is the time, z is the vertical coordinate (positive upwards), H is the water column height and h b 2 m is the mean height of the turbulent logarithmic bottom layer (Bedford and Abdelrhman 1987;Bouffard et al 2012;Valipour 2012). Baroclinic currents were calculated from (2):…”