Modeling summer circulation and thermal structure of Lake Erie

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/102215/1/jgrc20419.pd

Lake Erie hypoxia prompts Canada‐U.S. study

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95631/1/eost15589.pd

Summer thermal structure and anticyclonic circulation of Lake Erie

Peer Reviewedhttp://deepblue.lib.umich.edu/bitstream/2027.42/95057/1/grl29007.pd

Poincaré wave-induced mixing in a large lake

A 10,000-km2 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-mdensity 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. © 2012, by the Association for the Sciences of Limnology and Oceanography, Inc

Sediment resuspension mechanisms and their contributions to high-turbidity events in a large lake

High-resolution field data, collected during April to October of 2008–2009, were analyzed to investigate the quantitative contribution of sediment resuspension to high-turbidity events in central Lake Erie. Resuspension events were distinguished within high-turbidity events according to turbidity, fluorescence and acoustic backscatter timeseries, as well as satellite images. We observed 16 high-turbidity events, causing a total duration of ∼20 d (out of 344 d) with elevated nearbed turbidity (> 10 NTU). Of these events, 64% were correlated with algal biomass, with the remaining 18%, 5%, and 4% being attributed to sediment resuspension by surface waves, storm-generated currents and enhanced nearbed turbulence induced by high-frequency internal waves, respectively. This is the first time that resuspension by enhanced nearbed turbulence from high-frequency linear internal wave degeneration has been observed in a large lake. Resuspension was parameterized as a function of the instantaneous critical bottom velocity, bottom shear stress and the Shields parameter. From the in situ measurements, we suggest an extended Shields diagram for silty bed material that can be used to predict resuspension in other aquatic systems with similar sediment composition (∼20% cohesive sediment)

Near-inertial waves in Lake Erie

Near-inertial (Poincare) waves with a period T-p similar to 17 h are the dominant wind-induced internal wave motions in central Lake Erie and consequently have a substantial influence on lake circulation, mixing and biogeochemistry. However, due to the complex three-basin bathymetry in Lake Erie, the vertical and horizontal modal structure of these waves remain poorly understood. In this study, we analyze field data to show wind events energize frequent vertical mode-one Poincare waves. The horizontal modal structure was also investigated, in a sensitivity analysis, using a calibrated three-dimensional hydrodynamic transport model forced with observed and idealized spatially uniform wind events. Strong horizontal mode-one Poincare wave cells form in both the Central and Eastern Basins when wind events have a duration of 0.25 T-p to 0.5 T-p, are impulsive and periodic at T-p, or have anticyclonic rotation with a duration of T-p. Momentum transfer from longer wind events (> 0.5 Tp) will oppose the Coriolis-force rotated currents and damp Poincare wave generation. In agreement with theory, the most efficient wind events are observed and computationally modeled to have a duration of 0.25 T-p; causing an excitation peak at similar to 0.4 T-p and converting similar to 0.8% of the wind energy input to Poincare waves. The efficiency of wind work in generating Poincare wave kinetic energy is given by (1-cos (2 pi f t)) t(-1), where f is the inertial frequency and t is the wind duration. Therefore, the efficiency peaks during each nT(p) period, where n is a non-negative integer, and decreases significantly for longer wind events

Record-Breaking Lake Erie Hypoxia during 2012 Drought

Hypoxia has been observed in the central basin of Lake Erie for decades. To understand the impact of various controlling factors, we analyze a record of hypoxic extents for Lake Erie for 1985–2012 and develop a parsimonious model of their interannual variability. We find that the 2012 North American drought and accompanying low tributary discharge was associated with a record-breaking hypoxic event in Lake Erie, whereas a record-setting harmful algal bloom in 2011 was likely associated with only mild hypoxia. River discharge and the timing of nutrient input therefore impact western basin bloom growth and central basin oxygen demand in distinct ways that merit further investigation. Overall, April to June tributary discharge, May to July soluble reactive phosphorus loading, July wind stress, and June northwesterly wind duration explain 82% of the interannual variability of hypoxia, and discharge alone explains 39%, indicating that meteorological factors need to be considered in the development of nutrient management strategies, especially as both extreme precipitation events and droughts become more frequent under a changing climate