Boundary layer instability beneath periodic internal solitary waves

We investigated the stability of the bottom boundary layer (BBL) beneath periodic internal solitary waves (ISWs) of depression over a flat bottom through two-dimensional direct numerical simulations. We explored the effects of variation in wave Reynolds number $Re_{ISW}$ and wave period on the nature of the instability, and energy production in the separated BBL. The instability characteristics and rate of vortex shedding of the BBL were strongly dependent on $Re_{ISW}$. The BBL was laminar and convectively unstable at $Re_{ISW}$ 90 and 300, respectively. At $Re_{ISW}=300$, the convective wave packet was periodically amplified by each successive ISW, until vortex-shedding occurred. This implies noise-amplification behavior and suggests that the discrepancies in the critical $Re_{ISW}$, for vortex shedding between lab and different numerical simulations, are due to differences in background seed noise. Instability energy decreased under the front shoulder of the ISW, analogous to flow relaminarization under a favourable pressure gradient. At larger $Re_{ISW}=900$, the BBL was initially convectively unstable, and then the instability tracked with the ISW, characteristic of global instability, regardless of the ISW periodicity. The simulated initial convective instability at both $Re_{ISW}$ 300 and 900 is in agreement with local linear stability analysis which predicts that the instability group speed is always lower than the ISW celerity. Increased free-stream perturbations and larger $Re_{ISW}$ shift the location of vortex shedding (and enhanced bed shear stress) closer to the ISW trough, thereby potentially changing the location of maximum sediment resuspension from the ISW, in agreement with field observations at higher $Re_{ISW}$

Factors affecting the development and dynamics of hypoxia in a large shallow stratified lake: Hourly to seasonal patterns

[1] The examination of hypoxia in the hypolimnion of large lakes traditionally focuses on the assessment of its spatial and temporal extent and its effect on water quality. In Lake Erie, hypoxia typically occurs between July and October in the central basin; however, there is considerable interannual variability both spatially and temporally. The processes driving this interannual variability as well as the small-scale time variation in oxygen depletion (e.g., −0.7 to +0.3 mg L−1 d−1) were examined in a field study conducted in the western part of the central basin of Lake Erie in 2008 and 2009. Data were obtained from a spatial array of moorings as well as sampling cruises that examined the physical and biological conditions needed to investigate the dynamics of the oxygen depletion and create a vertical oxygen budget. The flux of oxygen through the thermocline to the hypolimnion was a significant source of oxygen equivalent to ∼18% of the total oxygen depletion in the hypolimnion over the stratified period. The total oxygen depletion in the hypolimnion was due to equivalent amounts of hypolimnetic oxygen demand due to respiration in the water column and flux of oxygen to the bottom due to sediment oxygen demand. This latter finding was strongly dependent on hypolimnion thickness in Lake Erie, which also appeared to be an important parameter driving the rate of oxygen depletion by controlling the vertical volumetric fluxes and hence the competition between vertical flux and community respiration in the hypolimnion of shallow lakes

Impacts of hydrodynamics and benthic communities on phytoplankton distributions in a large, dreissenid-colonized lake (Lake Simcoe, Ontario, Canada)

We quantified the vertical and horizontal variation of chlorophyll a (Chl-a) to test how benthic filter feeders (dreissenid mussels), rooted macrophytes, and hydrodynamics influence phytoplankton distributions in a large lake (Lake Simcoe, Canada). Water column Chl-a did not differ significantly among sites of different depth, distance offshore, or rooted macrophyte biomass, but among the nearshore sites (7.5–10 m deep) it was higher where mussel biomass was greater. This counterintuitive result may be explained by wind-driven horizontal circulation during our specific study periods together with the patchy distribution of the mussels in the lake. Information on mixing depths, vertical eddy diffusivity, and mussel biomass was used to predict when and where the grazing pressure of mussels would likely deplete near-bottom phytoplankton. Chl-a depletion was frequently predicted at sites with moderate or high mussel biomass and sufficient thermal stratification to impede vertical mixing but never at sites without thermal stratification. Observations were consistent with predictions in most cases. The results suggested that mussels at depths of 7.5–15 m (a depth range of generally high mussel biomass in the lake) may frequently suffer food limitation due to near-bottom depletion during the early and middle stratified season. A deep Chl-a maximum was documented and may be important for mussel nutrition at such times

A framework for ensemble modelling of climate change impacts on lakes worldwide : the ISIMIP Lake Sector

Empirical evidence demonstrates that lakes and reservoirs are warming across the globe. Consequently, there is an increased need to project future changes in lake thermal structure and resulting changes in lake biogeochemistry in order to plan for the likely impacts. Previous studies of the impacts of climate change on lakes have often relied on a single model forced with limited scenario-driven projections of future climate for a relatively small number of lakes. As a result, our understanding of the effects of climate change on lakes is fragmentary, based on scattered studies using different data sources and modelling protocols, and mainly focused on individual lakes or lake regions. This has precluded identification of the main impacts of climate change on lakes at global and regional scales and has likely contributed to the lack of lake water quality considerations in policy-relevant documents, such as the Assessment Reports of the Intergovernmental Panel on Climate Change (IPCC). Here, we describe a simulation protocol developed by the Lake Sector of the Inter-Sectoral Impact Model Intercomparison Project (ISIMIP) for simulating climate change impacts on lakes using an ensemble of lake models and climate change scenarios for ISIMIP phases 2 and 3. The protocol prescribes lake simulations driven by climate forcing from gridded observations and different Earth system models under various representative greenhouse gas concentration pathways (RCPs), all consistently bias-corrected on a 0.5 degrees x 0.5 degrees global grid. In ISIMIP phase 2, 11 lake models were forced with these data to project the thermal structure of 62 well-studied lakes where data were available for calibration under historical conditions, and using uncalibrated models for 17 500 lakes defined for all global grid cells containing lakes. In ISIMIP phase 3, this approach was expanded to consider more lakes, more models, and more processes. The ISIMIP Lake Sector is the largest international effort to project future water temperature, thermal structure, and ice phenology of lakes at local and global scales and paves the way for future simulations of the impacts of climate change on water quality and biogeochemistry in lakes.Peer reviewe

Application of a two-dimensional hydrodynamic and water quality model to Lake Erie

grantor: University of TorontoA longitudinal/vertical hydrodynamic and water quality model, CE-QUAL-W2, is applied to Lake Erie for the summer of 1994. Observed data used for model forcing and calibration include: meteorological buoys, current meters, thermistor chains, water level gauges and numerous water quality samples. Without major adjustment, the model accurately simulates water levels. Modifications to the eddy coefficient turbulence scheme were required to model the longitudinal currents, which are reasonably simulated in the neutrally stratified western basin. In the eastern basin and central basin hypolimnion, baroclinic currents dominate over observed barotropic currents. The temperature field and distinct thermocline are simulated to within 3°C of observed. Dissolved oxygen profiles are well modelled. A higher algal settling velocity was required for calibration of algal concentrations to near-shore observations because the model does not account for filter feeding by zebra mussels. The next phase of the project will be the inclusion of a zebra mussel algorithm.M.A.Sc

A diapycnal diffusivity model for stratified environmental flows

The vertical diffusivity of density, K-rho, regulates ocean circulation, climate and coastal water quality. K-rho is difficult to measure and model in these stratified turbulent flows, resulting in the need for the development of K-rho parameterizations from more readily measurable flow quantities. Typically, K-rho is parameterized from turbulent temperature fluctuations using the Osborn-Cox model or from the buoyancy frequency, N, kinematic viscosity, nu, and the rate of dissipation of turbulent kinetic energy, epsilon, using the Osborn model. More recently, Shih et al. (2005, J. Fluid Mech. 525: 193-214) proposed a laboratory scale parameterization for K-rho, at Prandtl number (ratio of the viscosity over the molecular diffusivity) Pr = 0.7, in terms of the turbulence intensity parameter, Re-b = epsilon/(nu N-2), which is the ratio between the destabilizing effect of turbulence to the stabilizing effects of stratification and viscosity. In the present study, we extend the SKIP parameterization, against extensive sets of published data, over 0.7 < Pr < 700 and validate it at field scale. Our results show that the SKIF model must be modified to include a new Buoyancy-controlled mixing regime, between the Molecular and Transitional regimes, where K-rho is captured using the molecular diffusivity and Osborn model, respectively. The Buoyancy-controlled regime occurs over 10(2/3)Pr(-1/2) < Re-b < (3 ln root Pr)(2), where K-rho = 0.1/Pr-1/4 nu Re-b(3/2) is Pr dependent. This range is shown to be characteristic to lakes and oceans and both the Osborn and Osborn-Cox models systematically underestimate K-rho in this regime. (C) 2013 Elsevier B.V. All rights reserved

Multi-Year Simulation of Western Lake Erie Hydrodynamics and Biogeochemistry to Evaluate Nutrient Management Scenarios

During the 1970s, harmful cyanobacteria (HFCB) were common occurrences in western Lake Erie. Remediation strategies reduced total P loads and bloom frequency; however, HFCB have reoccurred since the mid-1990s under increased system stress from climate change. Given these concurrent changes in nutrient loading and climate forcing, there is a need to develop management tools to investigate historical changes in the lake and predict future water quality. Herein, we applied coupled one-dimensional hydrodynamic and biogeochemical models (GLM–AED) to reproduce water quality conditions of western Lake Erie from 1979 through 2015, thereby removing the obstacle of setting and scaling initial conditions in management scenarios. The physical forcing was derived from surface buoys, airports, and land-based stations. Nutrient loads were reconstructed from historical monitoring data. The root-mean-square errors between simulations and observations for water levels (0.36 m), surface water temperature (2.5 °C), and concentrations of total P (0.01 mg L−1), PO4 (0.01 mg L−1), NH4 (0.03 mg L−1), NO3 (0.68 mg L−1), total chlorophyll a (18.74 μg L−1), chlorophytes (3.94 μg L−1), cyanobacteria (12.44 μg L−1), diatoms (3.17 μg L−1), and cryptophytes (3.18 μg L−1) were minimized using model-independent parameter estimation, and were within literature ranges from single year three-dimensional simulations. A sensitivity analysis shows that 40% reductions of total P and dissolved reactive P loads would have been necessary to bring blooms under the mild threshold (9600 MTA cyanobacteria biomass) during recent years (2005–2015), consistent with the Annex 4 recommendation. However, these would not likely be achieved by applying best management practices in the Maumee River watershed