Wind-Driven Dynamics and Numerical Modelling of Temperature Structure in the Nearshore Region of Lake Ontario

Abstract

Physical observations in Lake Ontario during 2010 were analyzed and numerical models were applied to study the seasonal temperature structure in the lake and assess the turbulence closure schemes that are used within numerical ocean models to parameterize mixing and dissipation at sub grid scales. These schemes require calibration and often have difficulty simulating and maintaining the sharp vertical thermal gradients. In Chapter 2, wind, lake temperature and current data was analyzed to characterize the effect of the imposed wind stress on the momentum balance of the lake, motion of the thermocline, and bed processes such as resuspension and dissipation. The effect of the wind was clearly observed in the residual pressure gradient from the momentum balance. Large magnitude positive pressure gradients preceded downwelling of the thermocline while weak positive and negative pressure gradients preceded upwelling. While it cannot be stated definitively that internal Kelvin waves did not contribute to large perturbations of the thermocline, the observations indicated that the motion of the thermocline-shelf intersection was primarily driven by the pressure gradient initiated by the local wind forcing. The stability of the thermocline was affected by the direction of motion of the thermocline shelf intersection, producing temperature overturns during upwelling events and while the temperature transitioned from upwelling to downwelling behaviour. In Chapter 3, The Finite-Volume Community Ocean Model (FVCOM) using the General Ocean Turbulence Model (GOTM) for vertical mixing was tested and calibrated extensively with the goal of reproducing the thin seasonal thermocline present in observations. Total temperature RMSE was reduced following adjustment of internal coefficients and the inclusion of new stability functions but the thermocline remained significantly more diffuse than observed, even while underestimating the diffusivity of heat by several orders of magnitude. In Chapter 4, GOTM was tested as a standalone model to investigate its performance outside of FVCOM. GOTM produced realistic values for the turbulent macro lengthscale, dissipation of turbulent kinetic energy, and diffusivity of heat indicating that the vertical mixing parameterization by GOTM is not the root cause of overly diffuse temperature reproductions in FVCOM.