Circulation and exchange in the Saginaw Bay - Lake Huron system: Observations and numerical modeling

Knowledge of lake circulation is essential for addressing many issues ranging from water quality to human and ecosystem health. Lake Huron, the third largest of the Great Lakes by volume, has been significantly affected by natural and anthropogenic activities. Since Lake Huron is a connecting waterway between the upper and lower Great Lakes, understanding Lake Huron circulation and thermal structure is also important for questions involving the lower lakes. In this study, we use a three-dimensional, unstructured grid hydrodynamic model to examine circulation, thermal structure, ice cover extent, and exchange in the Saginaw Bay - Lake Huron system during summer months for 3 consecutive years (2009-2011) and winter months for 2 years (2010 and 2013). The model was tested against ADCP observations of currents, data from a Lagrangian drifter experiment in the Saginaw Bay, observations of temperature from thermistor chains, and temperature data from the National Data Buoy Center stations. Mean circulation was predominantly cyclonic in the main basin of Lake Huron with current speeds in the surface layer being highest in August in summer and in January in winter. Circulation in the Saginaw Bay was characterized by the presence of an anti-cyclonic gyre at the mouth of the outer bay and two recirculating cells within the inner bay for both seasons. The ice cover data extracted from Moderate Resolution Imaging Spectroradiometer (MODIS) with relatively high spatial resolution and from the Great Lakes Ice Atlas were used to test against results obtained from an ice model. The results show that the ice model was able to simulate lake circulation and ice cover extent in winter season reasonably well. The percent coverage of ice reached a maximum of 38.3% and 38.7% in mid-February in 2010 and beginning of March in 2013 respectively. New estimates are provided for the mean flushing times (computed as the volume of the bay divided by the rate of inflow) and residence times (computed as e-folding flushing times treating the bay as a continuously stirred tank reactor) for Saginaw Bay for summer and winter seasons. The average flushing time (over the three months of summer and for all three years) was 23.0 days for the inner bay and 9.9 days for the entire bay. The corresponding values for the winter season are 43.2 days and 15.6 days respectively. The mean e-folding flushing time was 62 days for summer and 64.7 days for winter for the inner bay and 115 days for the summer and 114.2 days for the winter conditions for the entire bay. Empirical relations between the mean residence time and river discharge were proposed. To characterize the behavior of river plumes in the inner Saginaw Bay, the absolute diffusivity values in the along-shore and cross-shore directions were calculated using data from GPS-enabled Lagrangian drifters and simulation results based on particle transport models.