Evidence of Multiple Physical Drivers on the Circulation and Thermal Regime in the Green Bay of Lake Michigan

Circulation and thermal regime in Green Bay have important consequences for water quality, contributing to hypoxic conditions during the summer. Currents and temperatures in the Great Lakes exhibit a wide range of time and spatial scales, in response to atmospheric forcing, Earth's rotation, and geometry. Because of the interaction of drivers, water exchange between Green Bay and Lake Michigan has effects on water quality that are at least as important as those of watershed discharges. We are using field methods, hydrodynamic modeling, and frequency domain analysis to investigate the different drivers. Field methods showed that currents simultaneously flow in and out of the bay at different depths, and changes in temperature can be explained by oscillating influx of cold lake water at lower depths. We calculated spectra of temperature measured in summer 2011 in southern Green Bay, and of water currents across the mouth between the bay and the lake, simulated during the same period. Both sets of spectra showed numerous significant oscillations at frequencies higher and lower than the inertial frequency of about (17.5 h)-1. The higher frequencies include several of first few surface modes of Green Bay and Lake Michigan. A low frequency was also observed, with a period of 8-10 days, which previous studies explained as the first internal mode of Green Bay. Hydrodynamic model simulations showed that water exchange is significantly higher for winds blowing from the west than it is for winds blowing from the east.