The Impact of Surface Heat Flux and Wind on Thermal Stratification in Portage Lake, Michigan

Portage Lake is situated near the center of the Keweenaw Peninsula and is connected to Lake Superior via lengthy (> 7 km) navigation channels. Using moored thermistor records and meteorological data, we examine how changes in lake stratification are related to surface winds and heat flux. Frequent episodes of full water column mixing are observed throughout the summer. Convective mixing through surface cooling appears to be an important agent responsible for these events, as all occur during cold air outbreaks and when the net heat flux is directed out of the lake (negative). However, windinduced mixing is also implicated in contributing to some vertical mixing events, as evidenced by two events initiated during a period of strong winds and declining, but not yet negative, heat fluxes. Our analysis indicates that each time the water column restratifies, it tends to become more susceptible to convectively-driven overturn during cold air outbreaks. This tendency is quantified by the estimated time over which surface cooling due to a specified set of conditions, characteristic of a cold air outbreak, would reduce the temperature contrast between the upper and lower layers by half. This time declines by more than an order of magnitude for successive restratification events observed in the summer of 1999. Our analysis also reveals successive formation and dissolution of a diurnal surface mixed layer in an otherwise homogeneous water column during a 10-day period of August 1999. This is attributed to the combination of relatively light winds and negative daily net heat fluxes. INDEX WORDS: Lake stratification, air-lake interaction, lake heat fluxes.