J. Great Lakes Res. 33:143–155 Internat. Assoc. Great Lakes Res., 2007 The Impact of Surface Heat Flux and Wind on Thermal Stratification in Portage Lake, Michigan James H. Churchill 1,* and W. Charles Kerfoot 2 1 Department of Physical Oceanography Woods Hole Oceanographic Institution Woods Hole, Massachusetts 02543 2 Department of Biological Sciences Michigan Technological University Houghton, Michigan 49931 ABSTRACT. 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 meteoro- logical 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 dur- ing cold air outbreaks and when the net heat flux is directed out of the lake (negative). However, wind- induced 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 oth- erwise homogeneous water column during a 10-day period of August 1999. This is attributed to the com- bination of relatively light winds and negative daily net heat fluxes. INDEX WORDS: Lake stratification, air-lake interaction, lake heat fluxes. INTRODUCTION Although the Laurentian Great Lakes are often viewed as series of self-contained bodies, they are integral parts of larger ecosystems. These include the numerous embayments and smaller lakes that line the perimeters of the Great Lakes. These smaller bodies of water connected to the Great Lakes can serve as nursery areas for various fish species (Boehlert and Mundy 1988) and are areas of seasonal recruitment for coastal zooplankton that hatch from resting stages (Kerfoot et al . 1999, 2004; Kerfoot and Weider 2004). The biological communities residing within em- bayments, including those communities with links to the adjoining great lake, are undoubtedly influ- * Corresponding author. E-mail: jchurchill@whoi.edu 143 enced by physical processes occurring within the embayments. Processes likely to significantly im- pact resident biota include the evolution of stratifi- cation and the generation of bottom stresses. Understanding such processes is critical to forming an understanding, and a capability of properly mod- eling, the overall lake-embayment ecosystem. The focus of this paper is Portage Lake, Michi- gan, a water body with a mean depth of approxi- mately 9 m. This is part of the Keweenaw Waterway, a navigation passage with openings to Lake Superior that cuts across Michigan’s Ke- weenaw Peninsula (Fig. 1). In a previous paper (Churchill et al. 2004), we examined the wind-dri- ven exchange between the Keweenaw Waterway and Lake Superior. Our concentration here is on the evolution of thermal stratification within Portage