Impact of Water Mixing and Ice Formation on the Warming of Lake Superior: A Model‐guided Mechanism Study

Abstract

The Laurentian Great Lakes are one of the most prominent hotspots for the study of climate change induced lake warming. Warming trends in large, deep lakes, which are often inferred by the observations of lake surface temperature (LST) in most studies, are strongly linked to the total lake heat content. In this study, we use a 3D hydrodynamic model to examine the nonlinear processes of water mixing and ice formation that cause changes in lake heat content and further variation of LST. With a focus on mechanism study, a series of process-oriented experiments is carried out to understand the interactions among these processes and their relative importance to the lake heat budget. Using this hydrodynamic model, we estimate the lake heat content by integrating over the entire 3D volume. Our analysis reveals that (1) Heat content trends do not necessarily follow (can even be opposed to) trends in LST. Hence, using LST as a warming indicator can be problematic; (2) vertical mixing in water column may play a more important role in regulating lake warming than traditionally expected. Changes in the water mixing pattern can have a prolonged effect on the thermal structure; (3) Ice albedo feedback, even in cold winters, has little impact on lake thermal structure, and its influence on lake warming may have been overestimated. Our results indicate that climate change will not only affect the air-lake energy exchange but can also alter lake internal dynamics, therefore, the lake’s response to a changing climate may vary with time. Climate change induced lake warming in both tropical and high latitude lakes has been investigated recently by numerous studies (Burnett et al. 2003; Livingstone 2003; O’Reilly et al. 2003; Verburg et al. 2003; Coats et al. 2006; Vollmer et al. 2008; Adrian 2009; Arvola et al. 2009; Schneider et al. 2009; Schneider and Hook 2010; Fink et al. 2014; Gronewold et al., 2015). For example, O’Reilly et al. (2015) found that warming lakes are geographically distributed by using worldwide synthesis of in situ and satellite-derived lake data, revealing that the warming rate of seasonally ice-covered lakes is 0.72 C per decade while ice-free lakes are experiencing 0.53 C per decade from 1985 to 2009. It has been observed that east-central North America, including the Laurentian Great Lakes, is one of the most prominent hotspots where lake surfaces have warmed more rapidly than the ambient air during the summer (Austin and Colman 2007; Lenters et al. 2013). A warming trend of lake surface temperature (LST) in the Laurentian Great Lakes in the last century was reported by McCormick and Fahnenstiel (1999). Austin and Colman (2008) estimated an average summer (July, August, and September) LST warming rate of 0.278 C per decade over the last century, with a dramatic increase up to 0.118 C per year after the 1980s using the offshore buoy data. In addition, similar results from 1986 to 2002 were reported by Dobiesz and Lester (2009), estimating that LST of Lakes Huron and Ontario during August have been rising at annual rates of 0.084 C and 0.048 C, respectively, while the trend for Lake Erie, the shallowest of the Great Lakes, was smaller and insignificant. While it is suggested that the mechanism for summer warming in the shallow lakes, such as Lake Erie, is due to a rapid response of the lakes to synchronous increases in solar radiation and air temperature during the summer (Adrian et al. 1999; Gerten and Adrian 2000; Livingstone 2003; Straile et al. 2003; Piccolroaz et al. 2014), the mechanism responsible for large, deep lakes (Gorham 1964; Robertson and Ragotzkie 1990; Mazumder and Taylor 1994) such as Lake Superior is different. Due to the much larger heat capacity and thermal inertia, the deeper lakes integrate the effects of increasing air temperature over longer periods of time (Verburg et al. 2003; Arvola 2009; Piccolroaz et al. 2015; Zhong et al. 2016). For *Correspondence: [email protected] This is an open access article under the terms of the Creative Commons Attribution-NonCommercial-NoDerivs License, which permits use and distribution in any medium, provided the original work is properly cited, the use is non-commercial and no modifications or adaptations are made.