Sensitivity of summer Lake Superior thermal structure to meteorological forcing

We use a one-dimensional model, forced with realistic meteorological measurements, to determine to first order the sensitivity of summer surface water temperature, heat content, and vertical stratification scale to three forcing variables: air temperature, wind speed, and previous winter ice cover, all three of which have exhibited long-term trends over the last few decades. Summer-averaged surface temperature increases with increased air temperature, decreased ice cover, and decreased wind speed. Differences in heat content between model runs with differing initial temperatures (a proxy for winter ice cover) decrease over the course of the season, but significant differences present in late spring persist through the summer season. Interannual variability in wind speed is the predominant driver of variability in the vertical stratification scale. The thermal interaction between a lake and the atmosphere is complex, since several distinct processes contribute to the heat flux and the resulting change in water temperature. In turn, water temperature is perhaps the dominant environmental variable determining the structure and makeup of a lake’s ecosystem (Wetzel 2001). There is a growing consensus that the ‘‘stationarity assumption’’ that once pervaded limnology has given way to a new paradigm (Livingstone 2008), with an acknowledgement that the climate that determines lake thermal structure is changing. A wide array of recent work has documented long-term trends in lake temperature in both tropical lakes (O’Reilly et al. 2003; Verburg et al. 2003; Vollmer et al. 2005) and temperate lakes (McCormick and Fahnenstiel 1999; Coats et al. 2006; Schneider et al. 2009). Other work has shown that long-term climatic trends may also be responsible for observed trends toward lower ice cover (Palecki and Barry 1986; Magnuson et al. 2000; Assel et al. 2003). Recent work (Austin and Colman 2007) has shown that winter ice cover appears to play a primary role in determining summer surface water temperatures in Lake Superior, to the extent that most of the interannual variability in summer temperatures appears to be due to variability in previous winter ice cover, rather than in current summer atmospheric conditions. However, their treatment of the problem was largely statistical, consisting of an analysis of historical records, rather than a dynamic approach that established causality. The flux of heat into (or out of) a lake, and therefore its thermal structure, is predominantly set by incoming shortwave radiation and conditions in the adjacent atmosphere. In equatorial lakes, where the intensity of shortwave radiation and atmospheric conditions both display relatively weak annual signals, lakes stay close to being in thermal equilibrium with the adjacent atmosphere (Verburg et al. 2003; Vollmer et al. 2005). In this case, the individual heat flux terms may be large, but the net heat flux, and hence the variability in thermal structure, is relatively small. In addition, surface temperature in deep equatorial lakes appears to respond roughly proportionally to changes in regional air temperature, providing a useful measure of long-term climate change in those regions. In contrast, lakes in middle-latitude regions are constantly in a state of adjustment toward thermal equilibrium with the adjacent atmosphere (Edinger et al. 1968). Annual variability in the heat content is set by shortwave radiation, the magnitude of which is independent of water temperature, and other terms of the heat balance, specifically net longwave radiation, sensible and latent heat fluxes, which we will refer to as equilibrative heat fluxes, since all depend on the surface water temperature and serve at some level to drive surface water temperatures toward an equilibrium with the atmosphere (Edinger et al. 1968; Dingman 1972). The relative time scales at which the surface waters of a lake can react to the equilibrating heat fluxes and the strength of the annual cycle in shortwave forcing determine how close to equilibrium the lake surface temperatures will be. If the adjustment scale is long, the lake may constantly be adjusting to atmospheric conditions, never truly reaching equilibrium. The ability of a lake to approach this equilibrium can also be a function of lake depth, with shallow lakes approaching equilibrium readily and deeper lakes constantly adjusting toward equilibrium. Surface air temperatures around the globe have increased significantly over the last several decades, in part due to anthropogenic forcing (Hansen et al. 2006). This increase is thought to be stronger in midcontinent regions as well as middleto high-latitude regions of the globe. Increases in air temperature in the Lake Superior region have been on the order of 1.2 K over the last 25 yr (Austin and Colman 2007). Since several of the mechanisms responsible for heat transfer between the atmosphere and a lake are strongly dependent on air temperature (primarily downward longwave radiation and sensible heat flux, and indirectly, latent heat flux), it is well documented that changing air temperatures are going to play a significant role in determining lake thermal structure (Robertson and Ragotzkie 1990; Hondzo and Stefan 1993; Stefan et al. 1998). The exact form of the relationship is nontrivial; a 1 K increase in air temperature does not result in a 1 K increase in the equilibrium lake surface temperature (Robertson and Ragotzkie 1990; Peeters et al. 2002). Air * Corresponding author: [email protected] Limnol. Oceanogr., 56(3), 2011, 1141–1154 E 2011, by the American Society of Limnology and Oceanography, Inc. doi:10.4319/lo.2011.56.3.1141