CLIMATE CHANGE IMPACTS : LARGE-LAKE PHYSICAL / HYDROLOGICAL RESPONSES

5.1 Large lake physical / hydrological concerns

Climate change studies of large lakes address lake physical responses as well as potential water quality impacts. In many cases these factors are interrelated. We discuss some of the water quality concerns which may result from climatic changes in Section 6. In this section, we concentrate on some of potential effects on key physical components which have been investigated on large lakes. It is well understood from detailed analyses under current climatic conditions, that water quality issues in large lakes show a strong dependence on the lake physical characteristics. For example, meteorological forcing influences the air-water exchange, thermal response, circulation, waves as well as ice cover amongst other important components. Many of these factors vary seasonally. A major unknown is the magnitude of possible changes in these factors, however, preliminary investigations conducted on the Laurentian Great Lakes over the last decade have provided some indication of the direction of possible changes in the large lake cycles. In general terms, GCM models predict milder winters leading to longer thermal stratified seasons, less ice cover, a decrease in wind speed and a decrease in water levels resulting from higher evaporation. Some of these changes have implications for water quality and aquatic ecosystems.

5.2 Energy and hydrological components

Heat exchange analysis of large lakes is complex and involves consideration of heat gains and losses based on hydrological exchanges, radiative and turbulent heat exchanges at the air-water interface as well as smaller heat fluxes through the lake bottom and others (Eqn. 1). Long-term heat budget analyses under current climate conditions have been conducted on all of the Laurentian Great Lakes [e.g., Lake Superior (Schertzer, 1978); Lake Huron (Bolsenga, 1975);

Lake Erie (Derecki, 1975; Schertzer, 1987); Lake Ontario (Atwater & Ball, 1974); composite fluxes and heat storage (Schertzer, 1997)]. An examination of the potential change in the heat balance of the Great Lakes resulting from climate warming has been conducted using both climate analog (Schertzer & Sawchuk, 1990) and GCM climate scenarios (Croley et al, 1996;

Schertzer & Croley 1999).

Schertzer & Sawchuk (1990) provided a preliminary analysis of the thermal structure of the lower Great Lakes for an anomalously warm year (1983) to infer the potential response for thermocline characteristics and anoxia occurrence in Lake Erie which involved the computation of the changed surface heat exchange. Heat flux simulations for 1983, showed that under anomalously warm conditions associated with strong El Nino, large reductions in surface heat losses in winter and above average surface heat flux gains in summer can occur. On an annual basis, the lower Great Lakes buffered large surface heat gains in summer months through losses in other months. Observations for Lake Erie indicated higher surface water temperatures, significant reductions in extent and duration in ice cover, and earlier disappearance of the 4°C isotherm, signaling an earlier start to thermal stratification. In response to greater surface heating and low wind conditions, the thermocline formed higher in the water column, and stratification lasted longer than in other years with higher annual heat storage.

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5.3 Temperature cycles

The seasonal response of the lake thermal regime to climate forcing can be examined using intensive field programs (e.g., IFYGL, 198 1; Schertzer et al., 1987). Comparison of long-term temperature time-series from moored thermistor chains compared to long-term climatology have demonstrated the response of large lake surface temperature to global climate phenomena such as El Nino (Schwab et al., 1999; Schertzer & Hamblin, 2000) and severe storms such as Hurricane Opal (Hamblin et al., 1996). A hierarchy of temperature prediction models have also been applied to large lakes which have application to climate impacts analysis. Thermal models have been developed for l-d (lake-wide/basin applications), 2-d (cross-lake applications) and fully 3-d thermal models. In general, l-d models have been the most widely used for analysis of many large lake issues. McCormick (1994) examined the predictive ability and potential applications of four basic l-d models to describe large lake thermal structure (1) turbulence closure models; (2) deterministic solutions; (3) eddy diffusion models; and (4) integrated mixed layer models. While l-d eddy diffusion models have some limitations in describing mixed-layer dynamics, they have been successfully applied to, for example, simulation of thermal structures for large-lake water quality issues (e.g., Lam & Schertzer, 1987). Another l-d model, the Dynamic Reservoir Simulation Model (Imberger et al., 1978) was shown to be appropriate for mixed-layer dymamics where rotational effects are not dominant.

Boyce et al. (1993) examined the heat load on Lake Ontario and its thermal response under current conditions for a hypothetical climate change scenario based on CCC11 GCM output. Simulation of the daily lake-wide averaged vertical temperature profile was accomplished by applying the l-d Dynamic Reservoir Simulation Model (DYRESM) (Imberger

& Patterson, 198 1). Figure 2 shows horizontally averaged isotherms for Lake Ontario computed using the DYRESM model under (a) current climate conditions, and (b) a simulation based on the CCC11 GCM. The climate change scenario dramatically indicates that the simulated lake no longer experiences spring and fall convective overturn (4°C water at the surface). The stratified period is two months longer and maximum surface water temperatures are 4°C higher.

Minimum summertime temperature over the water column was computed to be 6°C.

McCormick & Lam (1999) examined the thermal response of Lake Michigan by applying GISS, GFDL and OSU climate change scenarios to a mixed layer model (Garwood, 1977;

McCormick, 1990). These simulations showed increased surface water temperatures throughout the year with the GISS and GFDL simulations suggesting year-round permanent thermal stratification above 4°C. The simulations showed surface temperature increases up to 4°C with cooler bottom temperatures in winter. This results from reduced vertical mixing due to stronger stratification and increase in the duration of a stratified water column. Warmer conditions may also result in an increased heat storage. Due to shallower and stronger thermoclines, more energy is required to generate large scale vertical mixing. As with the Lake Ontario simulations (Boyce et al., 1993) these results have significant implications for aquatic ecosystems and water quality for the Great Lakes. The predicted increase in surface temperature also has significant implications for economic sectors (e.g., for hydro-electric power generation and others).

Croley et al. (1996) applied the GLERL hydrology model to all of the Great Lakes to provide a preliminary assessment of possible changes to the lake heat balances. These investigations indicated significant changes to the lake heat fluxes and the thermal characteristics. Similar to the investigations described above, it was found that warmer climates can result in reduced frequency of buoyancy-driven water column turnovers. In many of the GCM steady-state climate scenarios, lake surface water temperatures often do

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not fall to 3.98”C (the temperature of the maximum density of water) during the colder half- year. As a result, buoyancy-driven vertical turnovers of the water column change from a

frequency of two times per year to once per year. Under the warmer climates, dates for interannual turnovers show a significant shift later in the year when they occur. Since this is related to a fundamental physical property of fresh water, it is highly likely that this will occur in any future climate that is sufficiently warm. This could result in significant environmental impacts since these turnovers are important for nutrient distribution, oxygenation of lake water.

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5.4 Circulation patterns

Advances in hydrodynamical modeling of large lakes has been considerable with continued developments in 3-d circulation models (e.g., Simons 1974, 1975, 1976a,b, 1980, 1985;

Bennett, 1977; Schwab & Bedford, 1994). It is recognized that accurate simulation of lake circulation is critical for understanding the complexities of the lake physical regime, for forecasting, and for increasing understanding of such important processes as aquatic ecosystem health and the fate and transport of pollutants. In the Laurentian Great Lakes, advances have included the progressive development and application of operational hydrodynamic and water quality models (e.g., Schwab & Bedford, 1995; Murthy et al., 1998) which can be used for climate impacts assessments. Beletsky et al. (1999) provided a preliminary assessment of the potential impacts of climate warming on current speeds and circulation patterns in the Great Lakes. Since the amplitude of currents is proportional to wind speed and density gradients and inversely proportional to depth, it is suggested that an increase in the duration of the stratified period may increase the duration and amplitude of density driven currents. Decreasing ice cover would be expected to increase transfer of momentum from wind to currents. With decreases in water levels there may be an increase in current amplitudes but this effect may be balanced by the effect of decreased wind speeds.

Generalization of potential climate effects on large-scale circulation patterns of large lakes is not currently possible due to the complexity in each lake and limitations of current model frameworks.

An important contribution in the assessments of possible changes in lake circulation resulting from climate warming is the description of uncertainties and limitations of the circulation models (Beletsky et al., 1999). Issues that need to be addressed include the development of a comprehensive climatology of current patterns under current conditions for comparison to climate simulations including quantification of the interannual variability. It is also recognized that the present generation of numerical models need improvements to increase the accuracy of temperature and circulation predictions. Suggested improvements include increasing horizontal resolution in models, accuracy in forcing functions, increasing the accuracy in model physics and sensitivity to initial conditions. Grid refinement is expected to improve simulation of complex nearshore-offshore processes such as coastal upwelling fronts and jets, thermal bar, internal wave and mesoscale eddies. At present, lake ice is not incorporated into the annual cycle of numerical models. Development of coupled ice-circulation models is an important area for climate impacts assessments since the winter thermal structure is recognized as an important factor in the overall annual heat balance of the lake and for determining the annual lake circulation patterns.

5.5 Wind-wave generation

Wind-waves are one of the most noticeable of the hydrodynamical motions. An extensive review on wind-waves and modeling has been compiled by Komen et al. (1994). Forecasting and hindcasting of waves is recognized as an important process for such applications as navigation and aquatic safety. From an engineering point of view, prediction of waves is essential for design structures and measures to control sediment erosion. Modeling of wind- waves is complex as waves respond to meteorological forcing (Schwab et al., 1984).

Solution of the wave equations requires simplifications and typical assumptions (e.g., negligible coriolis force, neglect of surface tension, fluid is inviscid, fluid is incompressible, h-rotation of waves, waves are periodical and follow stationary random processes). Liu (1999) provides a synopsis of wind-wave modeling on the Laurentian Great Lakes

concluding that there have been significant advances in understanding and modeling waves in the past decade for practical applications although detailed understanding of physical processes is still elusive. With respect to possible impacts of climate change, it is apparent that long-term wave climatologies are critical. Such climatologies based on measurements are not readily available for large lakes, however, construction of such a climatology is possible by application of hindcast procedures using historical wind data. In combination with some available wave measurements, an alternate approach for establishing a wave climatology is offered by “Wave Climate Synthesis” analysis (Andrews et al., 1983) used to derive long-term wave statistics directly from long-term wind statistics. This methodology is based on the concept of relating long-term marginal probabilities of significant wave-height and wind speed by means of a parametric model of conditional probability. Briefly, the conditional probability can be approximated by a Gamma distribution of significant wave height, given wind speed. Liu (1999) indicates that the parameters in the Gamma distribution are given in terms of the wave height in each sorted wind speed interval. With the averages and standard deviations of the wind height formulated as empirical functions of wind speed, then, once the long-term wind speed distribution is given, a corresponding long- term wave height distribution can be determined. This approach has been tested using NDCB buoy data for the Great Lakes (198 l-1984) by Liu (1986) and shows promise as a methodology for deriving long-term wave climatology for each lake. Application of this methodology may be useful for testing a range of climate change scenarios to assess trends in the historical time-series and for evaluating potential climate change impact on the wave climate. For example, one potential impact is that if the GCM scenarios for a decrease in wind speed is realized, then wind-waves would be expected to be reduced accordingly.

5.6 Ice cover extent and duration

As indicated above, coupled ice-hydrodynamical models have not been applied to the Great Lakes and this is a critical element for future comprehensive analyses of the heat budget, temperature and circulation responses to altered climate conditions. A knowledge of ice extent, thickness and duration on large lakes such as the Laurentian Great Lakes is important since the winter ice regime has impacts on the economies of both Canada and USA, the aquatic system, and local weather and climate (Assel, 1991). It is recognized that ice cover is a hazard for commercial navigation, hydroelectricity generation, shore erosion and aquatic ecosystems (e.g., fisheries). Assel (1999) provides a summary of the current knowledge of the ice climatology of the Laurentian Great Lakes. Although observations of shore ice thickness have been reported since 1901, systematic observations of large-scale patterns were not started until late 1950s. Remote sensing of ice characteristics have also included side looking airborne radar and satellite observations. Ice databases on the Great Lakes have allowed the development of comprehensive ice climatologies (Assel et al., 1983). Ice climatology databases have been essential for description of and statistics on ice type characteristics, ice distributions (extent and thickness) including date ranges of ice formation and decay. The long-term data for the Great Lakes has been valuable for identifying climatic trends and impacts on ice. For example, time-series analysis has demonstrated changes in ice-loss dates associated with changes in temperatures. Analysis of Grand Traverse Bay, Lake Michigan data shows earlier ice-loss dates associated with warmer March temperature starting in the 1940s and 1950s (Hanson et al. 1992; Skinner, 1993). A similar analysis using Lake Mendota data suggest a second shift to earlier average ice loss dates associated with a warming of January through March temperatures starting in the 1980s. A recent ice cover model (Croley & Assel, 1984) indicates that the decadal average ice covers for the 1980s and

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for the 1950s was less extensive than the ice cover for the 20 year period of the ice cover climate analog (Schertzer & Sawchuk, 1990).

6 CLIMATE CHANGE IMPACTS: WATER QUALITY RESPONSES