Water Science and Technology Directorate, Environment Canada, Canada Centre for Inland Waters, 867 Lakeshore Rd., Burlington, Ontario L7R 4A6

Summary

        This paper provides an overview of historical and current knowledge regarding important characteristics of the surface meteorology, hydrology, thermal structure, energy fluxes and heat storage, water movements and hydrodynamic modelling in Lake Superior. Relevant descriptions of measurements and model simulations are also presented. The following provides a brief synopsis of the state of the research, and the effect of improvements in measurements and techniques on the current knowledge-base on selected physical components of the lake.

Advances and Required Improvements in Measurements
        Historical analyses of the environmental climatology were largely based on shoreline meteorological observations. Current investigations have the benefit of in situ lake observations from NOAA and Canadian buoys. Combined observations from adjusted shore-based observations, limited lake buoys and use of satellite observations (e.g. AVHRR and QuikSCAT) have improved interpretations and analyses of dynamical processes. There are surveillance programs operational on the lake with meteorological buoys in ice-free periods, however, the spatial resolution of data is limiting for developing climatological time-series databases with high spatial resolution. Model simulations of nearshore dynamics such as upwelling and downwelling events and currents near the Keweenaw Peninsula have shown improved accuracy. Further improvements may be achieved through improvement in the coastal wind field possibly through coastal radar measurement systems or with mesoscale meteorological models. Modern observations and forecast systems for the Great Lakes over the past decades have allowed for greater advanced notice of storms with potentially extreme winds and waves which has increased the safety of ship transport on Lake Superior. Similarly, advancements in procedures to derive daily representations of water surface temperature using satellite as well as ice cover information as shown using GLSEA, will provide increased spatial information applicable to lakes research than traditional methods alone.

Advances in Hydrological Components and Evaporation Technique
        An understanding of the magnitude and variation of lake hydrological components and response to climate is essential for lake management and for realistic regulation of the lake water levels and outflow. Early research (e.g. Bennett, 1978) established the baseline characteristics for water budget components. Over-lake evaporation on a lake the immense size of Lake Superior is a challenge to measure. Initial estimates of lake evaporation using constant transfer coefficients similar to the Lake Hefner studies resulted in very high levels of condensation (negative evaporation) during the months with very stable atmospheric conditions. Energy budget, water budget, and improved mass transfer formulations using variable transfer coefficients and recently including corrections for static stability and wind shear have shown a convergence of monthly mean evaporation values. Application of advanced techniques such as fast response eddy covariance methods will provide improved estimates of the lakewide evaporation as well as an understanding of the dynamic responses of the evaporative process to atmospheric forcing.

Improved Knowledge of Surface Heat Exchange and Heat Content
        Knowledge of the surface heat exchange is an important boundary condition input to thermal and hydrodynamic models. Schertzer (1978) provided baseline values of the radiative and turbulent exchange components largely based on shoreline meteorological data. Bennett (1978) used water temperature profiles based on long-term lake surveys to approximate the lake heat storage for Lake Superior. Intensive observations conducted in 2005 from shoreline meteorological sites, lake meteorological buoys and in situ lake temperature profiles, offer a possibility to re-evaluate the magnitude and seasonal variation of both the heat flux components and lake heat storage.

Requirement for Improved Long-term Current Observation
        As indicated previously, advancements in simulation of the lakewide circulation and dynamics of the water movements in the nearshore, requires improved resolution of boundary conditions such as the wind field. Current investigations have focused on generalization of the summer, winter and annual circulation based on historical data (e.g. Beletsky, 1999a: 1999b). Observations have shown that the summer circulation is mostly cyclonic and the Keweenaw current is a persistent feature of summer circulation in Lake Superior. In general, there is insufficient observational data to discern the large-scale winter circulation in Lake Superior.

Advances in Keweenaw Circulation Modelling and Role of Eddies
        Recent research has seen the application of hydrodynamic models in the lakewide case and nearshore to simulate currents in the complex zone of the Keweenaw Peninsula. Mid-lake eddy fields have been observed in Lake Superior throughout the water column and throughout the lake during all seasons. Findings of Ralph (2002) raise several questions regarding the mechanisms of formation, propagation and dissipation as well as the complex 3-D structure. Eddies are postulated to have implications on how nutrients, sediments and contaminants are transported within large lakes and as well as the patchiness of phytoplankton and zooplankton.

Important Role for Remote Sensing
        AVHRR imagery, supplemented by higher resolution SPOT-HRV, LANDSAT TM, or RADARSAT scenes, can provide high spatial coverage and real-time detail necessary to monitor dynamic changes associated with many meso-scale lake phenomena (Budd et al., 1999). Application of AVHRR and SeaWiFS instrumentation may provide further remotely sensed data to investigate dynamic temperature and circulation processes.

Requirement for Research on Climate Change and Development of Improved Scenarios
        Modelling techniques for the simulation of climate change effects on lake hydrodynamics are available, however, data input to such models do not always exist (Lam and Schertzer, 1999). In the case of Lake Superior, Austin and Coleman (2007) indicated that while there have been theoretical and modelling studies on the lake, there is a paucity of direct observations from which to understand the actual responses. Annually averaged air temperatures from 1979 to 2005 have shown a positive trend, influencing a decreasing trend in ice extent in Lake Superior. Wind speeds from over-lake meteorological buoys show a tendency to higher average winds. These changes have an effect on the lake thermal response. July to September water temperatures have increased ~ 2.5 oC over the period 1979 – 2006 which is significantly in excess of regional atmospheric warming and has had the effect of lengthened the ice-free season resulting in earlier temperature stratification. Changes of this magnitude may have implications on other physical responses of the lake including aquatic ecosystem components.
Climate scenarios have been applied to Lake Superior largely focused on hydrological responses and ice extent. In general, these simulations have shown a potential toward reduced runoff, higher lake evaporation and a decrease in lake water level and decrease in ice extent. The magnitude of change can be quite different using scenarios from different GCM models. Simulation of potential climate impacts on water movements in Lake Superior requires improved climate scenarios and advances which may include lake-atmospheric coupling. Our understanding of physical limnological processes has increased greatly over the last three decades. Lake-wide models have been developed and the influence of meteorological and hydrological events is much more defined. However, further development in models at higher resolution in the nearshore will require an increase in spatial distribution of meteorological observations. Furthermore, these models have to be validated with hydro and thermodynamic variables in the lake. Analysis of trends in lake temperatures, water levels and ice cover strongly suggest that we are already experiencing warming in Lake Superior. Although future climate scenarios are uncertain, potential effects of climate change needs to be assessed using the deterministic lake models.