Circulation in the Great Lakes
With horizontal scales of hundreds of kilometres, depth scales of 100 m (except Lake Erie) and well-developed seasonal thermal stratification, the major Great Lakes have many of the physical phenomena associated with the coastal oceans and inland seas. The major physical difference is the closed boundary, the shoreline of the Great Lakes. The earth's rotation (Coriolis force) and basin topography strongly affect large-scale circulation. [source for current information: "Thermal Structure and Circulation in the Great Lakes", F. M. Boyce et al, Atmosphere-Oceans, 27 (4) 1989, 607-642].
The major difference between oceans and the Great Lakes is a consequence of fresh water having a maximum density at 4°C, significantly above the freezing temperature of 0°C. Overturning of the complete water column thus occurs in the fall when the surface waters cool to 4°C and again in the spring when the surface water warms from freezing through the 4°C range. A weak, stable stratification of the water column forms in the winter with water cooler than 4°C (lower density) at the surface. In the early phase of warming in the spring, a band of water next to the shore is heated above 4°C while the central part of the lake remains at 4°C and a thermal bar is formed due to the density differences. The thermal bar may persist through June in Lakes Ontario and Huron-Michigan, and even longer on Lake Superior, with surface water cooler than 4°C remaining over the deepest portions of the lakes. Eventually the entire lake surface warms and becomes thermally stratified. The stability of a layer of warm water floating on cool water restricts vertical circulation and affects large-scale horizontal circulation.
During the winter isothermal period the lake circulations are driven by the wind. Because the Great Lakes generally have smaller horizontal dimensions than the weather systems passing over them, the wind stress is essentially uniform across the basin. Close to shore, wind drag is experienced all the way to the bottom; this water is accelerated in the direction of the along-shore component of the wind. Since the lakes are closed basins there must be a return flow. The balancing return flow occurs in the middle of the basin, the circulation thus taking the form of a double gyre. Unlike the other major basins, the near-uniform depth of Lake Erie's central basin makes its circulation sensitive to the torque (curl) of the wind stress. The wind-forced circulation of the central basin may take the two gyre form or it may be a single basin-wide gyre in either direction, depending on the torque of the wind stress.
In the spring, as the water shoreward of the thermal bar increases in temperature, the onshore/offshore pressure gradients created by the density difference tend to push the warm water offshore. The effect of the earth's rotation (Coriolis force) is to deflect this offshore flow and set up a quasi-steady circulation with the warm water moving counter-clockwise (Northern Hemisphere) and following the bottom contours. Because of the stability of the air column above the lake (cool water and warm air), wind stresses are reduced and this thermally-driven horizontal circulation may persist for over a month.
During the summer stratified period, wind blowing over a lake will initially cause the warm surface layer to slide downwind over an undisturbed thermocline (lower layer). At the downwind shore, the warm water will force the thermocline down, and where the warm water moves offshore the thermocline must rise. Generally, the strongest currents occur between 1 and 10 km from shore and are associated with shore-parallel currents that move initially in the direction of the component of the wind parallel to shore. Then, over a time-scale measured in days, they reverse direction before dying out. Offshore, beyond 10 km, the currents are more variable and show a tendency in summer to rotate clockwise. Very close to shore, within the surf zone, along-shore currents are generated by the breaking surface waves.
The paragraphs above attempt to explain the general horizontal circulation in the Great Lakes. The inflow and outflow of the larger rivers, such as the Niagara River, will have some local effect on lake circulation. There may also be a hydraulic component of flow in shallow bays and narrows, caused by the difference in water level at the two ends of a channel. For example, currents of 2 to 3 knots have been observed at Little Current in the North Channel of Lake Huron.
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