Water circulation in the Great Lakes

Learn how the Great Lakes water circulation compares to oceans and seas, and how they’re affected by winter, spring, summer and other variables.

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Great Lakes compared to oceans and seas

The major Great Lakes have many of the physical phenomena associated with the coastal oceans and inland seas, including:

  • depth scales of 100 m (except Lake Erie)
  • horizontal scales of hundreds of kilometres
  • well-developed seasonal thermal stratification

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.

Freshwater maximum density

The major difference between oceans and the Great Lakes is a consequence of fresh water having a maximum density at 4°C. This is significantly above the freezing temperature of 0°C.

Overturning of the complete water column occurs in the:

  • fall, when the surface waters cool to 4°C
  • 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:

  • the central part of the lake remains at 4°C
  • a band of water next to the shore is heated above 4°C
  • 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. Surface water cooler than 4°C remains 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 will:

  • restrict vertical circulation
  • affect large-scale horizontal circulation

Winter isothermal period

During the winter isothermal period, the lake circulations are driven by the wind.

The wind stress is essentially uniform across the basin. This is because the Great Lakes generally have smaller horizontal dimensions than the weather systems passing over them.

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 takes 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.

Depending on the torque of the wind stress, the wind-forced circulation of the central basin may:

  • take the 2 gyre form
  • be a single basin-wide gyre in either direction

Spring warming

In the spring, 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 earth's rotation (Coriolis force) deflects the offshore flow and sets up a quasi-steady circulation. Warm water moves counter-clockwise (in the northern hemisphere) and follows the bottom contours.

Wind stresses are reduced because of the stability of the air column above the lake (cool water and warm air). This thermally-driven horizontal circulation may last for over a month.

Summer stratified period

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. Where the warm water moves offshore, the thermocline must rise.

Generally, the strongest currents:

  • occur between 1 and 10 km from shore
  • are associated with shore-parallel currents that move initially in the direction of the component of the wind parallel to shore
  • reverse direction before dying out over a time-scale measured in days

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.

Other variables

The general horizontal circulation in the Great Lakes may also be affected by:

  • the inflow and outflow of the larger rivers, such as the Niagara River
  • hydraulic components of flow in shallow bays and narrows
    • this is caused by the difference in water level at the 2 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