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Seiches and Thermoclines

December 28, 2010

Lake seiches are produced by the wind when it blows in a constantly for an extended period of time. Basically it piles up the water above the thermocline at the downwind end of the lake; these are also known as “wind tides.” It seems like the water sloshes back and forth in the like like the water in a bathtub but a better analogy might be a lava lamp. Scientists divide a stratified lake into three parts being the part above the thermocline, the thermocline, and the water below the thermocline. While the water does mix between layers, it does not mix readily.

As the warmer water above the thermocline is pushed down wind, the cooler water beneath flows upwind to replace it. This actually tilts the thermocline. Once the wind stops, the water seeks level again. In Lake Champlain, the top layer sloshes back and forth on about a four hour period until it finally settles back to level, if the winds stay calm. The cooler layer, in Lake Champlain, sloshes on a four day period, and while the upper layer only rises and falls a few inches, at most, the lower layer, and so the thermocline, rises and falls tens of feet over the four day cycle. Of course, the wind seldom blows for a fixed period, then stops for several days to allow the lake to react exactly this way, but measurements show that the thermocline does rise and fall by tens of feet somewhat in synch with the north and south winds.

Another consequence of the different flows of the warm and cool water is that there is a “shear” at the thermocline. Think of this like two blocks of wood touching each other and moving in the opposite directions. How this affects fish is anybody’s guess, but it seems certain that it would. The dissolved solids and gases in the water moves with the water in each direction. It seems like the dominant current in the broad lake would be flow towards the Richelieu River, but in fact the seiche can carry suspended debris to south end of the lake. Under the proper conditions a “water parcel” can travel on quarter the length of the Broad Lake in a period of two days.

Another interesting finding from the paper is that the thermocline is located at different temperatures on the northern and southern end of the lake. It is thought that this is due to the fact that the lake is shallower on the north end. At Valcour Island, the thermocline was measured to be at 61 degrees F, at Thompson’s Point, the thermocline was measured to be at 50 degrees F. The thermocline is defined as the depth where the temperature change is the steepest. This works out to be the boundary where the water masses above and below flow independently. Another possibility which showed up as theoretically possible, but which was never actually measured during the month of the study, was that the thermocline could reach the surface, exposing the lower layer to the atmosphere, affecting temperatures and dissolved gases, for what it’s worth.

How did they measure these things? They placed several moored sensor lines, attached to submerged buoys, and having temperature and current sensors positioned along the line. These were used to measure temperature and currents profiled against the depths.

The original paper which provided the information above is here:

Numerical studies of the 4-day oscillation in Lake Champlain – Kenneth Hunkins

Lamont-Doherty Earth Observatory of Columbia University, Palisades, New York – Thomas O. Manley

Marine Research Corporation, Middlebury, Vermont – Patricia Manley

Department of Geology, Middlebury College, Middlebury, Vermont – James Saylor

NOAA Great Lakes Environmental Research Laboratory, Ann Arbor, Michigan

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