When we think of weather processes, we usually consider what is occurring up in the atmosphere. Weather also has exerts its influences downward into the soil and water. Some atmosphere-ocean interactions are well known such as El Niño. Those of us who live along the Great Lakes are also very familiar with the lake effect snows that are caused by atmosphere-lake interactions. A lesser known weather impact involves the twice per year turnover cycle that occurs in deeper fresh water lakes. This phenomenon occurs in regions where the temperature drops below 39 degrees Fahrenheit (4 degrees C).
During certain times of the year, lakes in these colder climates develop a highly stratified density profile from the surface downward. In late summer when the temperature of a lake’s surface water has reached its annual maximum, a layering of water temperatures develops. The warmest water, which has the lowest density, stays on top. The temperature of the water decreases with depth reaching its minimum temperature at the greatest depth. The exact minimum varies from lake to lake, but never falls below 39 degrees F (4 degrees C).
The three major layers within lakes that experience turnover are:
• Epilimnion, the upper layer of circulating warm water, usually no more than 20 feet deep (6 m), where dissolved oxygen concentrations are moderate to high.
• Thermocline, the middle and often thin layer that has rapidly decreasing temperature and oxygen levels, which separates the upper and lower layers.
• Hypolimnion, the cold, deep-water, non-circulating layer in which oxygen is low or absent.
During the early fall, cold air masses drag polar air southward. This contact cooling combines with radiational cooling caused when more solar radiation is lost to space than what is gained during the shorter days. As a result of both contact and radiational cooling, lake surface waters start to cool from their summer peaks. The eventual drop in temperature of the water in the top epilimnion layer makes it denser than the waters below. When these surface waters reach around 50 degrees Fahrenheit (10 degrees C), the surface waters sink into the thermocline waters below. This begins erasing the temperature stratification that had built during summer.
As the water temperatures of the upper and middle layers continue to cool to equal those of the bottom hypolimnion layer, the full water mass reaches a uniform temperature. When the winds are strong and fairly constant in a direction for an extended time, the wind establishes water circulation within the lake. As surface waters are blown downwind, waters from below must rise along the windward shore to replace those waters pushed across the surface. Bottom waters must then rise to replace the ascending waters; and to complete the circuit, leeward shore surface waters, piled up by the wind, must sink to replace the ascending bottom waters.
The resulting circulations will, over time, completely overturn and mix the full lake water mass. This is what is called fall turnover. When the first deep waters rise to the surface, they release their sulphurous gases into the air, often producing a telltale rotten-egg odour. Eventually, the turnover mixes atmospheric oxygen into the full water mass, replenishing the oxygen in deep waters and cleansing the sulphurous gases. This allows fish to return to the depths where many will overwinter.
As the winter approaches in areas where subfreezing temperatures are common, the lake surface temperatures approach the freezing mark. Fortunately for aquatic life — and perhaps all life in these regions — water has a most unique and curious property. Unlike most compounds, water reaches its maximum density as a liquid just before becoming a solid. Under normal conditions, freshwater is most dense at 39 degrees Fahrenheit (4 degrees C ), and solid water or ice, being less dense than liquid water, floats. Thus, as lake waters move toward freezing, the denser but not frozen water sinks and the colder water turns to ice and remains above. The ice layer then prevents the winds from stirring the water, which allows the bottom dense water to remain at a constant non-freezing temperature. This is a good thing since it is in this bottom layer that fish and other organisms are able to survive the winter.
Think about the consequences for life and climate if ice were denser than water. Instead of floating at the surface, it would sink to the depths, and lakes would fill with ice each winter from the bottom up, eventually providing no liquid water in which fish and other animals could seek refuge. In deeper lakes, it is logical to assume that ice could remain at depth all year long, perhaps not melting, except for small puddles, even in the shallows. Not only would this lead to very different aquatic species inhabiting higher latitude lakes, but the local climate would be greatly altered wherever very cold water bodies remained through the warmer seasons.
Completing the cycle, with spring warming the ice melts and cold surface waters warm until they reach the temperatures of the bottom waters, again producing a fairly uniform temperature distribution throughout the lake. When this occurs, winds blowing over the lake again set up a full circulation system, this mixing known as spring turnover. As the warming continues, the three water layers again become established, and our cycle has been completed.
While researching lake turnover, I came across some shots of Lake Superior from space. The photo below shows ice cover on Lake Superior as of March 3, 2009. The photo was taken by the NASA Acqua satellite. It is rare these days for Lake Superior to freeze over. It is considered frozen over when 95 percent of the surface water is frozen.
In this true-color image, ice floating on the surface of Lake Superior ranges in color from white to pale gray-blue. The ice appears most solid along the southern shore of the western half of the lake. North of that solid band of ice, cracks reveal deep blue lake water. Dark lake water also appears in the eastern part of the lake, especially along its northern shore. As it does in the west, ice cover appears relatively unbroken at the extreme southeast end of the lake.
Here is a satellite image of Lakes Superior and Michigan, and a close up image of Lake Superior. The straight-line clouds in the first image are contrails. Contrails (short for "condensation trails") or vapour trails are artificial clouds that are the visible trails of condensed water vapour made by the exhaust of aircraft engines. In both images there is still some snow that can be seen on the ground.
Material for this blog posting was modified from that written by Keith C. Heidorn, http://www.islandnet.com/~see/weather/elements/turnlakes.htm as well as Provided by the SeaWiFS Project, NASA/Goddard Space Flight Center, and ORBIMAGE, and http://mywisconsinspace.com/2009/04/ice-cover-on-lake-superior/
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