The Thermocline and Underwater Waves

The waters of the loch are seldom at rest. One of the earliest scientific distinctions earned by Loch Ness was in 1904 when it was the site of the discovery of large scale water movements known as internal seiches. The loch's profile has an unusual trench-like regularity and is orientated North East and South West in line with the prevailing winds. The warm overlying water of the epilimnion is therefore easily transported down wind. The water then returns beneath the surface like a conveyer belt. At the same time, in a typical South West wind, the warm water is piled up at the North East end, tilting the thermocline down in that direction. As the wind begins to drop, the warm water begins to flow back, even against the wind, and gathering momentum overshoots the level and see-saws with a steady 54 hour oscillation for up to a couple of weeks. This internal seiche hardly affects the surface but the movement of millions of tons of water generates huge underwater waves on the thermocline over 40m high and travelling at a majestic 1km per hour. The turbulence of these internal waves is the main process of mixing heat downwards through the thermocline. They are at their height during the autumn when the loch ceases to gain heat and equinoctial gales keep the cooling epilimnion mixed to a uniform temperature. The difference in temperature between the epilimnion and the water beneath becomes less as the thermocline settles deeper but it will be Spring before the whole loch is mixed to a uniform 5.5o C.

This page copyright Shine, LNP

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