Tim Herrick, of Wanaka, asks :-
If there is a strong wing blowing down a lake, will the wind pressure and wave movement result in water being pushed to the downward end of the lake? Will this result in the lake level being higher at the downward end?
Claire Flynn, a MetService meteorologist, responded.
Strong winds can indeed push water from one end of a body of water to the other, and cause average lake levels to be higher at the downwind end. The size of this difference will depend on the strength of the winds, how long the strong winds persist, and the size and shape of the lake. In some of the Great Lakes in North America, differences of over 2 metres have been measured between one end of the lake and the other during periods of extremely strong winds.
The strong winds also churn up waves on the surface of the lake. The height of the waves produced again depends upon the strength of the wind.
If the wind then suddenly drops off, there is no longer anything to maintain this difference in lake level from one end to the other. This means that the water at the higher end will now flow â€˜downhillâ€™ due to gravity. As the water flows downhill, it will gain momentum. Because of this momentum, the water wonâ€™t just stop once the lake level is equal at both ends â€“ it will cause the level at the opposite end of the lake to rise a little, before gravity eventually takes over again, and the water flows back downhill in the other direction. You can think of this like a pendulum swinging back and forth, or water sloshing around in a bathtub. This is a type of â€˜standing waveâ€™, and is referred to as a â€˜seicheâ€™. You generally wonâ€™t be able to observe these waves with your own eyes, but it is able to be measured.
This type of difference in lake level is common on Lake Wakatipu by Queenstown. The lake level often varies by approximately 20cm. When the winds drop off, seiches have been observed to form in Lake Wakatipu. Due to the size and shape of the lake, the â€˜sloshingâ€™ usually occurs on a 27 minute cycle.
Thinking on a bigger scale, a similar phenomenon occurs in the Pacific Ocean. We get fairly persistent easterly winds over the Pacific Ocean in the tropics (trade winds). These easterly winds push water from east to west, causing water to collect at the western end. This results in a difference in mean sea level of about 20 â€“ 40cm between Indonesia in the west, and the west coast of South America in the east.
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