In winter, when the atmospheric temperature starts falling well below 0 degree C, the water at the surface of a tank or ocean initially at temperature of 4 degree C beings to radiate heat to the atmosphere, so the temperature of water starts falling. When the temperature of water at the surface falls, water contracts, so its density increases and therefore it sinks to the bottom. This continues till the temperature of the entire water reaches to 4 degree C. 
In short this happens due to anomalous expansion of water.
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