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Ocean Density

The density of fresh water is 1.00 (gram/ml or kg/litre) but added salts can increase this. The saltier the water, the higher its density. When water warms, it expands and becomes less dense. The colder the water, the denser it becomes. So it is possible that warm salty water remains on top of cold, less salty water. The density of 35ppt saline seawater at 15ºC is about 1.0255. For brevity this is written as “25.5”.

The relationship between temperature, salinity and density is shown by the blue isopycnal (of same density) curves in the figure below. In red, green and blue the waters of the major oceans of the Earth are shown for depths below 200 meters. The Pacific has most of the lightest water with densities below 26.0, whereas the Atlantic has most of the densest water between 27.5 and 28.0. Antarctic bottom water is indeed densest for Pacific and Indian oceans but not for the Atlantic which has a lot of similarly dense water.

www.seafriends.org.nz/oceano/seawater.htm