|MadSci Network: Earth Sciences|
A very astute question! The density of seawater is related to both its
temperature and salinity. Density is just the mass per unit volume of a
substance. When water is heated, the molecules it is made up of move a
part a bit, causing it to expand. Because the mass of the water doesn't
change, but its volume goes up, it becomes less dense. Conversely, when
water is cooled, the molecules move a little bit closer, and the water
Salinity is just the amount of "salt" dissolved in the water (it includes things like sodium, magnesium, calcium, and a bunch of other metals, mostly only present in very small amounts). So, if you take pure water and dissolve some salt in it, the mass of the water increases, but its volume stays the same: its density will thus increase.
The relationship between temperature, salinity and density of water has
been well described by oceanographers, and can be summarized by a graph:
Salinity is along the top, and temperature is along the side. The contours are density, and it is in a funny unit that oceanographers use called "sigma-t". Sigma-t is simply the density of the water (in kilograms per cubic meter) minus 1000. So, if the density is 1005 kilograms per cubic meter, sigma-t is 5. You can see that there are sigma-t contours for 0, 5, 10, 15, 20, 25, and 30.
This is kind of a complicated graph, but all you need to get from it is one thing: density is more strongly effected by salinity than by temperature. If you look across the range of temperature, sigma-t changes by no more than about 5, while if you look across the range of salinity sigma-t changes by about 30.
The typical case in the ocean is that water of lower density is found on top of water of higher density. That ought to make sense: higher density water is "lighter", and will float on top of lower density water. That is the case most of the time: water in the surface is heated by the sun, becomes lighter, and floats on top of the water below it. This usually results in warmer water being found over colder water- like you said. However, as I mentioned before, the density of seawater is mostly salinity, so salty water tends to underly fresh water. In some places, particularly the tropics, water evaporates from the surface. If there is not much in the way of freshwater inputs in that area (such as from rivers), it is possible for the surface water to become so salty it becomes heavier than the water below it. In that case it will "sink", moving warm salty water downward. That is quite common in lagoons in many tropical areas. A similar kind of thing happens in polar regions- when sea water freezes, the salt is left behind. So in the arctic, water can become very cold and salty, and sink downwards. This drives what we call the thermohaline circulation of the oceans.
For more information on the properties of seawater, I suggest you check out this excellent site from the University of Nebraska Lincoln.
hope that helps!
Rob Campbell, MAD Scientist
Try the links in the MadSci Library for more information on Earth Sciences.