The surface waters of the Arctic and Antarctic oceans become very cold and finally freeze. When seawater freezes to form sea ice, salt is squeezed out and moves down into the water. This means that sea ice is made up of mostly fresh water. The water just below the sea ice is saltier and more dense than the water below. Because it is more dense, it sinks.
The water that sinks is replaced by surface water that comes from areas closer to the equator where it is warmer. A demonstration that hot water floats on top of colder water. Image source. Salt in seawater. Antarctic sea ice is made up of mostly fresh water. The salt has been squeezed out, and sinks into the sea below. Next step learning: Find out about the special sea conditions in the Fiords of Fiordland where the sea has fresh water on the surface.
Skip to main content. Cold water sinks. On its journey around the globe, the water transports heat and salt from the tropics to the earth's poles, nutrients from the ocean's depths to the surface, and fresh water entering at the coast from rivers or melting glaciers out into the sea.
Even though the ocean is constantly in motion and there are lots of factors that affect how the ocean water moves, there is one natural phenomenon that has been contributing to ocean water movement for thousands of years: one large ocean current that connects all ocean basins, as well as the ocean surface and the deep ocean.
This current is sometimes called the global conveyor belt, for the way it circulates water all around the globe Figure 1.
If water could be tracked on its journey on the global conveyor belt, following the red path as warm water nears the surface, then cools and sinks to follow the blue path until it comes back to the surface again, we would find that it takes the water around 1, years to complete its travel all the way around the world. An obvious first guess as to what might cause this motion in the ocean is the wind. Wind blows over the ocean's surface, causing both waves and movement of water in a downwind direction.
And indeed, parts of the global conveyor belt are driven by the wind [ 3 ]. Wind systems, like the trade winds for example, consist of strong winds that constantly act on large surface areas of the ocean, supplying large amounts of energy and putting huge volumes of water into motion. However, another part of the motion of ocean water in the global conveyor belt is caused by something much less obvious: density differences in the water.
Density is a measure how heavy a certain amount of a substance is. The definition of density is thus mass per unit volume. For example, a cube of whipped cream has a much lower density, and thus a lower mass, than a cube of rock of the same size Box 1.
Apart from its importance in understanding ocean circulation , why would you want to know a substance's density? There are several reasons. Sometimes it is easier to measure a volume than the weight of something. For example when baking, you have probably come across those graduated measuring cups with different scales down the side, indicating how far you would have to fill the cup for a certain weight of sugar, flour, water, and others.
Why is it not enough to have just one scale on the cup for all of the ingredients? Because 1 cup of water takes less space than 1 cup of flour.
This is means that 1 cup of water is more compact, has a higher density, than 1 cup of flour. Imagine an ice cube tray filled with water all the way up to the rim. When you place this ice cube tray into the freezer and come back the next day, you will find that the ice cubes have grown and now bulge out of the tray. The water that you put into the ice cube tray now takes up a larger volume than it did before it was frozen.
So, if you wanted the ice cube tray filled with ice exactly up to the rim, the way you initially filled it, you would have to shave away the bulge in the ice, which would decrease the mass of water left in the tray. Usually when a liquid is heated the molecules acquire more energy and become more widely spaced, so in the same volume, the density decreases.
In fresh water the opposite may happen, depending on the temperature. When fresh water at 3. You know from the lake Figure 5 that ice i. But cooling a liquid usually packs the molecules more closely, which increases the density.
This means that below 3. This is an amazing physical property and is why pipes burst and water in cracks shatters rocks in cold temperatures. The molecular structure of a water molecule is shown in Figure 6. This results in a small net negative charge on the oxygen side of the molecule, and a small net positive charge on the hydrogen side. This is a polar structure in which molecules are weakly attracted to each other and form weak 'hydrogen bonds'. At low temperatures a more ordered packing of water molecules develops and the density is reduced.
Above 3. Seawater is saline and the salt affects the density. The variation in salinity in 7b clearly affects the temperature of maximum density shown in 7c. Much of our knowledge about how the oceans circulate is based on measuring the various parameters such as temperature and salinity from the surface to the sea floor. The water which makes up this range is called the water column.
The most important instrument used by oceanographers is called a CTD Figure 8 , which measures the conductivity and temperature of the seawater and depth pressure. Because pressure in the ocean is proportional to the weight of the water above, it is given by the hydrostatic equation:. The minus sign indicates that the vertical coordinate z depth is positive in an upwards direction. So by measuring pressure, Equation 2 can be arranged to get depth.
Other parameters can also be measured, such as sediment particle density, the amount of chlorophyll present in algae , and so on. If the ocean is m deep, as in Figure 7, a round trip is 10 m and one profile can take almost three hours.
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