Thermohaline circulation and North Atlantic Deep Water

At the edge of the Arctic Circle, where water freezes at the ocean surface, it is the process of freezing that drives the Great Conveyor. Ice is less dense than liquid water; that is why ice floats. Water becomes denser as its temperature decreases and it reaches its maximum density at a little above freezing. Freshwater is densest at 39.2°F (4°C) and seawater is densest at 35.6°F (2°C). The sea loses heat into the very cold air. This chills the sea surface and increases the density of the uppermost layer of water.

Thermohaline circulation and North Atlantic Deep Water

North Atlantic Deep Circulation

When salt water freezes its salt is stripped from it. A water molecule is arranged with its two hydrogen atoms separated by an angle of 104.5°, so they are both on the same side of the oxygen atom. The oxygen side of the molecule carries negative charge (written as O-) and the hydrogen side carries positive charge (H+).

Salt is sodium chloride (NaCl). Each of its atoms is charged and the salt molecule is held together by a covalent bond between them: Na+Cl-. When a salt molecule dissolves in water, the polar water molecules pull its sodium and chlorine apart. Na+ then attaches to the O- of a water molecule and Cl- attaches to H+. The diagram illustrates the process. When the water freezes, however, its molecules bind together, the H+ of one molecule linking to the O- of its neighbor. This leaves no room for the Na+ and Cl-. They are detached from the water molecules and join together.

Ice therefore consists of freshwater. When sea ice forms, the water adjacent to the ice carries the salt that was "squeezed out" as the water froze. Adding more salt increases the density of the liquid water.

The Great Conveyor. A system of ocean currents carries cold water toward the equator and warm water toward the poles, strongly influencing climate.

Water at the edge of the ice is denser than water farther away because its temperature is just above freezing and because it contains more salt. In the Norwegian Sea, this water sinks all the way to the ocean floor and flows south. Arctic bottom water (ABW) forms in the same way between Greenland and Norway. This dense water fills the basins of the Greenland and Norwegian Seas and spills through narrow channels in the submarine ridge that lies between Greenland, Iceland, and Scotland. It is then known as the North Atlantic Deep Water (NADW) and continues down the western side of the North Atlantic. Because it is driven by changes in the temperature and salinity of the water, this flow of water is often called the thermohaline circulation (THC).

What happens when salt water freezes. 1. Salt (NaCl) enters water (H2O). 2. Na attaches to O, Cl to H. 3. When water freezes, Na and Cl are detached. 4. Na and Cl rejoin to form NaCl.

salt salt

sodium and chlorine separate

sodium and chlorine separate

ice forming

sodium and chlorine rejoin to form salt sodium and chlorine rejoin to form salt

^^ When water freezes, Na and Cl are detatched.

The Great Conveyor

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