Saturday, February 6, 2010

Ocean Circulation, Part I

Whenever I do a CTD cast, I get a very precise snapshot of the ocean. I know the temperature and salinity of the water underneath me and exactly how they vary with depth. But it's only one moment, in one place. For the data to make sense, I need to understand how the ocean works.

The ocean is mostly stable. The ocean is heated by the sun, so the water on top is warmer than the water in the bottom. If you remember what you learned about convection in sixth grade, you know that you only get all of that motion when heat is added to the bottom of a fluid. When it's added at the top, you get layers of lighter (less dense) water over layers of heavier (more dense) water.

Still, the oceans circulate. One reason is that the tropics get more heat than the poles do. Since it's warm near the equator and colder everywhere else, heat has to circulate. Some of that circulation is done by the atmosphere, but some is done by the ocean.

Water sinks in the ocean in two areas: the North Atlantic and the Southern Ocean around Antarctica. The water that sinks in the North Atlantic is called North Atlantic Deep Water (NADW) and is very salty. Some of the water that sinks in the Southern Ocean goes down to around 1000m and is called Antarctic Intermediate Water (AAIW). The really cold water from the Southern Ocean sinks all the way down to the sea floor and is called Antarctic Bottom Water (AABW).

NADW is around 2° to 4° C (35.6° to 39.2° F). That sounds pretty cold to me! But AABW is colder: -2° to 0° C (28.4° to 32° F). Based on these different temperatures, we can see how far they travel.

Everywhere the map shows red is NADW, and the blue is AABW. Now you can see why processes in Antarctica are so important: water that forms here spreads all over the world.

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