Although global circulation in the atmosphere - the patterns of winds that move energy from the tropics towards the poles - is an important process of energy transfer, it is a relatively small player when compared to energy moved by ocean water. The importance of the ocean is a direct result of the energy needed to heat water and the energy water can release when it cools. This can be explained by the First and Second Laws of Thermodynamics and the molecular theory of matter. The Second Law states that any difference in temperature between the ocean and the atmosphere will cause heat to flow from the warmer to the colder. Therefore, warm tropical air will act to heat the oceans and cold arctic air will take energy from the oceans. The first law of thermodynamics requires that any energy going into the ocean must either increase its temperature, be stored as potential energy, or leave the ocean. If we assume that ocean temperature remains, on average, fairly constant, then we can assume the energy entering the oceans must equal what leaves. This implies that the energy gained by the ocean in the tropics must be lost by the ocean at higher latitude -- the net effect is a transfer of energy from the tropics to the poles. Finally, if one considers the molecular nature of liquid water and air, one can see that more energy is needed to heat a liquid than gas and so a warm liquid can "give-off" more energy when cools. One liter of water is approximately 1000 times heavier than a liter of air. Because the mass of matter is a measure of the number of molecules being weighed, one can estimate that liquid water will have about 1000 times more molecules than an equal volume of air. It makes sense then that it will take more energy to raise the temperature of 1000 molecules than it would to raise the temperature of one and that 1000 molecules have more energy to give back when they cool off than a single molecule that is the same temperature.

A simple example of this effect is a comparison of the climates of England and Labrador. Both regions border the Atlantic Ocean and are at the same latitude. One might expect similar climates. One would be wrong. Currents in the Atlantic -- the Gulf Stream and the Labrador Current -- radically affect the temperature of the ocean so that the average ocean temperature off Labrador is about 8° C ( 45° F) but is about 16° C (60° C) near England. The effects are dramatic. Palm trees grow in southern England, and it hardly ever snows. In contrast Labrador has long cold winters. Even the pines that have evolved to survive in the subarctic conditions of northern Canada are small (Lumber companies get one 2x4 per tree.). This contrast in conditions indicates the importance of ocean temperature (and the currents that determine ocean temperature) in controlling climate. In the section that follows, the forces that create the currents will be discussed, the currents that exist described, and the effects of currents on the climate of Antarctica examined in more detail.

Surface currents in the ocean originate in response to the consistent wind patterns associated with global circulation in the atmosphere. Generally speaking, a wind blowing across the surface of the ocean will exert a pull on the water. Friction between the air and water creates a drag on the surface and pulls the surface in the direction that the wind is blowing.

However, friction between the surface waters and water beneath the surface creates a drag in the opposite direction. The net effect of these opposing forces and the Coriolis's effect is to shift direction of the current so that it does not parallel the wind. Instead it flows at a 30 to 45° angle to the wind -- clockwise rotation in the northern hemisphere and counter clockwise in the southern. You may have felt the effects of this "deflection" of the surface current if you have visited the Shore in the summer. A south wind at the shore will blow surface waters away from beach.

Colder water from beneath the surface rises to replace the water that has been moved away and the near shore water temperatures drop. What happens when a north wind blows at the shore? Does cold water come with cooler air?

Because there is a consistent pattern to global scale atmospheric circulation, there is also a consistent pattern of ocean circulation. Trade winds which blow from northeast to southwest push water in the tropical oceans in a westerly direction. Winds of the westerlies (southwest winds) push water at mid latitude (35-55°) to the east. In the northern hemisphere these latitude-parallel currents eventually hit a continent and must deflect north or south to make room for water flowing behind them. In the North Atlantic the trade winds push water into the Caribbean and Gulf of Mexico raising the surface elevation of the water there 10 to 12 feet above the elevation of the ocean off Africa. Because water is a fluid, this is an unstable situation and water in the Gulf of Mexico flows eastward through the narrows between Florida and Cuba and then northward to replace waters been blown eastward by the westerlies. The Gulf Stream carries its warm, Gulf of Mexico water to Europe giving England, France, Spain and Germany -- countries located 10 to 20 degrees north of us -- climates considerably milder than what we experience here in Philadelphia. The northern west coast of North America is warmed by a similar current in the Pacific Ocean. However, where that current deflects southward it becomes a cool current and acts to cool the climate of southern California. If you ever have a chance to swim in the ocean at Los Angeles, you'll experience the coolness of the ocean.

The southern oceans behave somewhat differently. Equatorial currents are the same, and there is a similar general pattern to ocean circulation. However, the eastward moving current at 40 to 60° south latitude, driven by the westerlies of the southern hemisphere can circle the globe without being blocked by any continental land mass. Although the northern edges of this current are pushed northward by South America, Africa and Australia to create northward moving currents in the Pacific, South Atlantic, and Indian Oceans, the main body of the current flows continuously around Antarctica. The northward movement of the northern edge of the current pulls water away from the Antarctic coast causing deeper and colder water to rise to replace it. The cold ocean isolates the Antarctic continent from energy transfer processes that might bring tropical waters or air to the continent. Temperatures even in the most northern reaches of the continent rarely rise above freezing and the continent is locked in ice.

Glaciers cover all of Antarctica with the exception of a few dry valleys where so little snow falls that the ice of glaciers flowing into them evaporates before they are filled. In places the ice is more than 2 miles thick. However, Antarctica has not always been ice covered. Relatively recent geologic history suggests that Antarctica was free or nearly free of ice only 12 million years ago even though it occupied a position over the South Pole similar to its position today. The key difference is related to plate tectonics. In the past South America and Antarctica were closer together. A narrow Straits of Magellan prevented the free movement of currents in the southern ocean and caused a much larger portion of that current to be deflected northward against western South America and some would have been swept southward against the coast of Antarctica causing it to warm much like the Japan Current warms southern Alaska. A gradual increase in the distance between South America and Antarctica opened the Straits to free movement of the southern current, plunging Antarctica into the icebox conditions that continue today. There is circumstantial evidence, basically one of timing, that suggests that the cooling and glaciation of Antarctica was the initial condition leading to a general global cooling and the onset of major glaciations (Ice Ages) that have affected the planet for the last 2 to 3 million years.

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