Saturday, July 2, 2011

The All Important Winds and Currents – Part II

Winds and currents in 600 B.C. (and for two thousand years afterward) were as important to the seaman as an interstate freeway is to us today. These sea lanes crossed the world and provided the only means by which a sailing vessel “driven forth before the wind” could travel across the “great deep.”

In the Southern Ocean, and completely around the world, a wind and ocean current blow without impediment. This wind is called the Prevailing Westerlies and the current is called the West Wind Drift. These Westerlies, anti-trades, or Prevailing Westerlies, are the prevailing winds in the middle latitudes between 30 and 60 degrees latitude, blowing from the high pressure area in the middle or horse latitudes towards the poles. These prevailing winds blow from the west to the east.

The Westerlies are particularly strong, especially in the southern hemisphere, where there is less land in the middle latitudes to cause the flow pattern to amplify, or become more north-south oriented, which slows the Westerlies down. The strongest westerly winds in the middle latitudes can come in the Roaring Forties, between 40 and 50 degrees latitude. The Westerlies play an important role in carrying the warm, equatorial waters and winds to the western coasts of continents, especially in the southern hemisphere because of its vast oceanic expanse.

If the Earth were a non-rotating planet, solar heating would cause winds across the mid-latitudes to blow in a poleward direction, away from the subtropical ridge. However, on a rotating planet, the Coriolis effect caused by the rotation of Earth causes winds to steer to the right of what would otherwise be expected across the Northern Hemisphere, and left of what would be expected in the Southern Hemisphere. This is why winds across the Northern Hemisphere tend to blow from the southwest, but they tend to be from the northwest in the Southern Hemisphere.

When pressures are lower over the poles, the strength of the Westerlies increases, which has the effect of warming the mid-latitudes. This occurs when the Arctic oscillation is positive, and during winter low pressure near the poles is stronger than it would be during the summer. When it is negative and pressures are higher over the poles, the flow is more meridional, blowing from the direction of the pole towards the equator, which brings cold air into the mid-latitudes.

In the Southern hemisphere, because of the stormy and cloudy conditions, it is usual to refer to the Westerlies as the Roaring Forties, Furious Fifties and Shrieking Sixties according to the varying degrees of latitude.

Due to persistent winds from west to east on the poleward sides of the subtropical ridges located in the Atlantic and Pacific oceans, ocean currents are driven in a similar manner in both hemispheres. The currents in the Northern Hemisphere are weaker than those in the Southern Hemisphere due to the differences in strength between the Westerlies of each hemisphere. The process of western intensification causes currents on the western boundary of an ocean basin to be stronger than those on the eastern boundary of an ocean. These western ocean currents transport warm, tropical water polewards toward the polar regions. Ships crossing both oceans have taken advantage of the ocean currents for centuries.
The Antarctic Circumpolar Current, or the West Wind Drift, is an ocean current that flows from west to east around Antarctica. This West Wind Drift is the dominant circulation feature of the Southern Ocean and, at approximately 125 Sverdrups, the largest ocean current.

It should also be noted, that a sailing ship “driven forth before the wind” flows southward from the Arabian Peninsula in the Arabian Sea and into the Indian Ocean, then as it nears the West Wind Drift, it is forced eastward and into this very strong current of the Southern Ocean. It then travels around the planet in a west to east direction. When this West Wind Drift reaches the area known as the Drake Passage, between the southern tip of South America and the tip of Antarctica (the area of Espersnza and Arctowski), the northern portion of this current is forced upward along the west coast of South America, while the southern portion of the current flows through the narrow passage and into the Atlantic Ocean on its course around the planet.

The West Wind Drift at this point are traveling around 25 miles per hour. Interestingly, it slows majestically down to no current at all around the 30º south latitude along the Chilean coast in this area of the Tropic of Capricorn before picking up again and gaining speed back up to 25 miles per hour and flowing back across the Pacific toward Indonesia in a circular gyre—the South Pacific Gyre.

Thus, the perfect landing site along the coast would be at the area today known as the Bay of Coquimbo and the city of La Serena.

(See the next post, The All Important Winds and Currents – Part III, to see what existed at this location in 600 B.C.)

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