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3. Surface Circulation in the Gulf of Mexico

Water movements in the Gulf of Mexico are controlled by a

variety of interacting forces including fresh water inflow from land, currents set up by winds, currents and water transport induced by surface gravity waves, tidal currents, currents associated with

internal waves, and movement of water masses due to density

differences.

The general circulation pattern for the eastern Gulf consists of a clockwise Loop Current flowing in through the Yucatan Strait and out through the Florida Strait (Figure 10).

This is essentially an extension of the Yucatan Current which flows into the Gulf, circles to the right and flows out through the Florida Strait to join the Gulf Stream Current. A counterclockwise gyre off the west coast of Florida is a persistent feature, more defined during the winter months, however. The western half of the Gulf has no strong, semi-permanent currents but is characterized by a well-defined pattern of winter flow and a highly variable summer pattern (Nowlin, 1971). There is a general westward sweep of currents along the northern shelf west of the Mississippi Delta. Another mass of water moves northward along the Mexican coast to a zone of convergence off Texas. All the currents in the western Gulf seem to flow toward this general zone.

The actual area in which the north and south moving currents converge changes throughout the year in response to atmospheric conditions.

In

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Figure 10.

Generalized surface circulation in the Gulf of Mexico inferred from logs of ships at sea. The numbers refer to current speeds in knots, as estimated from ships' drift. (From Nowlin, 1971)

winter (Figure 11) the area is to the south of the U.S.-Mexican border.

The area shifts gradually northward during the spring and summer

months (Figure 12). Current velocities range in speed from about 0.4

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Tidal action in the Gulf of Mexico is damped and its range

of tides is reduced to an average of about 2 1/2 feet (U.S. Army Corps of Engineers, 1973b). Along much of the Texas coast the tides are of the daily type, i.e., diurnal. When the moon is near its maximum declination, the tide is diurnal and has the greatest range. When the moon is over the equator, the tide has the least range and there may be several days having two highs and two lows, i.e., a semi-diurnal tide. Although tides in the Gulf have a small range they do have important roles in modifying currents and accelerating the movement of water through narrow passages.

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Both wind waves and swell are appreciably lower in the

Gulf of Mexico than along the Atlantic and Pacific Coasts. In the Corpus Christi area waves of 20 feet or more have been observed for all months of the year (Brower, et. al., 1972). In general waves are not as high during the summer months when average wind speeds are less. An average wind speed of 12.7 knots for the Corpus Christi area usually results in approximately six foot waves.

Statistical

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Figure 11. Surface Currents in January (U.S. Army Corps of Engineers, 1973).

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Figure 12.

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Surface Currents in July (U.S. Army Corps of Engineers, 1973).

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