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In addition to the tropical cyclones, extratropical

cyclones that may vary greatly in intensity occur in this area primarily These storms have attained wind speeds as

during the winter months.

great as 30 to 50 knots. They originate in middle and high latitudes. forming on the fronts that separate different air masses. The Gulf of Mexico is an area of cyclone development during the cooler months due to the contrast in temperatures of the warm air over Gulf waters and the cold continental air over the United States.

From August 1959 through April 1970, 90 such extratropical cyclones have passed within 120 miles of the Mississippi Delta Region and could possibly have affected the area.

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A phenomenon known as "norther" is quite common in the

area in question during the winter months. A norther occurs when cold, polar air moves southward from the cold interior of the North American continent out over the warm waters of the Gulf. This unstable

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cold air mass, when heated from below, developed strong gusty northerly winds with considerable cloudiness and showers. During a typical winter as many as 30 such cold outbreaks reach the Gulf Coast. majority of these cold outbreaks, spilling out over the Gulf, produce winds in the 15-20 knot range but approximately one third of these cold outbreaks have winds over 34 knots with approximately half of these being vigorous enough to reach 48 knots. 1/

1/ Information obtained from Deputy Assistant Secretary of Environmental Affairs, Department of Commerce.

C. Physical Oceanography

1. Sea Surface Temperatures

Leipper (1954b) presents figures for average February and August sea surface temperatures for the Gulf of Mexico; they are reproduced on the following pages as Figs. 8 and 9. According to Leipper, the main feature of the average winter pattern is a gradual drop from approximately 75°F in the south to 65°F in the north in all parts of the Gulf. In the summertime, average temperatures are very nearly uniform at 84°F throughout the Gulf. Years of investigations have shown that considerable deviation from these average isotherms may occur at certain times. It is well known that periodically, intense cold spells along the Texas coast can cause sufficient cooling of coastal waters to result in extensive mortality of warm-water fish with narrow limits of temperature tolerance (Gunter and Hildebrand, 1951).

In shallow coastal waters and in estuarine and marsh areas, water temperatures more closely follow air temperatures without reaching the extreme limits exhibited by air temperatures during brief

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In the upper 50 meters (168 ft.), water in the central Gulf of Mexico typically has a salinity of very near 36.0 parts per thousand (ppt) (Leipper, 1954b; Nowlin, 1972).

Salinity of waters

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Average sea surface temperatures for February (after Fuglister). (From Leipper, 1954)

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Figure 9. Average surface sea temperatures for August (after Fuglister). (From Leipper, 1954)

over the continental shelf is generally lower and more variable. However, in coastal areas and lagoons of southern Texas where precipitation and runoff is low, salinities often equal or exceed those of the open sea. Research in the Port Aransas area by Jones, Copeland and Hoese (1964) showed that summer salinities (June-September) to be almost uniform, at 35 to 36 ppt from the coast to 25 nautical miles (M) offshore. Through the fall, coastal salinities decrease to about 30 ppt due to runoff and wind driven, lower salinity bay water entering the Gulf. Spring brings increasing coastal salinities and a breakdown of the uniform winter isohalines. June again brings the summer salinity

condition.

Highly variable nearshore salinities are also found along the upper Texas coast. Jeffery and Creager (1954) have noted that the salinity near Freeport has been found to vary from 10.5 ppt to 34 ppt daily in deep water. The average of 26 years salinity measurements at the Galveston tide station shows the salinity to be lowest (about 20 ppt) in May, followed by a sharp increase to 30 ppt in August; this is followed by a sharp decrease to about 25 ppt in October and a gradual fluctuating decline to the May value again (Jeffery and Creager, 1954).

Salinities near the Texas/Louisiana border are lower due to the influence of the extensive marsh bordering the area and higher Average salinities from one year of sampling by Louisiana Wild Life and Fisheries Commission (1971) were as follows:

river runoff.

Sabine Pass--17 ppt; Calcasieu Pass--17 ppt; lower Mud Lake on the
Mermentau River--5 ppt.

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