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Effects of a Cold-Air Outbreak on Shelf Waters of the Gulf of Mexico

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  • 1 Dept. of Oceanography, Texas A&M University, College Station 77843
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Abstract

Two surveys of the waters over an area of the continental shelf in the northwestern Gulf of Mexico were made during January 1966. The first observation period was just before a major outbreak of cold, dry air; the second was about 15 days later with the region still under the influence of this outbreak. Waters were well mixed to 100 m, or the bottom in shallower depths. During the 15-day period temperature decreased nearly 5C and salinity increased about 1‰ near the shore. Some 150 mi offshore, temperature decreased only 1–2C and salinity showed no significant change.

Study of the change in T-S relationships before and after the outbreak indicates the strong possibility that subsurface water types generally found beneath the Subtropical Underwater core in the Gulf were formed locally over the shelf by evaporation and sensible heat exchange to the atmosphere.

For the period between observations, the mean rates of change of vertically-averaged salinity, temperature and heat content were computed. Neglecting advection, the local rate of heat extraction averaged about 400 cal cm−2 day−1 within 50 mi from shore and generally increased to 700–1500 cal cm−2 day−1 at the offshore survey limits. These values agree reasonably well in magnitude and spatial distribution with estimates of heat fluxes using bulk formulas and meteorological data taken during the post-outbreak cruise. Average values of sensible and latent heat fluxes for the region were 255 and 542 cal cm−2 day−1, respectively, which generally agree with estimates by other workers for outbreak conditions. Such estimates are reviewed.

Abstract

Two surveys of the waters over an area of the continental shelf in the northwestern Gulf of Mexico were made during January 1966. The first observation period was just before a major outbreak of cold, dry air; the second was about 15 days later with the region still under the influence of this outbreak. Waters were well mixed to 100 m, or the bottom in shallower depths. During the 15-day period temperature decreased nearly 5C and salinity increased about 1‰ near the shore. Some 150 mi offshore, temperature decreased only 1–2C and salinity showed no significant change.

Study of the change in T-S relationships before and after the outbreak indicates the strong possibility that subsurface water types generally found beneath the Subtropical Underwater core in the Gulf were formed locally over the shelf by evaporation and sensible heat exchange to the atmosphere.

For the period between observations, the mean rates of change of vertically-averaged salinity, temperature and heat content were computed. Neglecting advection, the local rate of heat extraction averaged about 400 cal cm−2 day−1 within 50 mi from shore and generally increased to 700–1500 cal cm−2 day−1 at the offshore survey limits. These values agree reasonably well in magnitude and spatial distribution with estimates of heat fluxes using bulk formulas and meteorological data taken during the post-outbreak cruise. Average values of sensible and latent heat fluxes for the region were 255 and 542 cal cm−2 day−1, respectively, which generally agree with estimates by other workers for outbreak conditions. Such estimates are reviewed.

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