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W. D. Nowlin Jr. and C. A. Parker

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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W. D. Nowlin Jr., J. S. Bottero, and R. D. Pillsbury

Abstract

The kinetic energy levels Of horizontal oscillations near and above the inertial frequency are described based on 84 current records made during 5 years at Drake Passage. Instrument depths ranged from 280 to 3600 m. Moorings spanned the passage, though the best depth and time coverage is for a central location. For each variance spectrum of horizontal current, the frequency band above the local inertial and semidiurnal tidal peaks was fitted to the Garrett and Munk internal wave spectrum in the form of

The fit is determined by the log-log decay (p) of energy density (NE 0) with frequency (ω), where E 0 is the energy density at 1 cph (ω0) normalized by stability frequency N. Although these parameters were generally near the “universal” values, there was considerable spatial variability—more than the year-to-year variability. Values of p were found to increase with depth; values of E 0 were larger in the northern passage and at the continental slopes suggesting source regions for wave energy. The fitted relation was integrated between local inertial and the appropriate stability frequency to examine the total kinetic energy of horizontal motions in the internal wave field. This internal wave energy was normalized by stability frequency and plotted versus depth for three distinct bathymetric regimes. Both energy levels and vertical distributions differ, with greater energy and more depth variation over the rugged bathymetry of the northern passage.

The prominent inertial peak was examined on the basis of its frequency, peak height and peak width at 50% of peak values. Values of these parameters fell within the range of values obtained by Fu for the North Atlantic. The time and space variations of inertial oscillations was described based mainly on band-passed records centered at local inertial frequencies. The phase coherence for inertial oscillations decreases with increasing vertical instrument separation with a vertical coherence scale somewhere in the order of 700–1500 m. For horizontal separations of less than 20 km the phase coherence is significant in all cases, and for separations greater than 60 km the inertial waves are not coherent.

The partitioning of kinetic energy of horizontal oscillations between 2 hours and 2 days is examined. We earlier estimated that 49% of this energy is found in narrow pass-bands containing the dominant short period tides. Here we find that additionally 10 and 27% of that fluctuation kinetic energy is accounted for by inertial oscillations and internal waves, respectively.

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D. L. Murphy, D. F. Paskausky, W. D. Nowlin Jr., and W. S. Merrell Jr.

Abstract

Between 25 October and 11 December of 1973, 1015 surface drifters were launched at 36 stations in the American Mediterranean. Returns from 75 drifters (7.4%) have been received from 23 (64%) of the 36 stations. No flow into the Caribbean Sea through the Mona and Windward Passages was indicated. A net westward drift in the Caribbean Sea of 0.18 m s−1 was indicated. Movement in the Gulf of Mexico was consistent with observations and models of the Loop Current.

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R. G. Peterson, W. D. Nowlin Jr., and T. Whitworth III

Abstract

An equatorward meander in the Antarctic Polar Front at Drake Passage was observed to amplify and pinch off during January and early February 1979, forming a cold-core. cyclonic current fins with a radius of ∼50 km. This ring appeared to move directly across the Passage parallel to a submarine ridge before turning northeastward upon reaching a gap between the ridge and the South American continental rise. Its net motion was across the Polar Frontal Zone and the ring was last observed to be pushing through the Subantarctic Front. Geostrophic surface speeds of up to 90 cm s−1 relative to 3500 m wore observed where the northern sector of the ring and the Subantarctic Front were In proximity. A stability analysis of the banded flow regime across Drake Passage suggests that necessary conditions for baroclinic instability exist everywhere within the zonal current and those for barotropic instability exist adjacent to the fronts. The beat and salt anomalies of this ring relative to the Polar Frontal Zone are estimated as −8 × 1018 J and −2 × 1011 kg, respectively, and relative to the Subantarctic Zone as −3 × 1019 J and −8 × 1011 kg, respectively. The process of ring migration across these fronts is potentially important to the heat and salt budgets of the Southern Ocean.

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T. Whitworth III, W. D. Nowlin Jr., and S. J. Worley

Abstract

Estimates of the net transport through Drake Passage are made for three periods during which the year-long DRAKE 79 current meter array spanning the Passage was in operation. Relative geostrophic shears from hydrographic surveys in January 1979, April 1979 and January 1980 were referenced to directed speed measurements to give profiles of net speed. Direct measurements were averaged in time to make them more compatible with the spatially-averaged baroclinic shears. The agreement between directly-measured and baroclinic shears is generally good except in regions of large bathymetric relief and during periods when current cores were shifting past or between moorings.

The presence of cold-core rings during two of the DRAKE 79 hydrogaphic surveys resulted in intensified flow and increased transport within fronts, but did not affect the net transport through the Passage. The three latest estimates of net transport (117, 144 and 134 × 106 m3 s−1) are in close agreement with a previous estimate of 124 × 106 m3 s−1 made from 1975 data using the same technique. The consistency of thew four Estimates suggests that the net transport may be less variable than some previous calculations have implied.

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A. F. Treshnikov, R. D. Pillsbury, W. D. Nowlin Jr., E. I. Sarukanyan, and N. P. Smirnov

Abstract

During the austral summer of 1974–75 direct measurements of currents in the Drake Passage were made. One data set was collected by the Soviet Arctic and Antarctic Research Institute while another set was collected by Oregon State University as a part of the IDOE/ISOS program. The Soviet data from 61°11′S, 62°31′W beginning on 25 December and ending on 10 January are compared with the U. S. data collected at 60°45′S, 62°15′W. While there is a spatial separation of 44 km and a time separation of 47 days, there are significant similarities in the energy spectra and in the trends of the velocity components. Comparison between moorings at 59°03′S, 63°24′W and 58°46.5′S, 64°24′W yields similar results.

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