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G. O. Marmorino

Abstract

Towed thermistor chain measurements are used to describe a large patch of ocean fine- and microstructure. In contrast to the relatively quiescent salt-fingering conditions of Part I, the 3-km long by 5–10-m high patch is in a frontal zone where background shear is enhanced by an inertial were (Mied et al.). It is conjectured that an embedded, coherent structure resembling a series of breaking waves is creating the smaller-scale activity. The waves have lengths of 30–50 m which are not incompatible with Kelvin-Helmholtz shear instabilities growing in an observed low-Richardson number layer. Ongoing mixing in the patch is inferred from large values of Cox number, fluctuation length scales which scale with the Cox number, and gradient microstructure spectra which increase as (wavenumber)+1 and are often unresolved at the higher values of Cox number.

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G. O. Marmorino

Abstract

Coastal tide gage and meteorological records from Pensacola to Key West for the period January–April 1978 have been examined for low-frequency fluctuations. The dominant 6-day period signals in sea level, alongshore wind stress, and atmospheric pressure were coherent over the entire shelf and propagated southward, consistent with the movement of cold fronts through the area. Sea level response tagged the local wind stress by 18 h (in the north) to 9 h (in the south). In response to a 1 dyn cm−2 alongshore stress, sea-level amplitudes were largest (∼60 cm) where the shelf is widest (200 km) and undergoes an abrupt bend, and were ∼30 cm elsewhere; large transient alongshore sea level slopes, on the order of 10−6, were thus set up.

A linear steady-state shelf circulation model (Hsueh, 1980) is used to explore the sea level distribution that is in frictional equilibrium with a wind stress of given orientation. For a bottom resistance coefficient of 0.014 cm s−1, a value suggested by an analysis of February current measurements on the inner shelf, the model results resemble the observed response. Modeled responses are trapped to within the inner shelf (depths < 30 m) with an e-folding scale of ∼60 km. Experimentation with open-ocean forcing, idealized as a shelfbreak sea level distribution induced by various hypothetical configurations of the Loop Current, shows that changes in the equilibrium coastal response are negligibly small.

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G. O. Marmorino

Abstract

Records from a number of temporary tide gage stations spaced an average of 20 km apart new Cedar Key, Florida are used to examine the alongshore pressure gradient over length scales much smaller than before possible. In agreement with previous studies, an alongshore stress of 1 dyn cm−2 will produce a large-scale (over distances of several hundred kilometers) alongshore sea-level gradient on the order of 10−6, sea level rising downwind. However, cross-shore stress is also important and, at short periods (∼3.5 days), can be the more coherent forcing. Estimates of small-scale-level slope do appear to show systematic deviations from the larger scale slope, some of which seem to be related to alongshore variations in nearshore bathymetry.

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G. O. Marmorino

Abstract

Observations of currents and temperature at a mooring on the 18 m isobath, 30 km south of the Florida shoreline, are discussed for the 31-day period 15 August–15 September 1978. Tidal currents, having average amplitudes of ∼10 cm s−1, account for 85% of the observed kinetic energy. Low-frequency currents with maximum speeds of ∼10 cm s−1 appear to be at least partly driven by local wind-stress events of magnitude ∼0.2 dyn cm−2. Frictional effects give rise to a veering with depth (in the Ekman sense) of both the low-frequency flow and the energetic, counterclockwise-rotating twice-daily tidal currents. Propagation of both the tidal signal and low-frequency coastal sea-level fluctuations (amplitudes <5 cm) is westward through the study area.

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G. O. Marmorino

Abstract

Towed conductivity microstructure measurements are examined for evidence of salt fingering in the seasonal thermocline of the Sargasso Sea. Patches of limited-amplitude, narrow-bandwidth signals occur in particular fluid layers about 1 m thick and up to several kilometers in horizontal extent. The finger-like signals occur continuously in the patches but are incoherent between sensors spaced 30 cm apart horizontally. Conductivity gradient spectra rise above background levels in a limited range of wavelengths, peaking at 3 to 5 cm, in a fashion qualitatively similar to Schmitt's spectral model. The density ratio is made favorable for salt fingering within the microstructure patches by finescale variability in the salinity profile. This is consistent with observations by Gargett and Schmitt and also Mack. The vertical convective heat flux estimated from the (Delta;S)4/3 laboratory law, where ΔS is the vertical salinity difference across a patch, appears too large when compared with values inferred from either the microstructure measurements themselves or with a flux law based on the vertical gradient across a patch.

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G. O. Marmorino and C. L. Trump

Abstract

A preliminary study shows how a standard acoustic Doppler current profiler, tilted so two beams scan horizontally beneath the surface, can he used to map characteristics of a ship's wake. For example, the measurements appear to resolve the flow of surface water outward from the wake centerline. This centerline divergence and the resulting convergence along the edges of the wake help form bands of relatively calm water that often appear in radar imagery of the sea surface.

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Ya Hsueh, G. O. Marmorino, and Linda L. Vansant

Abstract

The wintertime, wind-driven Ocean circulation on the West Florida Continental Shelf is studied within the framework of a linearized storm-surge model. The model bathymetry incorporates a realistic shelf, extending from New Orleans to the southern tip of Florida, and a deep ocean region. The boundary condition at the coast is that there is no normal flow. At the open boundaries, located off the shelf in deep water, the adjusted sea level is fixed at zero.

It is found that 1) a coastally trapped response is achieved within one local inertial period following the imposition of the wind; 2) the curved coast forces a mass exchange between the coastal water and the deep ocean; 3) this exchange leads to the generation of a series of mesoscale eddies along the shelf edge; and 4) these eddies give rise to long-period, shelf-wide oscillations that persist beyond the local spin-up time.

A hindcast of the wind-driven flow on the West Florida Shelf for a particular period (11–25 March 1978) that contains the passage of a distinct cold front produces coastal sea-level and current fluctuations that are in reasonable agreement with observations.

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G. O. Marmorino, J. P. Dugan, and T. E. Evans

Abstract

Temperature microstructure variability has been determined from measurements of electrical conductivity (∼1.5 cm wavelength resolution) along two depths in the seasonal thermocline of the Sargasso Sea in July 1981. The microstructure sensors were attached to a thermistor chain, which was towed in and away from a frontal shear zone in the region of the Subtropical Convergence Zone. Averaged over the 170-km-long tow, the estimated dissipation rate of temperature variance, χ, was ∼10−8 °C2 s−1, but χ values ranged from 10−11 (noise level) to 10−5 in the most energetic events. Cox numbers, C, were calculated by making use of a local temperature gradient calculated over a fixed ∼1 m vertical spacing on the chain. Mean values of C were ∼10, but values as high as 105 were observed. The signals wear highly intermittent, varying by as much as five orders of magnitude over scales of the order of 10 m. Probability distributions of χ and C appeared to resemble the lognormal form only in cases where the data were carefully drawn from energetic events. Low values of a large-scale Richardson number (7 m vertical by 450 m horizontal averages) hare no consistent relationship to the occurrence of an event.

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