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Kevin D. Leaman

Abstract

The time-depth structure of the baroclinic diurnal tide has been examined with the aid of current and temperature profiles on the West Florida Continental Shelf. Of interest is the fact that the diurnal frequencies (e.g., the K1 and O1 tides) are near the “critical frequency” corresponding to the bottom slope and density stratification at the experimental location.

The baroclinic semidiurnal tide was rather weak and most of the semidiurnal tidal energy was contained in the barotropic currents. This large ratio of barotropic-to-baroclinic, semidiurnal tidal energy is in agreement with the results obtained by Koblinsky (1979) from previous (current meter) measurements in the same area.

In contrast, the baroclinic diurnal tide is quite strong and exhibits appreciable structural variations with time. The diurnal oscillations are predominantly of low vertical modal order, and there is no evidence of the concentrated “beams” of internal tidal energy which have sometimes been observed in other areas (e.g., Torgrimson and Hickey, 1979). However, the diurnal structure is modulated in a fashion which seems to be more complicated than can be accounted for by a simple “beating” effect between the K1 and O1 constituents. This relatively rapid modulation in amplitude and vertical structure indicates that there was present a significant transient component in either the generation or propagation of the internal diurnal tide. It is shown that variations in the vertical shear of low-frequency currents which occurred were in the correct sense and were potentially of sufficient amplitude to produce a subcritical bottom slope for the diurnal constituents during one period of the experiment. In this same period, there is clear evidence of near-bottom intensification of the diurnal oscillations. The data also show that the internal diurnal oscillations are propagating up-slope, away from the shelf break.

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Kevin D. Leaman

Abstract

Vertical profiles of horizontal ocean currents are used to study the vertical structure and temporal behavior of internal waves in the ocean, particularly those near the local inertial frequency. The polarization, or direction in which the horizontal velocity vector of an internal wave rotates with depth, is an important feature of the vertical structure, since it provides information on the direction and magnitude of the vertical wave energy flux. Analysis of a time series of profiles at one location over smooth topography shows that the observed wave polarization and phase propagation in the vertical are consistent, at least within the limits of the observational technique that was employed, with the linear dispersion relation for internal waves. The fact that the waves are polarized in the clockwise sense with increasing pressure shows that they have a net downward energy flux. A spectral decomposition of the profiles into clockwise and anti-clockwise components provides an estimate of 0.2–0.4 erg cm−2 s−1 (0.2–0.4 × 10−3 J m−2 s−1) for this net downward energy flux and gives an estimate of bottom reflection coefficients for these waves as a function of vertical wavenumber. The horizontal kinetic energy spectrum as a function of stretched vertical wavenumber and the dropped, lagged, rotary coherence between profiles separated in time are compared with predictions based on a model derived by Garrett and Munk (1975). Within the expected error there is good agreement between the data and the model. Some profiles obtained over a region of rough bottom topography indicate that the rough bottom may be acting as an energy source for the near-inertial waves.

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Friedrich Schott and Kevin D. Leaman

Abstract

In the Golfe du Lion, south of France, favorable conditions for deep winter convection exist and were documented by the MEDOC experiments during 1969–75. A renewed investigation of that regime with upward-looking moored acoustic Doppler current profilers (ADCPs) was carried out during 24 January–5 March 1987, to record the three-dimensional currents associated with the deep mixing. While in the earlier studies initial deep convection did not begin until fairly late in the winter season, a very strong Mistral around 10 January 1987 had already generated a 1arge deep-mixed patch, homogeneous down to around 2000 m at deployment time. Three ADCPs, two working at 150 kHz and one at 75 kHz, were moored in a triangle of 15 km sidelength at 550–780 m depth. Full records at 1-hour ensemble time intervals, 400 pings per ensemble, 8 m bin lengths were obtained by the 75 kHz and one of the 150 kHz ADCPs.

In mid-February, a second Mistral hit the region. With the onset of strong winds and surface cooling the occurrence of short-period current fluctuations, in the period range of hours, was observed which lasted for the duration of the negative heat flux.

The vertical currents recorded by the ADCPs during this period included downward events of 5–10 cm s−1 velocity with weaker upward motion in between. These events appeared to occur simultaneously over the depth range of several 100 m covered by the ADCPs. An interpretation of these events as frozen structures, advected by with the mean current, yielded a horizontal scale estimate of only order 1 km. The mean vertical velocity during the Mistral week due to the integrated effect of these events was of order 1 cm s−1 downward.

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Kevin D. Leaman and Robert L. Molinari

Abstract

The effect of local topography in modifying the structure and variability of the Florida Current is examined using shipboard acoustic Doppler and PEGASUS acoustic current profiler data. PEGASUS absolute velocity data were obtained during 16 cruises in the Florida Current at 27°N as part of the Subtropical Atlantic Climate Studies (STACS) program. The ensemble average of all PEGASUS velocity data shows that the effect of the constriction imposed on the mean Florida Current by Little Bahama Bank can be detected up to 30 km into the Straits of Florida. A simple model is proposed to explain how this effect can produce the subsurface maximum of northward flow commonly observed in the eastern Straits.

PEGASUS and acoustic Doppler data obtained during the March 1984 STACS cruise are ~used to describe the temporal and spatial variability of the flow. It is shown that intermittent southward flow can exist in a band 10–15 km wide off Little Bahama Bank; one such event was detected during this cruise. The PEGASUS data suggest that these events are associated with meandering of the Florida Current. These results may explain earlier observations in satellite synthetic aperture radar images of small-scale vortices moving southward across the mouth of Northwest Providence Channel.

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Kevin D. Leaman and Jessie E. Harris

Abstract

Data obtained from a line of absolute current profiler (PEGASUS) and CTD stations at 26.5°N, east of Abaco Island in the northern Bahamas, are used to estimate the average transport, and overall level of variability, of the deep (below 800 m) southward flow associated with the so-called “Deep Western Boundary Current”(DWBC) at this latitude. From April 1985 to September 1987 a total of 11 sections were made along a line extending from near the island boundary to 85 km offshore as part of the Subtropical Atlantic Climate Studies (STACS) program. In all but two cases (when PEGASUS data were obtained over the total depth), absolute velocities were determined from 0 to about 3000 db while CTD derived relative velocities were determined over the total depth.

Using the combined datasets, it is possible to reference the relative geostrophic velocity with PEGASUS data, thereby obtaining an estimate of the absolute southward transport of the deep flow. The average value of transport determined in this manner is about 35 Sv (1 Sv ≡ 106m3s−1. Most of this large transport is located in a deep (2000 m) offshore core, with average southward velocities of ∼0.20 m s−1. This core is not co-located with the “Deep Western Boundary Current” core as defined by tracer studies. It is suggested here that this core may either be part of a southern deep recirculation gyre similar to those proposed farther north (e.g., nearer the Gulf Stream) or alternatively represents the augmented DWBC flow found in some theoretical models.

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Kevin D. Leaman and Peter S. Vertes

Abstract

Over a period of several years, RAFOS floats were launched into three levels of the deep western boundary current (DWBC) east of the northern Bahamas in order to identify and study any local recirculations that might be present in addition to the thermohaline-driven component of the current. These float trajectories reveal the presence of recirculations that are clearly caused by features of the lateral and bottom topography. In particular, the San Salvador Spur exerts a major influence on the paths of these floats. Although the floats exhibit a complicated set of motions, some order is imposed by relating periods when floats move directly along the boundary versus periods when they leave the launch site “anomalously” (i.e., to the cast or northeast) due to motions of the DWBC core. Comparison to current meter records along 26°30(N near the launch site shows that floats in the latter group were deployed when the DWBC core was located offshore.

The “eruption” of floats into the interior recirculation at the San Salvador Spur causes a reduction (by a process similar to what elsewhere has been termed “arrested dispersion”) in the mean rate at which the floats, and presumably other tracers, move southward along the boundary. The ”effective southward spreading rate” of these floats is estimated as 1.97 cm s−1, in reasonable agreement with analogous results from tracer studies in the same region.

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Kevin D. Leaman and Edward V. Browell
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Rainer J. Zantopp and Kevin D. Leaman

Abstract

The existence of a tight T-S relationship in the southwestern North Atlantic is used to convert temperature measurements from moored sensors to dynamic heights. Seven hydrographic cruises with intensive CTD coverage during 1980–81 allow us to establish a close correlation between temperature and specific volume anomaly, which then is integrated vertically as a function only of temperature to derive dynamic heights. The systematic errors arising from the method are smaller than the natural variability of temperature from the mesoscale field.

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Kevin D. Leaman and Friedrich A. Schott

Abstract

Deep winter convection in the northwestern Mediterranean (Gulf of Lions) and the subsequent formation of Mediterranean Deep Water were observed using advanced oceanographic instrumentation during a six-week long experiment in 1987. The severe 1987 European winter forced an intense outbreak of the Mistral, a cold, dry wind blowing down the Rhone valley, in mid-January. Surface cooling and evaporation were of sufficient intensity to cause an initial episode of deep convection shortly before the experiment described here began. However, several more Mistral events took place during the experiment.

During several cruises into the area, CTD and absolute horizontal velocity profiles were measured in the mixed area over the Rhone fan as well as across the southern front of this region; in addition, continuous records of shipboard meteorological and oceanographic parameters (air temperature, surface salinity, etc.) were made.

An early-February Mistral apparently did not produce enough surface cooling to reinitiate convection. In contrast, further convection during an intense, mid-February Mistral was observed in both hydrographic and current-meter data. The 0.02°C cooling observed in CTD data at 1950 m, as well as in data from moored temperature sensors at other depths by the end of this storm, is consistent with what would be expected if the estimated surface heat loss were mixed over the 2200 m depth of the water column.

Analysis of CTD data shows that the presence of unstably stratified surface layers was correlated with periods of strong surface cooling. An inverse relation appears to exist between the thickness and density excess of these layers. Comparison with surface cooling rates suggests that these layers could be formed in periods as short as one hour.

Finally, comparison of our results to historical data suggests that deep water formed in the northwestern Mediterranean has become progressively warmer and saltier over the past several decades.

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Dennis A. Mayer, Kevin D. Leaman, and Thomas N. Lee

Abstract

A linear relationship exists between sea level and the north component of the depth-averaged tidal velocity in the Straits of Florida. This relationship is used as a one-dimensional model to predict barotropic tidal currents across the Straits near 27°N. Predictions are independent of the choice of a sea-level reference site between Key West and Patrick Air Force Base. The model, when compared with three sets of depth-averaged velocity obtained from current profilers, can account for at least 70% of the variance in the diurnal and semidiurnal tidal bands. The predicted diurnal tidal current is dominant and can account for more than 80% of the predicted tidal energy. Twice a year the one-dimensional model yields a maximum amplitude of 12 cm s−1 ± 3.5 cm s−1 (rms). This corresponds to a tidal transport of 5.1 × 106 m3 s−1 ± 1.5 × 106 m3 s−1 (5.1 Sv).

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