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Charles L. Johnson and Thomas B. Sanford

Abstract

0bservations of vertical profiles of horizontal velocity made around the island of Bermuda during the FAME experiment reveal anomalies in the internal wave field associated with the island bathymetry. Compared to similar data taken at open-ocean sites, the near-Bermuda data exhibit lower horizontal kinetic energy levels, especially in the near inertial frequency band. Also, whereas the open ocean data consistently show the dominance of clockwise over anticlockwise polarized (with depth) energy, implying a near surface energy source, the Bermuda profiles frequently consist of mostly anticlockwise polarized energy. The near island internal wave field possesses a significant inshore over alongshore shear anisotropy, which, together with the anticlockwise polarization, might signify energy generation at the sea bottom with subsequent propagation upward radially away from the island. No relation is found between the amount of shear anisotropy and the energy level of the wave field. The shear anisotropy and temperature finestructure appear to be related to horizontal shear of the time-mean current and proximity to the island.

During the period when a large-scale eddy was impinging on the island, significant coherence was observed between vertical gradients of temperature and velocity components in the wavelength band 30–50 m. The phase between temperature and velocity variations was consistent with nearshore internal wave generation followed by upward and outward propagation. The observed coherence and phase are compared to that expected from a horizontally anisotropic, vertically asymmetric internal wave field. It is found that such a wave field can account for the observed temperature finestructure by internal wave distortion of the time-mean vertical temperature gradient. In contrast, horizontal advection by internal waves of a time-mean horizontal temperature gradient produces insignificant temperature finestructure. The narrow band coherence presumably results from scale-dependent generation or propagation.

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D. Y. Lai and T. B. Sanford

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Velocity Profiles and current meter measurements taken near Site D(39°10′N, 70°00′W) on the continental rise south of New England are used to study the variability of the near-inertial wave field along a sloping bottom. While the typical vertical scales of the waves are on the order of 100 m, some energetic downward propagating near-inertial features are observed with unusually large vertical scales, on the order of the ocean depth. Comparison with an internal wave model on a linear bottom slope shows that these energetic waves are dominated by the lowest three downward and seaward printing dynamical “slope” modes. The lowest mode arrives first at Site D from the north, the higher modes follow several days later.

The observed scales and propagation directions suggest that the energetic near-inertial waves were generated by a hurricane and then reflected at the steep continental slope to the north of Site D. The low-order, flat-bottom modes that usually dominate the far-field response of a hurricane are changed by the sloping bottom into “slope” modes, which then propagate toward deeper water. Energy intensification of these modes towards the bottom suggests that sloping bottoms may play a significant role in the near-inertial wave field below the main thermocline.

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M. C. Gregg and T. B. Sanford

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No abstract available.

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Thomas B. Sanford and Robert J. Serafin

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No abstract available.

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Gregory C. Johnson and Thomas B. Sanford

Abstract

Data from a CTD station and three expendable current profiler drops at the center of the sill of the Faroe Bank Channel are used to examine the structure of the northwestward outflow of cold, relatively fresh, dense water from the Norwegian Sea into the Atlantic Ocean. A bottom boundary layer is present and exerts a bottom stress estimated at 3.5 Pa using observations in the log-layer. The shear at the interface between the outflow water and the water above is sufficiently strong to overcome the stratification and generate shear instabilities. The large stress at the bottom boundary creates an Ekman layer and thus a secondary cross-channel flow to the southwest there. A flow of similar magnitude but to the northeast is found in the high shear region at the interface. Hence, these data suggest a spiral velocity pattern in the outflow, created by the Ekman flow in the bottom boundary layer and cross-channel flow at the interface. This proposed circulation scheme explains the pinching of the density field observed at the southwest channel wall in CTD sections across the channel.

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Ren-Chieh Lien and Thomas B. Sanford

Abstract

Twenty Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats in the upper-ocean thermocline of the summer Sargasso Sea observed the temporal and vertical variations of Ertel potential vorticity (PV) at 7–70-m vertical scale, averaged over O(4–8)-km horizontal scale. PV is dominated by its linear components—vertical vorticity and vortex stretching, each with an rms value of ~0.15f. In the internal wave frequency band, they are coherent and in phase, as expected for linear internal waves. Packets of strong, >0.2f, vertical vorticity and vortex stretching balance closely with a small net rms PV. The PV spectrum peaks at the highest resolvable vertical wavenumber, ~0.1 cpm. The PV frequency spectrum has a red spectral shape, a −1 spectral slope in the internal wave frequency band, and a small peak at the inertial frequency. PV measured at near-inertial frequencies is partially attributed to the non-Lagrangian nature of float measurements. Measurement errors and the vortical mode also contribute to PV in the internal wave frequency band. The vortical mode Burger number, computed using time rates of change of vertical vorticity and vortex stretching, is 0.2–0.4, implying a horizontal kinetic energy to available potential energy ratio of ~0.1. The vortical mode energy frequency spectrum is 1–2 decades less than the observed energy spectrum. Vortical mode energy is likely underestimated because its energy at vertical scales > 70 m was not measured. The vortical mode to total energy ratio increases with vertical wavenumber, implying its importance at small vertical scales.

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Mark D. Prater and Thomas B. Sanford

Abstract

A meddy, an eddy formed from Mediterranean source water, was surveyed in detail with two types of expendable profilers and a CTD instrument. The muddy comprised two distinct, vertically aligned tenses with a combined thickness of 650 m. Both lenses were stratification minima. The upper lens was warmer, fresher (12.25°C, 36.5 psu), and more circular., the lower lens was cooler, more saline (12.1°C, 36.65 psu), and more elliptical, oriented alone a northeast by southwest line. The upper lens, homogeneous out to a radius of 6 km, had a radius of maximum velocity of 9 km. Its relative vorticity was −0.85 f, and its Ertel potential vorticity, 4 × 10−12 (m s)−1, was 17 times below ambient levels due to the combined effects of negative relative vorticity and vortex stretching. The meddy contained more kinetic energy than available potential energy (energy Burger number of 2.5). Compared with historical meddies, it had a larger Burger number and a more negative vorticity Rossby number.

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H. Thomas Rossby and Thomas B. Sanford

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A time series of velocity profiles derived from three methods are used to describe the variations of current in time and in the vertical. Absolute velocity profiles were Obtained by acoustically tracking a falling probe; relative profiles were derived from motional electric fields (EM method) measured by a second free-fall instrument and from density observations using the dynamic method. The two free-fall profile methods agree within 0.01 m s−1 rms averaged over depth intervals in which the observations were separated in time by less than 10 min. Although the rms differences between profiles increases to about 0.02 m s−1, due to the fact that one device falls at one-third the speed of the other, the agreement between methods was sufficiently good that the eight acoustic profiles and six EM profiles were combined to yield a time series lasting 4 days. These profiles, taken near Bermuda In May 1971, were divided into two sets having a mean time separation of 2 days. Each set of profiles was fitted to a time-mean or steady profile and a rotary component of inertial frequency. Using lagged correlation and vector spectral analysis, it is shown that the inertial energy propagates downward at a group velocity having a vertical component of about 0.5 mm s−1. These results suggest a surface or near-surface energy source and a lack of modal structure to the inertial currents. The steady component agrees within 0.02 m s−1 rms with the geostrophic profile computed every 200 m and both have the same shear over the interval 200–1200 m.

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Thomas B. Sanford, James F. Price, and James B. Girton

Abstract

Three autonomous profiling Electromagnetic Autonomous Profiling Explorer (EM-APEX) floats were air deployed one day in advance of the passage of Hurricane Frances (2004) as part of the Coupled Boundary Layer Air–Sea Transfer (CBLAST)-High field experiment. The floats were deliberately deployed at locations on the hurricane track, 55 km to the right of the track, and 110 km to the right of the track. These floats provided profile measurements between 30 and 200 m of in situ temperature, salinity, and horizontal velocity every half hour during the hurricane passage and for several weeks afterward. Some aspects of the observed response were similar at the three locations—the dominance of near-inertial horizontal currents and the phase of these currents—whereas other aspects were different. The largest-amplitude inertial currents were observed at the 55-km site, where SST cooled the most, by about 2.2°C, as the surface mixed layer deepened by about 80 m. Based on the time–depth evolution of the Richardson number and comparisons with a numerical ocean model, it is concluded that SST cooled primarily because of shear-induced vertical mixing that served to bring deeper, cooler water into the surface layer. Surface gravity waves, estimated from the observed high-frequency velocity, reached an estimated 12-m significant wave height at the 55-km site. Along the track, there was lesser amplitude inertial motion and SST cooling, only about 1.2°C, though there was greater upwelling, about 25-m amplitude, and inertial pumping, also about 25-m amplitude. Previously reported numerical simulations of the upper-ocean response are in reasonable agreement with these EM-APEX observations provided that a high wind speed–saturated drag coefficient is used to estimate the wind stress. A direct inference of the drag coefficient CD is drawn from the momentum budget. For wind speeds of 32–47 m s−1, CD ~ 1.4 × 10−3.

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Thomas B. Sanford, Robert G. Driver, and John H. Dunlap

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A freely failing current meter called the Absolute Velocity Profiler (AVP) is described. This profiler is an expansion of a previously developed instrument, the Electro-Magnetic Velocity Profiler (EMVP), with the additional capability of acoustic Doppler (AD) measurements to determine the reference velocity for the EM profiles. The AVP measures the motional electric currents in the sea and the Doppler frequency shin of bottom-scattered echoes. The EM measurements yield a profile of the horizontal components of velocity relative to a depth-independent reference velocity; the AD measurements determine the absolute velocity of the AVP with respect to the seafloor. The EM profile is obtained from the sea surface to the bottom, and the AD measurements are obtained within about 60–300 m of the seafloor. The combination of the EM and AD measurements yields an absolute velocity profile throughout the water column. Performance analyses show the method is accurate to within 1–2 cm s−1 rms. The profiler also measures temperature and its gradient.

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