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Zachariah R. Hallock

Abstract

A hydrographic survey (CTD) was conducted in the vicinity of the Iceland–Faeroe Island oceanic front (IFF) north of the Faeroe Islands during October 1980. It consisted of CTD transects on three horizontal scales ranging from kilometers to hundreds of kilometers.

Intense interleaving of different water masses is found in the IFF in the presence of horizontal current shear. Significant alongfront variability on scales of about 50 km is present, consistent with earlier findings. Estimates of cross-front heat flux of 5.16 × 104 W m−2 and salt flux of 1.58 g m−2 s−1 are greater than those found for the Antarctic Polar Front but are of the same order as eddy heat flux across the IFF found by Willebrand and Meincke. Evidence suggests that intrusive interleaving in the IFF on 50 m vertical scales is driven by double-diffusive convection.

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Zachariah R. Hallock

Abstract

Changes in round-trip acoustc travel time (τ) measured between a bottom moored inverted echo sounder and the sea surface can be interpreted as changes in dynamic height (D) with suitable calibration information. The τ, D, and isotherm and isopycnal depths (Z) have been calculated using hydrographic (CTD) data from three regions: the Norwegian Current, the Sargasso Sea and the eastern Gulf Stream. Regressions of D and Z on τ were performed. The slope for the Norwegian Current is − 1.70 ± 0.01 [dyn cm (m s)−1], for the Sargasso Sea − 3.89 ± 0.16 [dyn cm (m s)−1] and for the Gulf Stream − 3.13 ± 0.07 [dyn cm (m s)−1]. The quasi-random scatter about regression curves is found to be primarily the result of variability in the seasonal thermocline, where present.

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Zachariah R. Hallock and William J. Teague

Abstract

A theoretical model of expendable bathythermograph (XBT) fall rate is reviewed, and a new form of fall-rate equation is proposed to include new-surface transient effects. Comparisons are made of T-7 XBT and CTD (conductivity, temperature, and depth) depths of thermohaline features off Barbados. Fall-rate equation coefficients are derived and compared with the manufacturer-supplied coefficients. As other investigators have found, the Sippican equation consistently underestimates probe depth by as much as 35 m at 760 m. Analysis yields a new equation, Z=6.798t−0.002383t 2−4.01, for depths greater than about 10 m. Considerable probe-to-probe variability is noted and is found to be primarily the result of differences in the linear term or terminal velocity of the probes; variation in effective drag resulting from probe irregularities is the likely cause. Recommendations for additional work are made.

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Zachariah R. Hallock and Robert L. Field

Abstract

Internal-wave energetics derived from moored acoustic Doppler current profiler (ADCP) observations on the New Jersey shelf are described. Horizontal and vertical velocity components from three 40-day ADCP records acquired near the shelf edge south of the Hudson Canyon were bandpass filtered to isolate high-frequency internal waves. In this band (0.5–7.5 h−1) the vertical component of velocity is significant and is integrated to yield elevation anomaly. Using the ADCP data with buoyancy frequency profiles from nearby CTD station data, kinetic energy (KE), baroclinic potential energy (PE), and energy flux were calculated. Results show depth-averaged KE and PE are nearly equal and of order 10−3 J kg−1, and there are significant northwestward fluxes, generally parallel to the bathymetry gradient, during most of the record, with depth-integrated magnitudes sometimes exceeding 100 W m−1. Vertical energy fluxes are small relative to horizontal fluxes. Vertical profiles of time-averaged flux suggest a dominant first internal-wave mode. Depth-integrated, time-averaged fluxes range from 9 to 24 W m−1, with the highest values occurring 18 km southwest of the Hudson Canyon. Two-dimensional probability density functions are estimated for energy flux. Group velocity of a large-amplitude internal-wave packet is estimated at 0.36 m s−1.

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Lynn K. Shay, Thomas M. Cook, Zachariah R. Hallock, Brian K. Haus, Hans C. Graber, and Jorge Martinez

Abstract

As part of the Naval Research Laboratory and Office of Naval Research sponsored Physics of Coastal Remote Sensing Research Program, an experiment was conducted in September–October 1996 off Virginia Beach. Ocean surface currents were measured using the high-frequency (25.4 MHz) mode of the Ocean Surface Current Radar at 20-min intervals at a horizontal resolution of 1 km over an approximate 30 km × 44 km domain. Comparisons to subsurface current measurements at 1–2 m beneath the surface from two broadband acoustic Doppler current profilers (ADCP) revealed good agreement to the surface currents. Regression analyses indicated biases of 4 and −3 cm s−1 for cross-shelf and along-shelf currents, respectively, where slopes were O(1) with correlation coefficients of 0.8.

Nine months of sea level heights from the NOAA National Ocean Survey Chesapeake Bay Bridge Tunnel tidal station revealed an energetic M 2 tidal component having an amplitude of 37.5 cm and a phase of 357°. The S 2 tidal constituent had an amplitude of 7 cm and a phase of 49°. By contrast, the diurnal band (K 1, O 1) tidal constituents were considerably weaker with amplitudes of 1–5 cm. From 19 days of HF-derived surface currents, the M 2 and S 2 tidal current amplitudes had a maximum of about 50 and 8 cm s−1 at the Chesapeake Bay mouth, respectively. Explained variances associated with these four tidal current constituents were a maximum of 60% at the mouth and decreased southward. Analyses at the ADCP moorings indicated that the semidiurnal tidal currents were predominantly barotropic with cross-shelf and along-shelf currents of 18 and 10 cm s−1. Energetic semidiurnal tidal currents were highly correlated over the HF-radar domain, and the phase angles indicated a consistent anticyclonic veering of the M 2 tidal current with along-shelf distance from the mouth. Normalized tidal current vorticities by the local Coriolis parameter (f), which represent a proxy for the Rossby number, were ±0.25f near the mouth and ±0.05f in the southern part of the domain for the M 2 constituent. Simulations from a linear, barotropic model were highly correlated with observed M 2 tidal currents at 85 points within the HF-radar domain, consistent with the premise of weakly nonlinear flows.

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