Clifford L. Trump

Convergence and Downwelling at a River Plume Front

Abstract The small-scale structure of the circulation and hydrography at the frontal boundary of the Connecticut River plume in Long Island Sound has been resolved using a novel combination of instruments: a towed acoustic Doppler current profiler (ADCP) and a rigid array of current meters and conductivity–temperature sensors. Observations were made during the latter half of the eastward ebb tide, when the river plume was well established and the front was moving to the west at approximately 0.3 m s−1. Two across-front transects revealed a horizontal convergence rate in the across-front velocity components at 0.6 m of 0.05–0.1 s−1. This was associated with a salt-induced horizontal density gradient of 10−2 kg/m4. Observations obtained during a period in which the towed ADCP was caught in the zone of maximum surface convergence showed significant downwelling with a near-surface maximum of 0.2 m s−1. Vertical velocities of this magnitude are consistent with observed magnitudes of the convergence rate at 0.6...

Gulf Stream surface convergence imaged by synthetic aperture radar

On July 20, 1990, the north edge of the Gulf Stream (36.7°N, 72.0°W) was sampled by the R/V Cape Henlopen and simultaneously imaged by the Jet Propulsion Laboratorys airborne synthetic aperture radar (SAR). Hydrographic measurements show an abrupt surface front separating warm, salty Gulf Stream water in the south from a filament of cool, fresh (<33 practical salinity unit (psu)) water to the north. The filament lies within the stream and is likely water entrained from the continental shelf. The southern boundary of the filament is marked by increased surface wave breaking in a 100- to 200-m-wide zone, accumulations of Sargassum, and an orthogonal velocity change of 20 cm/s. The front is manifested in a sequence of SAR images as a narrow line having returns 1–2 dB higher than background. (A second, transient SAR line occurs near the northern filament boundary.) The observations are compared with model calculations of the surface wave hydrodynamics and radar scattering. The ocean waves are driven by southwesterly 8-m/s winds and interact with the front to produce primarily an enhancement of 2- to 3-m waves over a ≲200-m-wide region centered downwind of the front. Using a composite scattering radar model along with measured breaking-wave statistics, we show that the observed modulations in the radar backscatter can be accounted for through breaking-wave and tilted Bragg wave scattering effects. These results further show that SAR images of the ocean surface can be exploited for detailed study of particular ocean processes.

Internal-Inertial Waves in a Surgasso Sea Front

Abstract This work examines the presence of internal-inertial waves in a front in the North Atlantic subtropical convergence zone. Results of Doppler shear profiler and towed thermistor chain surveys are displayed to document the position and magnitude of the front. Objective maps of the total measured velocity are computed and subtracted from the observed velocity fields. The remaining wave signal is processed to yield horizontal (towed) and vertical (dropped) kinetic energy spectra across the front. From these, rotary spectra are also computed along the line of tow and in the vertical to determine the horizontal and vertical anisotropy. It is found that several nearly monochromatic waves are propagating northward and southward from the front with horizontal length scales of ∼32–50 km. It was also discovered that the region of anticyclonic frontal vorticity exhibits an excess of downgoing energy at the longest vertical wavelength thus sampled (∼50 m), while the region of cyclonic vorticity possesses more...

Calibrating a Gyrocompass Using ADCP and DGPS Data

Abstract Accurate values of ship’s heading, usually obtained from a gyrocompass, are vital in calculating absolute currents from ship-mounted acoustic Doppler current profilers (ADCPs). This note presents a straightforward method of calibrating a ship’s gyro by comparing two independent estimates of ship’s absolute velocity: one from bottom-track ADCP data (in combination with gyro data), and the other from differential global positioning satellite data. From a dataset collected in June 1993, 5-min-averaged data were isolated having these two estimates of ship’s velocity. These data showed that the speed estimates agreed within 0.2% but that the direction estimates varied by ±5° (standard deviation 2.4°) in a manner that was strongly a function of ship’s gyro heading. Correcting the data for this relationship reduced the standard deviation of the components of the ship-velocity differences by a factor of 2 and removed clear biases in their means.

Observations of an inshore front associated with the Chesapeake Bay outflow plume

Abstract Preliminary observations are reported of a recurring front located near Cape Henry, Virginia, USA. The front occurs on the right-hand side, looking seaward, of the buoyant plume discharging from the Chesapeake Bay and separates the plume from a band of relatively dense seawater confined against the Virginia coast. The front thus appears to be of a type similar to the inshore plume front reported by Sanders and Garvine for the Delaware Bay. Similar to an estuarine tidal intrusion front, the Cape Henry front evolves to a prominent V-shaped planform during flood tide and subduction of fluid along the front may provide a means for recirculation of near-surface material.

Effects of ship's roll on the quality of precision CTD data

Abstract Analyses of raw precision CTD data indicate apparent temperature and conductivity structure resulting from varying drop rates related to ships roll. The perturbations are proportional to vertical gradients and probably result from either of two mechanisms, heating of the water near the probes by the instrument arriving from shallower (warmer) water, or by entrained wake overtaking decelerating probes. Accurate fine structure data probably cannot be recovered from cast data and the best solution is to eliminate the effects of platform motion through some sort of feed-back control on the winch.

Near-Surface Current Measurements Using a Ship-Deployed “Horizontal” ADCP

Abstract An experiment was performed to measure the near-surface current by aiming horizontally two of the beams from an acoustic Doppler current profiler (ADCP) deployed at 0.6-m depth from an anchored (but rolling) ship. The results compare favorably with independent current measurements made at 2-m depth but appear to resolve as well a vertical current shear associated with the shallow wind-drift layer. The approach, therefore, has potential for investigating the current profile in the upper meter or two of the water column.

An occluded coastal oceanic front

Field observations, including hydrographic, microwave imaging radar, and HF radar measurements, reveal the evolution of a complicated frontal interaction between three water masses on the continental shelf near Cape Hatteras, North Carolina, during a period of incursion of water from the Gulf Stream. The water masses were found to be separated by intersecting frontal lines configured in a manner analogous to an occluded atmospheric front. The densest water lay between inshore and offshore fronts that gradually merged or occluded in the generally downstream direction, leaving a single surface front. The overall frontal structure appeared as a distinct Y-shaped feature in the radar imagery, similar to historical imagery of the study area. The interpretation of the observations is aided by the use of a two-dimensional numerical model. The model is initialized with two fronts idealized from the ocean measurements. The model fronts quickly sharpen and begin to move together, eventually occluding into a single surface front. As a result of the occlusion, the water mass having intermediate density subducts and intrudes under the most buoyant water, carrying with it strong horizontal and vertical shears, and a frontal band of diverging currents is created in the densest water mass. The model thus suggests that in the ocean there will be an increase in hydrographic and velocity fine structure downstream of the frontal occlusion point.

Radar imaging of sand waves on the continental shelf east of Cape Hatteras, NC, U.S.A.

Abstract Imaging radars, under certain environmental conditions, can provide an extensive description of shallow submarine topography. In this investigation, sand waves were observed in shallow water and under light winds, weak flow, and highly stratified conditions with an L band synthetic aperture radar and X band real aperture radar. An analysis of the radar data reveals that regularly spaced modulations seen in the imagery are a result of bathymetric forcing. These modulations appear as a group of bright linear east-west trending features approximately 5 km in length and spaced 230 m apart with observed peak modulations exceeding predicted modulations by 7 dB. Bathymetric measurements extracted from shipboard ADCP data confirm the existence of sand waves in this region. Results from the ADCP data reveal an east-west orientation of the sand wave crest with lee slopes facing north. Mean wavelengths are 230 m and the heights are roughly 2.5 m. The radar modulations lead the sand wave crest, by approximately 135 m suggesting a possible upstream hydrodynamic effect, which is consistent with an observed Froude number less than one. This study shows that bathymetric effects are observed in radar imagery at low current speed, light winds, and strong stratification, demonstrating the critical nature that topographic and stratified hydrodynamic effects have on radar image interpretation in the littoral environment.

Mapping small-scale along-front structure using ADCP acoustic backscatter range-bin data

A quasi-three-dimensional view of a scallop-shaped river plume front is derived using acoustic Doppler current profiler (ADCP) backscatter measurements recorded in individual range bins. The data show that the scallops consist of broad troughs of plume water separated by narrow cusps of ambient water and have an along-front wavelength of about 13 m. Having this view across such an inhomogeneous region is helpful in interpreting the corresponding ADCP velocity measurements, so that an examination of just those ADCP profiles made in the ambient water shows the expected strong sinking motion, which reaches about 25 cm s−1 at 1–2-m depth.