Zachariah R. Hallock

A comparison of multifrequency HF radar and ADCP measurements of near-surface currents during COPE-3

A high-frequency multifrequency coastal radar operating at four frequencies between 4.8 and 21.8 MHz was used as part of the third Chesapeake Bay Outflow Plume Experiment (COPE-3) during October and November, 1997. The radar system surveyed the open ocean east of the coast and just south of the mouth of Chesapeake Bay from two sites separated by about 20 km. Measurements were taken once an hour, and the eastward and northward components of ocean currents were estimated at four depths ranging from about 0.5 m to 2.5 m below the surface for each location on a 2 by 2 km grid. Direction of arrival of the signals was estimated using the MUSIC algorithm. The radar measurements were compared to currents measured by several moored acoustic Doppler current profilers (ADCPs) with range bins 2-14 m below the water surface. The vertical structure of the current was examined by utilizing four different radar wavelengths, which respond to ocean currents at different depths, and by using several ADCP range bins separated by 1-m intervals. The radar and ADCP current estimates were highly correlated and showed similar depth behavior, and there was significant correlation between radar current estimates at different wavelengths and wind speed.

Current and tide observations in the southern Yellow Sea

Determination of mean currents and tides are particularly difficult in coastal regions. Flows are often nongeostrophic and short lived. Bathymetry is frequently not adequately known and can greatly influence coastal dynamics. To better quantify tides and currents in the southern Yellow Sea, three pressure gauges and three acoustic Doppler current profilers (ADCPs) were bottom moored for 4–6 month intervals at depths ranging from 77 to 89 m. Tidal range is over 2 m, and maximum current velocities are between 50 and 80 cm/s at the mooring locations. The present data are analyzed for 13 significant tidal constituents which are found to account for ∼85% of the sea surface height variability. M2 is the dominant constituent, followed by S2 and K1. Flow is dominated by the tides. Mean currents are relatively small, ranging from ∼1.5 cm/s at the northern mooring to ∼4 cm/s at the southern mooring. Delineations of barotropic and depth-dependent currents are made from the ADCP measurements in terms of mean and eddy kinetic energy. Currents are found to be most depth dependent in the near-surface layers. Approximately 85–90% of the eddy kinetic energy is depth independent.

Observations of the response of a buoyant estuarine plume to upwelling favorable winds

[1] The response of the buoyant plume from the Chesapeake Bay to upwelling favorable winds and during conditions of low freshwater discharge is examined using an offshore array of bottom-deployed acoustic Doppler current profilers (ADCPs) as well as ancillary data. During periods of northward (upwelling favorable) winds the observed response is strong offshore flow (U > 0.2 m/s) in a shallow layer of plume water and a rapid reversal of the near-surface flow from the usual southward flow associated with inertial turning of the plume. The offshore migration of the plume front is deduced from sequential arrivals at the various mooring locations. The orientation of the plume front is approximately alongshore (north-south), suggesting that the upwelling response is two-dimensional. A plume thickness calculated from the ADCP data is relatively constant at about 4 m as the plume moves across the moorings. The offshore propagation speed of the plume front of about 0.20 m/s matched the advection speed predicted by Ekman theory using the calculated plume layer thickness. The mooring velocity profiles provide a view of the across-plume structure in a reference frame fixed to the front. This reveals a pattern of circulatory motion with sinking of plume and ambient water at the front and an ascending motion occurring inshore, suggesting a recirculation between the seaward and inshore portions of the plume. These results are consistent with recent model simulations of the response of a river plume during upwelling favorable winds.

Evidence for a North Pacific Deep Western Boundary Current

Four current meter moorings arrayed across the Japan Trench near 35°N yielded 2 years of deep current velocities in the vicinity of the Kuroshio where it separates from the Japanese coast. One- and 2-year means are interpreted in the context of other observations, including concurrent hydrography, as well as the results of earlier investigations. Principal findings are as follows: the presence of a persistent, southward deep western boundary current (1–2 cm s−1) along the slope (2000–4000 m) inshore of the Japan Trench; a generally northward deep flow (∼4 cm s−1) over the trench, beneath the Kuroshio; and southward and eastward deep flows farther southeast of the Kuroshio separation. These deep, mean flows, particularly the one inshore of the Japan Trench, may be partly of thermohaline origin. Eddy-driven local recirculations and bottom intensification are also likely.

The Fall Rate of the T-7 XBT

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−0.002383t2−4.01, 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.

Variability of Frontal Structure in the Southern Norwegian Sea

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.

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.

Kuroshio sea surface height fluctuations observed simultaneously with inverted echo sounders and TOPEX/POSEIDON

Sea surface height anomalies measured by inverted echo sounders with pressure gauges (IES/PGs) and TOPEX/POSEIDON are compared at six locations in the northwest Pacific near the Kuroshio where it separates from the Japanese coast. The instruments are positioned along a TOPEX/POSEIDON descending ground track near 35°N, 143°E. Good correlation is found between measurements, with best agreement near the Kuroshio, where the range of sea surface height change is largest. Correlations between IES/PG and TOPEX/POSEIDON sea surface height variations range from 0.70 to 0.97. Tides modeled and removed from the TOPEX/POSEIDON data at the inverted echo sounder sites compare well with tides measured by the IES/PGs. TOPEX/POSEIDON and IES/PG data are complementary in describing mesoscale variability in the Kuroshio region.

Subinertial Slope-Trapped Waves in the Northeastern Gulf of Mexico

Abstract Current velocity from moored arrays of acoustic Doppler current profilers (ADCPs) deployed on the outer shelf and slope, south of Mobile Bay in the northeastern Gulf of Mexico, shows evidence of alongslope, generally westward-propagating subinertial baroclinic Kelvin waves with periods of about 16 and 21 days, amplitudes of 5–10 cm s−1, and wavelengths of about 500 km. The observed waves were highly coherent over the slope between about 200 and 500 m and accounted for a significant amount of the current variability below 200 m. The source of the waves could be attributed to effects of the Loop Current on the west Florida slope but is more likely due to direct forcing by Loop Current–generated eddies impacting the experimental area.

Remote Observation of the Spatial Variability of Surface Waves Interacting With an Estuarine Outflow

This paper explores the application of phased-array high-frequency (HF) radars to identify locations of enhanced local waveheights. Measurements of the near-surface current velocities and waveheights were obtained from HF radars deployed near the mouth of the Chesapeake Bay in the fall of 1997. The radar-derived near-surface velocities were compared with the upper bin (2-m depth) of four upward-looking acoustic Doppler current profilers (ADCPs). The slopes of the linear correlations were close to one and the root-mean-square (rms) differences were similar to previous studies. Significant waveheight (Hs) estimates from both radars were compared with a laser height gauge. The largest differences were observed during low winds due to overestimates at one of the radar stations and during storms when the laser measurement failed. Further analysis focused on the HF radar results from the more reliable of the two sites. The rms difference between this radar and the in situ sensor was 0.29 m. Synoptic observations of Hs over the Chesapeake Bay revealed regions of current-induced wave shoaling and refraction. Hs over the estuarine outflow increased between 19-50% relative to the incident Hs in light onshore winds (~5 m/s). In stronger winds (>10 m/s), Hs also increased by up to 25% when there was a tidal outflow in the surface layer, although the near-surface currents were responding to both the wind and the ebbing tide. Hs was not enhanced when the outflow was below a thicker layer (>5 m) of wind-forced onshore flow