Shipboard Acoustic Doppler Current Profiler Research Articles

Current distributions in the southern East China Sea in summer

Current measurements using a shipboard acoustic Doppler current profiler (ADCP) and two satellite‐tracked buoys, together with conductivity‐temperature‐depth (CTD) casts, were carried out over the continental shelf in the southern East China Sea in July 1995. Two transects for ADCP measurements were defined and divided into segments of ∼56 km. In order to remove diurnal and semidiurnal tidal flows from observed flows, four round‐trip ADCP surveys were conducted along each segment over diurnal tidal periods of ∼24 hours and 50 min. The outflow through the Taiwan Strait, i.e., the Taiwan Current, with velocities of 35–40 cm s−1, was clearly observed to flow northeastward along the coast of China. The volume transport was estimated as at least 1 Sv. The Taiwan Current probably increased width and decreased velocity in the central East China Sea. Bifurcation into two branches of the Taiwan Current shown by previous studies was not observed. Another current entering onto the continental shelf northeast of Taiwan, i.e., the Kuroshio Branch, was found with velocities >15 cm s−1 in the surface, and it helped to make an anticyclonic eddy. The main portion of the Kuroshio Branch flowed northeastward along the 100 m isobath, with a volume transport of ∼0.3 Sv. The Taiwan Current was clearly separated from the Kuroshio Branch by the area of very weak or southward (reverse) flows.

The flow pattern north of Taiwan and the migration of the Kuroshio

The migration of the Kuroshio and mesoscale flow pattern north of Taiwan were studied using the historical ship-board acoustic Doppler current profiler (Sb-ADCP) current velocity and hydrographic measurements, moored current velocity, water temperature and salinity time series, snapshot current velocity observations and infrared images. The Kuroshio migrated both seasonally and intra-seasonally, with the former being more pronounced. The flow pattern north of Taiwan was significantly impacted by the seasonal migration of the Kuroshio. In summer the Kuroshio generally moved away from the shelf, colliding with the zonally running shelf breaks of the East China Sea (ECS) and splitting into an eastward mainstream and a northwestward branch current. Southwest of the branch current, a counterclockwise circulation was found along the edge of the shelf northeast of Taiwan, through which the subsurface Kuroshio water intruded forming a cold dome. In winter the Kuroshio moved close to and sometimes onto the northern shelf of Taiwan. The intrusion of the Kuroshio dominated the flow pattern in the region, causing the disappearance or obscuration of the counterclockwise circulation and cold dome. A sharp horizontal temperature front accompanying the horizontal velocity front was found on the northern shelf of Taiwan, through which the current flowed mainly northward against the winter monsoon. Intra-seasonal variations in flow patterns north of Taiwan and the intra-seasonal migration of the Kuroshio were also studied. In summer the counterclockwise circulation and cold dome migrated shoreward/seaward with the intra-seasonal migration of the Kuroshio. In winter the Kuroshio either intruded onto the shelf just off the northern tip of Taiwan or slightly further north. The resulting flow patterns north of Taiwan varied with the path of the intruding Kuroshio. Though the impact of the Taiwan Strait outflow on the flow pattern north of Taiwan was not negligible, it was not as great as the impact of the Kuroshio. Conversely, the intrusion of the Kuroshio strongly influenced the Taiwan Strait outflow. In summer counterclockwise circulation blocked the Taiwan Strait outflow and directly interacted with the Kuroshio. In winter the outflow joined with the on-shelf Kuroshio north of Taiwan.

Continuity of the poleward undercurrent along the eastern boundary of the mid-latitude north Pacific

Several recent data sets improve our view of the poleward undercurrent of the California Current System. As part of a triennial National Marine Fisheries Service (NMFS) survey of Pacific whiting, a series of 105 shipboard acoustic Doppler current profiler (ADCP) velocity sections across the shelf break from 33 to 51°N at about 18 km meridional spacing were collected from July to August 1995. Significant (>0.05 m s −1) subsurface poleward flow occurred in 91% of the sections. A mean cross-shelf section using the entire data set has statistical significance, revealing an undercurrent core >0.1 m s −1 from 200–275 m depth 20–25 km off the shelf break. The mean poleward volume transport in a 125–325 m layer is 0.8±0.2×10 6 m 3 s −1 . We focus particular attention on the Cape Blanco to Cape Mendocino region, and the NMFS results are compared with shipboard ADCP three weeks later from a study of coastal upwelling processes near Cape Blanco. ADCP streamfunction maps are derived and strongly suggest that one portion of flow is continuous over the 440 km meridional extent of the analysis region. Other portions of the flow show evidence of offshore turning, separation, and the formation of anti-cyclonic eddies. We also note that isopycnic potential vorticity from alongslope CTD stations during the NMFS survey appears to be a tracer for the poleward flow.

A separating coastal upwelling jet at Cape Blanco, Oregon and its connection to the California Current System

The coastal upwelling region near Cape Blanco, Oregon (43°N) off the west coast of the United States was studied using a towed CTD on SeaSoar, a shipboard acoustic Doppler current profiler (ADCP), satellite sea surface temperature maps and surface drifters during August 1995. The equatorward upwelling jet was inshore of the shelfbreak north of Cape Blanco, meandered gently offshore around the Cape, and then veered sharply offshore just to the south of the Cape. Analysis of vertical sections of density, velocity and “spiciness” confirmed the separation of an upwelling jet and front; the jet was followed from over the shelfbreak north of the Cape to 100 km offshore downstream of the Cape. The separating jet originates from the coastal upwelling jet leaving the shelf, but is augmented by an offshore branching of the poleward undercurrent. By combining information from ADCP streamfunction maps with analysis of hydrographic data, including distributions of the tracer-like quantity “spiciness”, a conceptual model of the three-dimensional circulation near the Cape emerges. A mid-shelf upwelling jet encounters a coastal promontory then turns offshore, stretching and deepening as it crosses the continental margin. The jet then turns back shoreward (cyclonically) where the deepened equatorward flow encounters the top of the poleward undercurrent flowing along the continental slope. This causes a portion of the undercurrent to turn offshore to join and strengthen the equatorward transport in the separated jet. Throughout the separation process the coastal upwelling front and jet are continuous, robust features. Separation of a coastal upwelling jet is an important mechanism for cross-shelf transport from the coast to the deep ocean. Cape Blanco appears to be the northernmost point where an equatorward jet regularly separates from the coast to become an oceanic jet and thus serves as the northern boundary of the region where an intense meandering current, characteristic of the California Current System, is found offshore. Drifters released late in the upwelling season are swept offshore and equatorward near the Cape, interact with the spatially complex circulation associated with the separated jet, and then return to the continental margin with the seasonal reversal in winds and near-surface currents.

Direct observations of velocity and transport in the passages between the Intra‐Americas Sea and the Atlantic Ocean, 1984–1996

Shipboard acoustic Doppler current profiler observations of the velocity in the upper 200 m of the water column collected during 1984–1996 using the National Oceanic and Atmospheric Administration R/V Malcolm Baldrige are used to examine the velocity structure and transport in the passages between the Atlantic Ocean and the Intra‐Americas Sea (IAS). Data were collected during 23 cruises along the following sections: across the Straits of Florida, in the Northwest Providence Channel (NWPC), across the northern passages into the Caribbean Sea (Windward, Mona, and Anegada), across the eastern Caribbean along 63°30′ W, thereby forming a closed quadrangle, and in the Grenada Passage. The Florida Current, the eastern Caribbean, and the Grenada Passage share a similar mean velocity structure characterized by high‐velocity, surface‐intensified flows with strong vertical and horizontal shears. The northern Caribbean passages (NWPC, Windward, Mona, and Anegada) share a different common mean velocity structure, with subsurface velocity maxima directed into the IAS, and surface‐intensified counterflows along one side of each passage. On average, there is a transport balance in the upper 200 m between waters entering and exiting the IAS, with the 16.5±2.4 Sv (1 Sv = 106 m3/s) transport of the Florida Current at 27°N comprised of 0.4±0.8 Sv from the NWPC, 2.2±1.5 Sv from the Windward Passage, 2.8±2.1 and 2.4±2.8 Sv from the Mona and Anegada passages, respectively, and 9.5±4.7 Sv across the eastern Caribbean, for a total of 17.3 Sv. The four passages north of 17°N (from NWPC to Anegada Passage) have a combined transport of 7.8 Sv, nearly half of the transport of the Florida Current in the upper 200 m. Of the 9.5 Sv flowing through the eastern Caribbean between 11°N and 17°N, 4.9±2.6 Sv, or more than half, come from the Grenada Passage. This is significant to the subject of cross‐equatorial exchange of mass, heat, and salt, as the Grenada Passage is where the highest transport of waters originating in the southern hemisphere is thought to enter the Caribbean.

On the circulation in the upper layer of the western equatorial Atlantic

Hydrographic observations of pressure, temperature, salinity, dissolved oxygen, and shipboard acoustic Doppler current profiler measurements are used to study the upper layer circulation in the western equatorial Atlantic Ocean, limited to the region bounded by the 10°S and 14°N latitudes between the longitudes 30°W and 52°W. Data were obtained during four World Ocean Circulation Experiment cruises, carried out in January‐March 1993, January‐March 1994, September‐October 1995, and April‐May 1996. In the upper layer, the continuity of the northwestward flowing North Brazil Current along the American continent toward the Caribbean Sea is confirmed in boreal spring. Furthermore, part of the North Brazil Current also continues northwestward in the subthermocline layer during short periods in boreal spring, contrary to previous estimates. The North Equatorial Countercurrent (NECC) is present in boreal spring west of 40°W, fed with water of Northern Hemisphere origin only. The southeastward flowing current observed at 3°N–44°W is fed by the North Brazil Current retroflection and by a cyclonic recirculation of the southern edge of the North Equatorial Current. The upper layer of this current, at 3°N–44°W, feeds the Equatorial Undercurrent (EUC) and the NECC, when its subthermocline layer feeds the EUC and the North Equatorial Undercurrent. Near‐surface eastward flow is present above the EUC during all cruises, yielding to a strong increase of the eastward warm water transport.

Continental Slope Flow Northeast of Taiwan

Hydrographic observations and current measurements with a Shipboard Acoustic Doppler Current Profiler over the continental shelf–slope junction northeast of Taiwan during 10–17 August 1994 allow the construction of the mesoscale flow pattern generated by the collision of the Kuroshio and a stretch of the continental shelf that has turned to run nearly east–west. The pattern is made up of a deflected Kuroshio mainstream to the east, an intrusion of Kuroshio water onto the continental shelf region, a counterclockwise circulation over Mien-Hwa Canyon (MHC) immediately northeast of Taiwan, a deep southwestward countercurrent along the northern wall of MHC, and a seaward outflow of continental shelf water around the northern coast of Taiwan. The hydrography features a cold dome over the west side of MHC that consisted of subsurface Kuroshio water. A temperature–salinity plot of all the station data shows the incorporation in the neighborhood of Taiwan of continental shelf water into the Kuroshio.

Diagnosis of the Three-Dimensional Circulation Associated with Mesoscale Motion in the California Current

A high-resolution upper-ocean survey of a cyclonic jet meander and an adjacent cyclonic eddy in the California Current region near 388N, 1268W was conducted as part of the summer of 1993 Eastern Boundary Currents program. Temperature and salinity were measured from a SeaSoar vehicle, and velocity was measured by shipboard acoustic Doppler current profiler (ADCP). SeaSoar data show a density front at a depth of 70‐100 m with strong cyclonic curvature. The geostrophic velocity fields, referenced to the ADCP data at 200 m, show a strong surface-intensified jet (maximum speed of 0.9 m s 21) that follows the density front along a cyclonic meander. Relative vorticities within the jet are large, ranging from 20.8 f to 11.2 f, where f is the local Coriolis parameter. The SeaSoar density and ADCP velocity data are used to diagnose the vertical velocity via the Q-vector form of the quasigeostrophic omega equation. The diagnosed vertical velocity field shows a maximum speed of 40‐45 m d21. The lateral distribution of vertical velocity is characterized by two length scales: a large (;75 km) pattern where there is downwelling upstream and upwelling downstream of the cyclonic bend, and smaller patches arrayed along the jet core with diameters of 20‐30 km. Geostrophic streamline analysis of vertical velocity indicates that water parcels make net vertical excursions of 20‐30 m over 2‐3 days, resulting in net vertical velocities of 7‐15 m d21. Water parcels moving along geostrophic streamlines experience maximum vertical velocities in the regions of maximum alongstream change in relative vorticity, an indication of potential vorticity conservation.

Diagnosing a meander of the shelf break current in the Middle Atlantic Bight

Two wintertime hydrographic crossings of the shelf break current in the Middle Atlantic Bight are analyzed. Separated by 3 days, the two sections sampled opposite phases of a meander of the current. The shipboard acoustic Doppler current profiler (ADCP) data reveal a strong, convergent jet during the first crossing, transporting 0.32 Sv of water equatorward. During the second crossing the transport is the same but the jet is weak and divergent. The measured Rossby numbers imply that the jet is significantly nonlinear in the convergent state. The associated potential voracity Q distributions differ substantially; the core of the weak jet is characterized by a region of uniform Q, whereas the strong jet contains no such feature. Surface thermal imagery indicates that the leading edge of a steep meander trough was sampled during the first crossing, followed by a broad crest during the second crossing. This is consistent with the convergent versus divergent nature of the flow in the two sections. A nearby Gulf Stream ring likely caused the steepening of the trough. After integrating the spatially low‐passed thermal wind shear, the resulting geostrophic sections are compared to the similarly low‐passed ADCP fields. This, together with the scales of the meander deduced from the surface imagery, indicates that the trough was in gradient wind balance, whereas the crest was predominantly geostrophic. These observations are consistent with the structure and dynamics of modeled baroclinic jets.

Geochemical and carbon isotopic characterization of particles collected in sediment traps from the East China Sea continental slope and the Okinawa Trough northeast of Taiwan

This paper reports results from various geochemical and carbon isotopic analyses of sediment particles collected at different water depths from an array of 5 sediment traps. The traps were deployed at places characteristic of distinct seafloor topography on the East China Sea continental slope and within the Okinawa Trough northeast of Taiwan during different durations from July 1993 to December 1995. Apparent sediment mass flux (ASMF) measurements show that particle fluxes are invariably high and fluctuate greatly within the canyons, as compared to those on the slope and in the Okinawa Trough, suggesting episodic sediment resuspension and a strong bottom transport from the East China Sea continental shelf to the Okinawa Trough within the canyons. The great terrigenous input thus overshadows any appreciable geochemical and isotopic changes in particles collected inside the canyons, which are otherwise seen in particles collected on the slope region where in situ biogenic input is enhanced by the intrusion of the nutrient-laden Kuroshio subsurface waters. In addition, the shipboard Acoustic Doppler Current Profiler (sb-ADCP) survey reveals that the persistent intrusion of the Kuroshio current then developed to a counter-clockwise circulation throughout all depths in the water column on the East China Sea continental slope. The countercurrent sweeps and redistributes the dispersed fine particles across the East China Sea continental slope, funnels massive sediments through the canyons, and deposits them on the northern slope of the Okinawa Trough. Our results are consistent with previous physical and hydrological observations. We thus conclude that all the spatial and temporal variability of geochemical and carbon isotopic characteristics of sediment particles collected in the region northeast of Taiwan can be attributed primarily to the nature of sediments sourced from the East China Sea continental margin, and the in situ biogenic modification in the water columns, as well as the dynamic transport of the dispersed fine particles by the counter flows developed in the region.

Upper layer currents in the western tropical North Atlantic (1989–1991)

Shipboard acoustic Doppler current profiler (ADCP) measurements and hydrographic observations of temperature, salinity, and dissolved oxygen are used to examine the upper water column flow field in the North Brazil Current (NBC) retroflection region of the western tropical Atlantic Ocean. Observations are presented from six cruises, one conducted in August 1989 and the other five conducted during the Western Tropical Atlantic Experiment between January 1990 and September 1991. The upper water column is divided into two layers, an upper thermocline layer located between the surface and the 24.5 σθ isopycnal surface and a lower subthermocline layer located between the 24.5 and 26.75 isopycnals. In the upper layer the NBC retroflects north of the equator to form the eastward flowing North Equatorial Countercurrent (NECC). During the six cruises the retroflection appeared complete. However, data coverage did not extend shoreward of the 200‐m isobath, so the possibility of a continuous flow over the shelf still remains. There were also indications of several NBC rings that had apparently separated from the NBC retroflection and drifted to the northwest toward the eastern Caribbean Sea. North of the NBC retroflection and the NECC, the North Equatorial Current (NEC) flows west as the southern limb of the subtropical gyre. Part of the NEC is observed to retroflect cyclonically to join the eastward NECC flow. In the lower layer, beneath the NBC, the North Brazil Undercurrent retroflects to feed the eastward North Equatorial Undercurrent (NEUC). To the north a deeper component of the NEC recurves to also contribute to the NEUC.

Measurement of surface currents using sequential synthetic aperture radar images of slick patterns near the edge of the Gulf Stream

Two‐dimensional surface currents are estimated over an area of ∼100 km2 near the inshore edge of the Gulf Stream by correlating the surface slick patterns observed on two synthetic aperture radar (SAR) images collected about 20 min apart. The currents obtained from this analysis are found to agree well with shipboard acoustic Doppler current profiler (ADCP) measurements at 10 to 20‐m depths. The orientations of the long, linear slicks observed in this region differ from the mean current direction by about 16° but coincide with the direction of maximum radial deformation or strain, as calculated from the observed current field. The calculated current gradients indicate a mean surface convergence of about 2×10−5 s−1 and a cyclonic vorticity of about 1×10−4 s−1, values which fall within the range of previous estimates in the study area. These results suggest that slick patterns may be used to infer surface current magnitude and direction from a time sequence of SAR images but that care must be exercised in inferring current directions from the orientations of slick patterns within a single image.

Upper ocean currents in the northern Arabian Sea from shipboard ADCP measurements collected during the 1994–1996 U.S. JGOFS and ONR programs

A large upper-ocean velocity data set was obtained using a shipboard acoustic Doppler current profiler (ADCP) on seventeen RV T.G. Thompson cruises during the JGOFS and ONR expedition to the northern Arabian Sea from September 1994 through January 1996. Seven of the cruises followed a large area survey track centered over the Arabian Basin, four cruises conducted SeaSoar surveys on either side of the Findlater jet axis, and six cruises were for the deployment and maintenance of moored instrumentation, together providing some 380 cruise-days and 96,000 track kilometers of coverage. The ADCP data extended over the upper 250–400 m of the water column depending upon the temporal/spatial distributions of acoustic scatterers. The velocity data revealed several items that differed significantly from the historical perspective. Maximum current magnitudes in this area varied from more than 1 m s -1 along the Arabian coast to 10–20 cm s -1 well offshore. Perhaps the most important result was the complete dominance of the velocity field by eddies that had offshore correlation length scales of roughly 100 km, a spectral peak at around 300 km, and kinetic energies that ranged from 70 to more than 90% of the total kinetic energy. The total kinetic energy was highest within about 300 km of the shore and decreased significantly in magnitude and vertical extent offshore. Within the coastal region, the temporal variability was such that currents of 50 cm s -1 or more could completely reverse within a two-week period, seemingly independent of any local atmospheric forcing. Mean and seasonal velocities also differed from historical results. There was a large anti-cyclonic feature located for most of the year south of Ras ash Shabatat (∼58°E) which intensified during the southwest monsoon. There was a strong jet-like current off Ras al Hadd (∼22°N), which also intensified during the southwest monsoon, although itflowed southwestward against the wind. In contrast to the historical ship drift data, which indicated that the surface currents followed the monsoonal winds, the ADCP data over the upper 200–400 m was highly variable, with overall seasonal means that were often directed against the wind. Except within 100–200 km of the Arabian coast, the mean currents were generally toward the southwest with a mean transport in the upper 150 m between the coast and 10°N of approximately 9×10 6 m 3 s -1.

The effect of a cyclonic eddy on the distribution of lithogenic particles in the southern East China Sea

Off northeastern Taiwan, the collision between the Kuroshio Current (KC) and the East China Sea (ECS) shelf forms a branch current (Kuroshio Branch Current) on the shelf, and the counterclockwise deflection of the subsurface Kuroshio water annually generates a cyclonic eddy, about 70 km in diameter, over the shelf-slope in summer. In this study, the total suspended matter collected from varying water depths over the eddy during August 1994 has been analyzed for Al, which is used as an indicator element to trace lithogenic particle transport. Simultaneously, the measurements of current field, using the Shipboard Acoustic Doppler Current Profiler, and hydrography have also been conducted on the same cruise during the seawater sampling. These offer an opportunity for direct examination of the relationships between circulation patterns and distributions of lithogenic particles to further establish a transport model for lithogenic particles. However, we must emphasize that this is a representative case for the export of terrigenous suspensions delivered from rivers, although the observation was made only in summer. The spatial distribution of particulate Al (PAI) in the upper water displays an Ω shape with high concentrations of PAI close to the Taiwan coast and around the eddy center. It suggests that Taiwan-derived riverborne particulates are the main source for the lithogenic particles in the upper water of the study area. Moreover, the PAI in the upper water is essentially constrained by the cyclonic eddy, and the entrainment and subsequent accumulation of lithogenic particles occur at the eddy center. The 6 μg/l contour of PAI in the upper water almost matches the western boundary of the main stream and branch of the KC across the ECS shelf-slope, and two concentration provinces of PAI can be identified from this contour: one to the west is larger and variable and the other to the east is smaller and uniform. This indicates that the main stream and branch of the KC may confine terrigenous particles to the inshore area where mixing of the lithogenic particle-rich coastal water and the clearer Kuroshio water occurs and may act as a barrier to the offshore transport of terrigenous particles to regions farther away from the coast, toward the north and east. In this study, a conceptual model for the transport of lithogenic particles in the water of the southern ECS is proposed according to the spatial distributions of PAI. This model suggests that the cyclonic eddy at the turning site of the KC may be the important conduit for the indirect transport of terrigenous particles from the ECS shelf and slope to the southern Okinawa Trough or the Pacific Ocean. This is also a valuable example of the role of the eddy system in regulating the distribution and transport of terrigenous particulates.

Use of Single-Ping Bottom-Track ADCP Data to Characterize Small-Scale Bathymetry

Abstract A shipboard acoustic Doppler current profiler, in its bottom-track mode, estimates the radial velocity of the transducer relative to the bottom and the range from the transducer to the bottom for each beam of each ping. These data, along with attitude data (pitch, roll, and heading), can be used to map the X, Y, and Z positions of the beam/bottom intercepts. This paper shows how these intercepts can then be used to characterize the size, shape, and orientation of small-scale bathymetric features. The method is applied to data collected over a field of sand waves located 40 km east–northeast of Cape Hatteras, North Carolina, in 30 m of water. Forty-five sand waves were crossed. The sand waves were oriented in an east–west direction, had characteristic heights of 3 m, wavelengths of 230 m, and northern slopes steeper than southern slopes (e.g., 3.8% versus 2.4%). Some sand waves were sampled repeatedly and extended for several kilometers.

Observations of the Ekman Balance at 8°30′ N in the Arabian Sea during the 1995 Southwest Monsoon

The Ekman transport is estimated from two sets of hydrographic and shipboard acoustic Doppler current profiler (ADCP) velocity observations made during June and September 1995, during the southwest monsoon in the Arabian Sea. Both sets of measurements were made along latitude 8°30′ N, designated as World Ocean Circulation Experiment (WOCE) line I1W, from Somalia to Sri Lanka. The Ekman transport estimates calculated from ageostrophic velocity were southward: 17.6 ± 2.4 106 m³ s−1 in June and 7.9 ± 2.7 106 m³ s−1 in September. These direct estimates were in good agreement with those predicted by the Ekman balance using both shipboard and climatological winds. The vertical structure of the ageostrophic velocity and the stratification were quite different between the two occupations of the transect. The wind‐driven momentum was confined to a very shallow layer in June (about 50 m) and the surface layer was strongly stratified, with a maximum salinity layer at depths between 50 and 70 m. The ageostrophic velocity penetrated much deeper in September (to about 160 m) and the pycnocline was correspondingly deeper. In both cases, the Ekman transport penetrated beneath the mixed layer, to the top of the pycnocline.

Anti-cyclonic circulation driven by the estuarine circulation in a gulf type ROFI

Baroclinic residual circulation processes are examined in gulf type Regions Of Freshwater Influence (ROFIs), which have large rivers discharging into a rounded head wider than the Rossby internal deformation radius. Theoretical and observational investigations concentrate on Ise Bay, Japan, with supporting data from Osaka Bay and Tokyo Bay. Simplified analytical solutions are derived to describe the primary features of the circulation. Three dimensional residual current data collected using moored current meters and shipboard acoustic doppler current profilers (ADCPs), satellite imagery and density structure data observed using STDs, are presented for comparison to the theoretical predictions. There are three key points to understanding the resulting circulation in gulf type ROFIs. First, there are likely to be three distinct water masses: the river plume, a brackish upper layer, and a higher salinity lower layer. Second, baroclinic processes in gulf type ROFIs are influenced by the Earth's rotation at first order. Residual currents are quasi-geostrophic and potential vorticity is approximately conserved. Third, the combined effects of a classical longitudinal estuarine circulation and the Earth's rotation are both necessary to produce the resulting circulation. Anti-cyclonic vorticity is generated in the upper layer by the horizontal divergence associated with upward entrainment, which is part of the estuarine circulation. The interaction between anti-cyclonic vorticity and horizontal divergence results in two regions of qualitatively different circulation, with gyre-like circulation near the bay head and uniformly seaward anti-cyclonicly sheared flow further towards the mouth. The stagnation point separating the two regions is closer to (further away from) the bay head for stronger (weaker) horizontal divergence, respectively. The vorticity and spin-up time of this circulation are-(ƒ−ω 1)/2 and h/2w 0 , respectively, where ƒ is the Coriolis parameter, ω 1 is the vorticity of the lower layer, h is the depth of the upper layer and w 0 is the upward entrainment velocity across the pycnocline. Under high discharge conditions the axis of the river plume proceeds in a right bounded direction, describing an inertial circle clearly seen in satellite images. Under low discharge conditions the river plume is deflected in a left bounded direction by the anti-cyclonic circulation of the upper layer.

Observations of strong inertial oscillations after the passage of Tropical Cyclone Ofa

One week after Tropical Cyclone Ofa passed near Samoa, near‐inertial oscillations of almost 1 m s−1 in amplitude were observed with a shipboard acoustic Doppler current profiler (ADCP). The velocity was nearly uniform with depth in the upper 80 m. The strongest currents in this layer were found about 300 km from Ofa's track, farther than expected from previous observations and models. Below 80 m was a layer 50–100 m thick in which the shear exceeded 0.01 s−1. Strong near‐inertial oscillations, with currents over 0.6 m s−1 from 130 to 200 m, extended through the shear layer to at least 300 m, the limit of the ADCP measurements. The phase of the oscillations propagated upward, consistent with downward propagation of energy. A simple slab model driven by an idealized wind field approximately matches the observed amplitude and phase of the oscillations in the surface layer within 150 km of Ofa's track, but not the stronger current 300 km from the track.

Correlation scales, objective mapping, and absolute geostrophic flow in the California Current

The spatial covariances of the time‐dependent density and geostrophic velocity fields off southern California are determined from a unique set of repeated hydrographic observations (44 cruises) made by the California Cooperative Oceanic Fisheries Investigations from 1984 to 1994. The covariances and objective analysis are used to combine direct velocity observations, from shipboard acoustic Doppler current profiler (ADCP) measurements made on a recent survey (October 1993), with hydrographic observations. The analysis reduces ageostrophic noise in the ADCP velocities by smoothing and enforcing horizontal nondivergence; additionally, the velocities are mapped over scales that are dynamically consistent with the hydrography. Maximum surface geostrophic flow in the California Current in October 1993 is about 35 cm s−1, 50% larger than that estimated assuming a 500‐m level of no motion. Absolute flow at 500 m is O(10 cm s−1) and indicates that the surface eddy field penetrates through the thermocline. Uncertainty in the geostrophic reference calculated from the ADCP measurements is of O(4 cm s−1). The velocity residual (objectively analyzed minus raw ADCP estimates) exhibits smaller correlation scales than the geostrophic flow.

Intensive surveys of the Azores Front: 1. Tracers and dynamics

The hypothesis that fronts are sites of active subduction is examined using density, temperature, salinity, and horizontal velocity data from a trio of surveys of the Azores Front done in May 1991 and March 1992. These surveys were made using a SeaSoar equipped with a conductivity‐temperature‐depth profiler and a shipboard acoustic Doppler current profiler. The potential density and potential voracity indicate that dense water from the north side of the front may be sliding down beneath the surface outcrop. This apparently subducting isopycnal has a great deal of temperature and salinity variability. Horizontal velocity is nearly parallel to isopycnals, indicating that the time rate of change and vertical advection must be small. The thermal wind balance is observed to be valid, especially in the region of the largest horizontal density gradients. Shear at the base of the mixed layer is likely due to near‐inertial motions. The potential vorticity is dominated by the planetary vorticity, except at the front, where vertical shears (the tilting term) become large. The tilting term acts to reduce the magnitude of the potential vorticity at the front, in agreement with simple theoretical models. The magnitude of the tilting term is similar to the total vorticity in the seasonal thermocline.