Richard Limeburner

Discharge of the Changjiang (Yangtze River) into the East China Sea

Abstract Hydrographic and current meter data collected in the East China Sea during June 1980, August 1981, and November 1981, have been used to describe the spatial and temporal structure of the Changjiang (Yangtze River) discharge over the inner and mid continental shelf off eastern China. In summer during high runoff, the freshwater discharge near the mouth of the Changjiang exhibited a bimodal distribution, with the freshest water extending in a band to the south along the coast, and a relatively shallow, low salinity plume-like structure extending offshore on average towards the northeast. Relatively warm, saline bottom water characteristic of the Taiwan Current Warm Water flowed into the Changjiang estuary along a submerged relic river valley. Direct current measurements suggest that significant transient offshore fluxes of relatively fresh Changjiang water can also occur from the southward-flowing coastal current south of the Changjiang estuary. In winter, the reduced Changjiang discharge flowed southward in a narrow band confined to the coast. Some possible physical causes for the observed structure of the Changjiang discharge are presented.

Modern Huanghe-derived muds on the outer shelf of the East China Sea: identification and potential transport mechanisms

Abstract Outer shelf muds south of Cheju Island contain calcite in their clay fraction, derived from the Huanghe (Yellow River), not the Changjiang (Yangtze River). Radiometric dating suggests that at least the surface of this mud deposit is contemporaneous, although the rate of accumulation may be slow. Part of the sediment may be derived from present-day erosion of the ancient Huanghe submarine delta off Jiangsu Province. Suspended matter, light transmission, and hydrographic measurements indicate a southsoutheast transport of sediment in the East China Sea during winter, when frequent storms can resuspend sediment and force Yellow Sea water to the southeast along the Chinese coast. The spatial distribution of maximum bottom stress associated with the dominant M2 and M4 tides also suggest that tidal currents could resuspend and transport fine-grained material from the inner shelf as well as from the eastern side of the Yellow Sea. Thus the present accumulation of modern sediment south of Cheju Island probably is the result of two separate transport mechanisms acting in concert.

Annual Cycle and Variability of the North Brazil Current

Abstract Current meter observations from an array of three subsurface moorings located on the Brazil continental slope near 4°N are used to describe the annual cycle and low-frequency variability of the North Brazil Current (NBC). The moored array was deployed from September 1989 to January 1991, with further extension of the shallowest mooring, located over the 500-m isobath near the axis of the NBC, through September 1991. Moored current measurements were also obtained over the adjacent shelf for a limited time between February and May 1990. The NBC has a large annual cycle at this latitude, ranging from a maximum transport of 36 Sv (Sv ≡ 106 m3 s−1) in July–August to a minimum of 13 Sv in April–May, with an annual mean transport of approximately 26 Sv. The mean transport is dominated by flow in the upper 150 m, and the seasonal cycle is contained almost entirely in the top 300 m. Transport over the continental shelf is 3–5 Sv and appears to be fairly constant throughout the year, based on the available...

Comparison of winter and summer hydrographic observations in the Yellow and East China Seas and adjacent Kuroshio during 1986

Abstract Two regional hydrographic surveys conducted in January and July 1986, aboard the R.V. Thompson and R.V. Washington illustrate the seasonal change in water properties from winter to summer in the Yellow and East China Seas (YECS) and adjacent Kuroshio. In January 1986, water over the shelf in the YECS was locally well mixed in the vertical, and the horizontal distribution of water properties was dominated by a large tongue or plume of relatively fresh Yellow Sea Cold Water (YSCW) flowing southeastward along the Chinese margin into the East China Sea. To the east of this plume, along the Korean margin, was found the more saline Yellow Sea Warm Water (YSWW). The Kuroshio front in the East China Sea was located at the shelf break, separating the warmer, more saline Kuroshio water from the relatively well-mixed cooler, less saline coastal water. Evidence of mixing between these two water masses was observed but limited to near the shelf break. In July 1986, water over the shelf in the YECS was strongly stratified everywhere except within tidally mixed areas near the coast. The surface water distribution in the YECS was dominated by a bubble or lens of Changjiang dilute water located to the northeast of the Changjiang mouth, and the bottom YSCW intensified and extended southward to the shelf break. The relatively fresh coastal water from the East China Sea shelf extended far past the shelf break over the Kuroshio near the surface, and in turn, Kuroshio water intruded onto the shelf near the bottom. Mixing between the Kuroshio and coastal water was found over much of the mid- and outer shelf and upper slope, spanning a cross-stream distance of 75 km. The seasonal freshening due to the Changjiang discharge contributed directly to the summer increase in freshwater transport in the upper Kuroshio. In addition, evidence of deep vertical mixing within the Kuroshio itself was found near 32.0°N, 128.2°E, most likely due to a mesoscale eddy found near there and internal tidal mixing over the slope.

Physical oceanography of the Amazon shelf

Abstract The Amazon shelf is subject to energetic forcing from a number of different sources, including near-resonant semi-diurnal tides, large buoyancy flux from the Amazon River discharge, wind stress from the northeasterly tradewinds and strong along-shelf flow associated with the North Brazil Current. Although the large volume of river discharge produces a pronounced salinity anomaly, the water motions on the shelf are dominated by the other forcing factors. Tidal velocities of up to 200 cm s−1 are generally oriented in the cross-shelf direction. Tide-induced mixing influences the position and structure of the bottom salinity front that separates the well-mixed nearshore region from the stratified plume. High concentrations of suspended sediment trapped along the frontal zone increase the stability of the tidal boundary layer and thus reduce the bottom stress. At subtidal frequencies, motion is primarily along-shelf toward the northwest, both in the plume and in the ambient, high-salinity water of the outer-shelf. The plume is generally 5–10 m thick, with a salinity of 20–30 psu. The along-shelf velocity within the plume varies as a function of the along-shelf wind stress. This variability results in large temporal variations in plume structure and freshwater content on the shelf. The net northwestward motion of the Amazon plume and of the ambient shelf water appears to be the result of a large-scale pressure gradient associated with the North Brazil Current system.

Separation of Tidal and Subtidal Currents in Ship-Mounted Acoustic Doppler Current Profiler Observations

A simple method is developed to analyze current measurements obtained from a moving platform. The need for this is motivated by the now common use of the ship-mounted acoustic Doppler current profiler (ADCP) to acquire absolute velocity data during an oceanographic survey of a given region. The full potential of this new measurement technique is severely hindered when the presence of high-frequency phenomena (e.g., tidal or inertial motions) prevents a clear visualization of the lower-frequency current structure of interest. Our analysis method is based on a spatial interpolation scheme, using arbitrary functions, that allows for the tidal current time variability, which then permits the tide-induced motions to be subtracted from the ADCP data to yield the subtidal current field. The method also allows nearby moored and drifter current measurements (if available) to be combined with the shipboard ADCP observations in a single analysis to obtain the best description of the tidal and subtidal currents. To illustrate this method, we present results from the analysis of ADCP data taken during oceanographic surveys in two different continental shelf regions, the East China Sea and the Amazon shelf. A 5-day conductivity-temperature-depth (CTD) and ADCP survey was made in the East China Sea near the mouth of the Yellow Sea during January 1986. There the currents were essentially barotropic and dominated by the semidiurnal tide. The ADCP-derived cotidal chart for the M2 (12.42 hours) component agrees well with existing charts derived empirically from sea level observations or from regional numerical models. The ADCP-derived steady flow is also consistent with the observed density field and indicates little flow in or out of the Yellow Sea and a transport of about 1 Sv toward the Tsushima Strait. Two CTD-ADCP surveys lasting 21 and 23 days were conducted over the Amazon shelf during March and May, 1990. Simultaneously moored current observations were also obtained at three locations along a cross-shelf array 200 km north of the Amazon River mouth, for a common period of at least 2 months (February 12 to April 13). Over the shelf, tidal and subtidal currents were comparable in magnitude. Our analysis indicates that the tidal currents were essentially barotropic, semidiurnal, and polarized in the cross-shelf direction, increasing in magnitude toward the inner shelf where current values of more than 2 m/s are common. The ADCP-derived steady currents were aligned in the along-shelf direction and strongly influenced by the North Brazil Current (NBC). During both ADCP surveys, the northwestward flowing NBC was transporting more than 2 Sv over the shelf at depths shallower than 100 m.

The Amazon River Plume during AMASSEDS: Spatial characteristics and salinity variability

The Amazon River discharge forms a plume of low-salinity water that extends offshore and northwestward over the north Brazilian shelf. Observations acquired as part of A Multidisciplinary Amazon Shelf SEDiment Study (AMASSEDS) are used to characterize the spatial structure and temporal variability of the Amazon Plume. Four shipboard conductivity-temperature-depth (CTD) surveys spanning the shelf from 1°S to 5°N during rising (March 1990), maximum (May 1990), falling (August 1989), and minimum (November 1991) discharge show the Amazon Plume is typically 3 to 10 m thick and 80 to over 200 km wide. Northwest of the river mouth, the plume is often characterized by a wedge of low-salinity water adjacent to the coast and a separate tongue of low-salinity water extending offshore over the middle to outer shelf. A bottom front separating the low-salinity plume water from oceanic water is consistently located between the 10- and 20-m isobaths. A moored array deployed about 300 km northwest of the river mouth from February to June 1990 included inner and midshelf moorings in 18 and 65 m of water on which temperature-conductivity measurements were made. The moored observations reveal salinity variations within the Amazon Plume of over 10 psu on timescales of days to weeks. This variability includes intermittent events in which plume water pools up in the vicinity of the river mouth and the plume width can exceed 200 km. These accumulation events are apparently due to wind events with a southeastward component which impede or block the normally northwestward freshwater transport. The resulting bulges in the plume are then released northwestward when the wind reverses. Volume budgets indicate the Amazon Plume entrains roughly twice the river discharge between the river mouth at the equator and 3°N. Estimates of gradient Richardson numbers from the moorings suggest entrainment, due to the strong semidiurnal tidal currents, occurs where the plume intersects the bottom and over the outer portion of the plume, where salinities approach oceanic values.

A Numerical Study of Stratified Tidal Rectification over Finite-Amplitude Banks. Part I: Symmetric Banks

Abstract Tidal rectification over a two-dimensional finite-amplitude symmetrical bank is studied using the Blumberg and Mellor primitive equation coastal ocean circulation model (ECOM-si). In the homogeneous case, the nonlinear interaction of tidal currents with the variable bottom topography generates an along-isobath residual circulation over the bank, which tends to increase as either the slope or height of the bank is increased. In the stratified case, internal waves at tidal and higher frequencies are generated over the sloping sides of the bank. Tidal mixing occurs in the bottom boundary layer, leading to horizontal tidal mixing fronts. The resulting stratified tidal rectification associated with the tidal mixing front, the generation of internal tides, and the modification of internal friction due to stratification leads to a subsurface intensification of the along-isobath residual current at the front and at the top of the bottom mixed layer over the slope, and a cross-bank double cell circulation...

Variability of the near-surface eddy kinetic energy in the California Current based on altimetric, drifter, and moored current data

Low-pass-filtered velocities obtained from World Ocean Circulation Experiment (WOCE) surface drifters deployed in the California Current off northern California during 1993-1995 have been compared with surface geostrophic velocity estimates made along subtracks of the TOPEX/POSEIDON altimeter and with moored acoustic Doppler current profiler (ADCP) data. To obtain absolute geostrophic velocities, a mean sea surface height (SSH) field was estimated using the mean drifter velocities and historical hydrographic data and was added to the altimetric SSH anomalies. The correlation between collocated drifter and altimetric velocities is 0.73, significant at the 95% level. The component of the drifter velocity which was uncorrelated with the altimetric velocity was correlated with the wind in the Ekman transport sense. Monthly averages of eddy kinetic energy (EKE), estimated using all drifter and altimeter data within the domain (124°-132°W, 33°-40.5°N), show energy levels for the drifters that are about 13% greater than those for the altimeter. Drifter, altimeter, and ADCP measurements all exhibit similar seasonal cycles in EKE, with the altimeter data reaching maximum values of about 0.03 m 2 s -2 in late summer/fall. Wavenumber spectra of the altimeter velocity indicate that the velocity fluctuations were dominated by features with wavelengths of 240-370 km, while the ADCP data suggest that the temporal scales of these fluctuations are at least several months. Between 36° and 40.5°N, the region of monthly maximum EKE migrates westward to about 128°W on a seasonal timescale. This region of maximum EKE coincides with the maximum zonal SSH gradient, with increased EKE associated with increased southward flow. A simple model shows that much of the seasonal cycle of the SSH anomalies can be produced by linear processes forced by the curl of the wind stress, although the model cannot explain the offshore movement of the front.

The M2 tide on the Amazon Shelf

As part of A Multidisciplinary Amazon Shelf Sediment Study (AMASSEDS), moored and shipboard current measurements made over the Amazon shelf during 1990–1991 have been analyzed to determine the dominant semidiurnal tidal constituent, the M2. These results have been combined with coastal sea level data from within the Amazon and Para Rivers, the adjacent shelf, and with satellite-derived tidal elevation data from off the shelf to provide a more complete description of the M2 tide in this complex river/shelf system. Near the Amazon River mouth the M2 tide propagates across the shelf and through the mouth as a damped progressive wave, with its amplitude decreasing and phase increasing upriver. Over the adjacent shelf north of Cabo Norte, the M2 tide approaches a damped standing wave, with large amplitudes (greater than 1.5 m) near the coast due to near resonance within the coastal embayment formed by the Cabo Norte shoal to the south and Cabo Cassipore to the north. The observed M2 tidal currents are nearly rectilinear and oriented primarily across the local isobaths. Comparisons between tidal observations in both the North Channel and the Cabo Norte-Cabo Cassipore embayment and a simple variable-width channel tidal model indicate that (1) most of the M2 tidal energy dissipation occurs over the mid- and inner shelf (in water depths less than 20 m) and (2) fluid muds found there cause a significant reduction (of order 50%) in the effective bottom friction felt by the M2 tide. The approximate resonant period of the Cabo Norte-Cabo Cassipore embayment is 11.9 hours, and at resonance the average energy dissipation per forcing period is roughly 2.2 times the average mechanical energy in the embayment. This damping rate is large enough that the tidal amplification is rather insensitive to forcing frequency, so that the response of the embayment to forcing over the semidiurnal band should be essentially the same. The vertical structure of the M2 tidal current is examined at one outer shelf site located in 65-m water depth. The observed semimajor axis increases logarithmically with height above bottom within the lowest 1–2 m and reaches a maximum in excess of 0.5 m/s at approximately 11 m above bottom. The mean ellipticity is small (less than 0.1) and positive, indicating clockwise rotation of a nearly rectilinear current, and the semimajor axis is oriented within 10° of the local cross-isobath direction. The M2 phase increases with height above bottom, with flood at the bottom leading flood at the surface by about 1 hour. A simple, local homogeneous tidal model with time- and space-dependent eddy viscosity simulates the observed near-bottom velocity reasonably well, however, the model suggests that stratification above the lowest few meters may significantly affect the tidal boundary layer structure at this site. The M2 energy flux onto the Amazon shelf and into the Amazon and Para Rivers has been estimated using current and surface elevation data and the best fit variable-width channel model results. The net M2 energy flux into the mouths of the Amazon and Para Rivers is 0.47×1010W and 0.19×1010W, respectively. A net M2 energy flux of about 3.3×1010W occurs onto the shelf between the North Channel of the Amazon River and Cabo Cassipore. This stretch of the Amazon shelf accounts for about 1.3% of the global dissipation of the M2 tide.