Transport and Fate of River Discharge on the Continental Shelf of the Southeastern United States

We have analyzed 8 years of wind and salinity data from a frontal zone in a region of the inner continental shelf off the southeastern United States. The changes in low‐salinity water stored in the frontal zone have been parameterized by analyzing the monthly rate of change in freshwater content. When the rate of change in freshwater content was negative, we interpreted this as a loss of low‐salinity water from the frontal zone. When this parameter was compared with seasonally averaged alongshore wind stress, the rate of loss was independent of the alongshore wind stress magnitude until threshold of about 0.1 dyne cm−2 was reached. Above the threshold there was a clear relationship between northward alongshore wind stress and rate of loss of freshwater from the inner shelf. Experimental evidence suggests that horizontal currents in the inner‐shelf frontal zone have cyclonic shear with increasing depth. When wind stress is northward and offshore, near‐surface low‐salinity water is transported offshore by Ekman transport while near‐bottom high‐salinity water is transported shoreward

Cape Romain and the Charleston Bump: Historical and Recent Hydrographic Observations

A review and analysis of historical and new hydrographic data are presented for the Charleston Bump region. An area of doming isotherms is identified primarily between 31.5° and 34.5°N and the 200 and 400 m isobaths. The highest incidences of doming are found off Long Bay (86%), Cape Fear (38%), and Cape Romain (25%). Evidence suggests that low salinity shelf water collects in the doming area off Long Bay in July and that seasonal fluctuations in the depth of the main thermocline layer in this area are linked to Gulf Stream transport and local winds. At times there is a gradual offshore‐onshore movement of the Gulf Stream opposite Long Bay roughly following the 400 m isobath and at other times an abrupt eastward movement near 32°N. Much of the time there appears to be a direct seasonal relationship between historical seasonal velocity fields and offshore deflection with higher (lower) velocities corresponding to greater (lesser) deflection

Coastal Upwelling Off The Rias Bajas, Galicia, Northwest Spain I: Hydrographic Studies

Coastal upwelling occurs off the Rias Bajas of Spain between April and October. Superimposed on large-scale upwelling is a mesoscale regime of enhanced upwelling induced by topographic influences. We show that the region surrounding Cape Finisterre has intensified topographically induced upwelling. The rias have a relatively unobstructed connection with the open ocean. Upwelling of high nitrate water on the continental shelf by Ekman transport sets up pressure gradients at the mouth of the rias that induce upwelled water to flow into the rias. Deep water in the rias responds directly to cycles of upwelling and downwelling on the continental shelf

Influences of physical oceanographic processes on chlorophyll distributions in coastal and estuarine waters of the South Atlantic Bight

Coastal and estuarine waters of the South Atlantic Bight are highly productive, with primary production of 600-700 gC/m2/y. While controls and fate of this production are conceptually well understood, the importance of meteorology and physical circulation processes on phytoplankton has not received equivalent attention. Here, we describe the effects of wind stress and tidal currents on temporal and spatial distributions of phytoplankton biomass represented as chlorophyll a (chl a). Moored instruments were deployed and shipboard sampling was conducted in the North Edisto estuary (South Carolina) and adjacent inner shelf waters during four, two-week field studies in May and August 1993, and June and September 1994. Local wind regimes induced upwelling- and downwelling-favorable conditions which strengthened or reduced vertical density stratification in the coastal frontal zone, respectively, and shifted the location of the front. Chl a in shelf waters was more or less homogenous independent of the wind regime, while chl a on the estuary delta was generally vertically stratified. Within the estuary, chl a concentrations were positively correlated with the alongshore component of wind stress; chl a was not correlated with the weaker cross-shelf component of wind stress. Highest chl a occurred during strong downwelling-favorable events. The quick response time to wind forcing (6-12 hrs) implied a direct effect on chl a distributions and not a stimulation of growth processes. The source of the elevated chl a in response to wind forcing was apparently resuspension of settled and epibenthic algal cells. Tidal currents also influenced the vertical distribution and concentration of chl a. Time series sampling on the estuary delta showed that, with increasing velocity of ebb and flood tide currents, the relative contributions of pennate and centric diatoms with attached detritus and sand grains also increased, indicating that tidal resuspension of settled and epibenthic microalgae also occurred. Vertical stratification of chl a (highest concentrations near the bottom) began to degrade upon mixing by tidal currents with velocities as low as 10 cm/sec. Homogenization of 5-7 m water columns was fully achieved at velocities of 20-30 cm/sec. The data document the direct and comparatively immediate (timescales of minuteshours) impact of tidal and wind energy on concentrations and distribution patterns of phytoplankton in coastal and estuarine waters of the South Atlantic Bight

Impacts of Atmospheric Nitrogen Deposition and Coastal Nitrogen Fluxes on Oxygen Concentrations in Chesapeake Bay

Although rivers are the primary source of dissolved inorganic nitrogen (DIN) inputs to the Chesapeake Bay, direct atmospheric DIN deposition and coastal DIN concentrations on the continental shelf can also significantly influence hypoxia; however, the relative impact of these additional sources of DIN on Chesapeake Bay hypoxia has not previously been quantified. In this study, the estuarine‐carbon‐biogeochemistry model embedded in the Regional‐Ocean‐Modeling‐System (ChesROMS‐ECB) is used to examine the relative impact of these three DIN sources. Model simulations highlight that DIN from the atmosphere has roughly the same impact on hypoxia as the same gram‐for‐gram change in riverine DIN loading, although their spatial and temporal distributions are distinct. DIN concentrations on the continental shelf have a similar overall impact on hypoxia as DIN from the atmosphere (~0.2 mg L−1); however, atmospheric DIN impacts dissolved oxygen (DO) primarily via the decomposition of autochthonous organic matter, whereas coastal DIN concentrations primarily impact DO via the decomposition of allochthonous organic matter entering the Bay mouth from the shelf. The impacts of atmospheric DIN deposition and coastal DIN concentrations on hypoxia are greatest in summer and occur farther downstream (southern mesohaline) in wet years than in dry years (northern mesohaline). Integrated analyses of the relative contributions of all three DIN sources on summer bottom DO indicate that impacts of atmospheric deposition are largest in the eastern mesohaline shoals, riverine DIN has dominant impacts in the largest tributaries and the oligohaline Bay, while coastal DIN concentrations are most influential in the polyhaline region

Variability of Surface Pigment Concentrations in the South Atlantic Bight

A 1‐year time sequence (November 1978 through October 1979) of surface pigment images from the South Atlantic Bight (SAB) was derived from the Nimbus 7 coastal zone color scanner. This data set is augmented with in situ observations of hydrographic parameters, freshwater discharge, sea level, coastal winds, and currents for the purpose of examining the coupling between physical processes and the spatial and temporal variability of the surface pigment fields. The SAB is divided into three regions: the east Florida shelf, the Georgia‐South Carolina shelf and the Carolina Capes. Six‐month seasonal mean pigment fields and time series of mean values within subregions were generated. While the seasonal mean isopleths were closely oriented along isobaths, significant differences between seasons in each region were found to exist. These differences are explained by correlating the pigment time series with physical parameters and processes known to be important in the SAB. Specifically, summertime concentrations between Cape Romain and Cape Canaveral were greater than those in winter, but the opposite was true north of Cape Romain. It is suggested that during the abnormally high freshwater discharge in the winter‐spring of 1979, Cape Romain and Cape Fear were the major sites of cross‐shelf transport, while the cross‐shelf exchange during the fall of 1979 occurred just north of Cape Canaveral. Finally, the alongshore band of high pigment concentrations increased in width throughout the year in the vicinity of Charleston, but near Jacksonville it exhibited a minimum width in the summer and a maximum width in the fall of 1979

Hydrographic Variability of Southeastern United States Shelf and Slope Waters During the Genesis of Atlantic Lows Experiment: Winter 1986

Continental shelf waters are particularly responsive to winter storm events mainly because of their shallow depths. Those of the southeastern United States (the South Atlantic Bight (SAB)) are especially responsive because they are broad and shallow. Also, the Gulf Stream serves as a continual source of warm water at the outer boundary. Thus the SAB receives strong meteorological (wind stress and heat loss) and oceanographic (advective) forcing. During the Genesis of Atlantic Lows Experiment (GALE) the response of shelf waters to winter storm events and Gulf Stream forcing was observed. The mean conditions showed a mixed water column with areas of stratification near the coast and at the shelf break. The nearshore area was stratified only during weak offshore winds, and the shelf break area was stratified during southward winds with accompanying onshore Ekman flow. On the inner shelf, advective buoyancy flux was similar in value to heat flux buoyancy and the buoyancy equivalent of wind mixing. Over the shelf break the advective buoyancy flux was 4 times the other forms of buoyancy flux and controlled the observed potential energy variability. A simple box model heat budget used to separate the effect of Gulf Stream eddies and meanders, and Ekman flow and air‐sea heat exchange on the shelf heat content showed that the observed heat content variability was caused by intrusion of Gulf Stream water. The intrusions may be caused either by onshore Ekman flow during southward winds or Gulf Stream meander events

Chandra Observation of the Cluster Environment of a WAT Radio Source in Abell 1446

Wide-angle tail (WAT) radio sources are often found in the centers of galaxy clusters where intracluster medium (ICM) ram pressure may bend the lobes into their characteristic C-shape. We examine the low redshift (z=0.1035) cluster Abell 1446, host to the WAT radio source 1159+583. The cluster exhibits possible evidence for a small-scale cluster-subcluster merger as a cause of the WAT radio source morphology. This evidence includes the presence of temperature and pressure substructure along the line that bisects the WAT as well as a possible wake of stripped interstellar material or a disrupted cool core to the southeast of the host galaxy. A filament to the north may represent cool, infalling gas that's contributing to the WAT bending while spectroscopically determined redshifts of member galaxies may indicate some component of a merger occurring along the line-of-sight. The WAT model of high flow velocity and low lobe density is examined as another scenario for the bending of 1159+583. It has been argued that such a model would allow the ram pressure due to the galaxy's slow motion through the ICM to shape the WAT source. A temperature profile shows that the cluster is isothermal (kT= 4.0 keV) in a series of annuli reaching a radius of 400 kpc. There is no evidence of an ongoing cooling flow. Temperature, abundance, pressure, density, and mass profiles, as well as two-dimensional maps of temperature and pressure are presented.Comment: 40 AASTeX pages including 15 postscript figures; accepted for publication in Ap

UGC 7388: a galaxy with two tidal loops

We present the results of spectroscopic and morphological studies of the galaxy UGC7388 with the 8.1-m Gemini North telescope. Judging by its observed characteristics, UGC7388 is a giant late-type spiral galaxy seen almost edge-on. The main body of the galaxy is surrounded by two faint (\mu(B) ~ 24 and \mu(B) ~ 25.5) extended (~20-30 kpc) loop-like structures. A large-scale rotation of the brighter loop about the main galaxy has been detected. We discuss the assumption that the tidal disruption of a relatively massive companion is observed in the case of UGC7388. A detailed study and modeling of the observed structure of this unique galaxy can give important information about the influence of the absorption of massive companions on the galactic disks and about the structure of the dark halo around UGC7388.Comment: 8 pages, 5 figure