Jackson O. Blanton

Tidal current asymmetry in shallow estuaries and tidal creeks

Abstract We examine tidal asymmetry in a 100-km long coastal plain estuary, a 15-km long tidal creek closed at the end, and a small side channel of a coastal plain estuary closed at low water. Current meter and water depth data were harmonically analyzed for M2 and the M4 and M6 overtide. The results are displayed in a plot of tidal current versus water level and the effects of the M2, M2+M4, and M2+M4+M6 are compared in order to show how overtides affect tidal asymmetry in the different systems. The long estuary showed a gradual increase in tidal current asymmetry. The current takes on a more progressive nature as it goes into the estuary, as indicated by increasing lags between high or low water and the corresponding slack water. On the other hand, the tidal creek has its reflection point at the head, a relatively short distance from the observations, which reflects the incident tidal wave more efficiently. The tidal creek has a high salt marsh which is covered only an hour or so before high water. Since its intertidal areas flood and drain near the time of high water, maximum flood and ebb currents occur a short time before and after high water slack throughout the tidal creek. Most of the tidal energy is dissipated at the end of the creek where M6/M2 of the tidal current almost doubles. The small side channel has tidal flats that become covered soon after the time of low water. The relative time of maximum flood and ebb currents occurs much closer to the time of low-water-slack, which contrasts significantly to the timing in the tidal creek channel surrounded by high salt marsh.

Phytoplankton dynamics within Gulf Stream intrusions on the southeastern United States continental shelf during summer 1981

Abstract During July and August 1981 subsurface intrusion of upwelled nutrient-rich Gulf Stream water was the dominant process affecting temporal and spatial changes in phytoplankton biomass and productivity of the southeastern United States continental shelf between 29 and 32°N latitude. Intruded waters in the study area covered as much as 10 1 km including virtually all of the middle and outer shelf and approximately 50% of the inner shelf area. Within 2 weeks following a large intrusion event in late July, middle shelf primary production and Chl a reached 3 to 4 gC m − d −1 and 75 mg m − , respectively. At the peak of the bloom 80% of the water column primary production occurred below the surface mixed-layer, and new primary production (i.e., NO 3 -supported) exceeded 90% of the total. Chl a-normalized photosynthetic rates were very high as evidenced by high mean assimilation number (15.5 mg C mg Chl a −1 h −1 ), high mean α (14 mg C mg Chl a −1 Ein −1 m), and no photoinhibition. As a result of the high photosynthetic rates, mean light-utilization index (Ψ) was 2 to 3 times higher than reported for temperature sub-arctic and arctic waters. The results imply a seasonal (June to August) middle shelf production of 150 g C m −1 , about 15% higher than previous estimates of annual production on the middle shelf. Intrusions of the scale we observed in 1981 may not occur every summer. However, when such events do occur, they are by far the most important processes controlling summer phytoplankton dynamics of the middle and outer shelf and of the inner shelf in the southern half of the study area.

Composition, productivity and nutrient chemistry of a coastal ocean planktonic food web

A 3 year field study was conducted to investigate patterns, magnitude and variability of primary production; the abundance, biomass and composition of producers and consumers; and the relative importance of physical and chemical variables associated with these parameters, in inner shelf waters of the South Atlantic Bight. Discrete interval, time series and continuous measurements were made along a transect and at two process-oriented stations during summer and winter 1985–1988. A quasi-permanent density front constrains low salinity (<34‰) waters to within ca 10 km of the coast. These waters contain abundant autotrophic and heterotrophic communities. Primary production is high, 6–7 × 102gC m−2 year−1, and is apparently subsidized by rapid nutrient recycling in the water column, sediments and adjacent salt marshes. Silicate is notable for its excess concentrations year-round and supports substantial diatom productivity. Correlation analyses suggest that Si strongly influences phytoplankton biomass, whereas growth rates are coupled to availability of light and NH4. Despite evidence of considerable variability in primary production over daily to interannual scales, plankton biomass is relatively constant. Experimental studies suggest a tight coupling between primary producers and microconsumers, and support the hypothesis that substantial fractions of primary and secondary production are recycled within the water column.

Circulation, mixing, and exchange processes in the vicinity of tidal inlets: A numerical study

The circulation in the vicinity of an idealized tidal inlet connecting a continental shelf and a coastal sound is examined. The circulation is forced by an M2 tide and a weakly buoyant discharge. The buoyant discharge forms a plume in the coastal ocean and induces a distinct anticyclonic circulation at the plume edge that is maintained throughout the tidal cycle. We focus on the plumes onset and its evolution over 5–10 tidal cycles. Over the timescales considered, the plume was roughly circular, slightly skewed in the along-coast direction. The model solution yielded high vertical Ekman numbers Eϵ ∼ O(5) in the vicinity of the inlet mouth, decreasing seaward from the inlet to an order of magnitude smaller (Eϵ ∼ 0.25) at the seaward edge of the plume. Passive particles released in the region seaward of the inlet mouth were used to describe the exchange between the coastal region and the inlet. A marked asymmetry between ebb and flood flows is observed in the vicinity of the inlet, with jet-like ebbing currents and weaker potential-flow-like flooding currents. Over a tidal cycle, net exchanges between the coastal ocean and the inlet are found to be spatially and temporally dependent; that is, particle trajectories depend on the release point and the time of the release in the tidal cycle. The near-inlet residual circulation shows significant differences in the absence of stratification.

A numerical study of the continental shelf circulation of the U.S. South Atlantic Bight during the autumn of 1987

Abstract The autumn circulation on the inner- and mid-shelf of the U.S. South Atlantic Bight (SAB) is examined numerically. Using data collected in 1987 during the Fall Experiment (FLEX) the alongshore structure of the currents and the coastal sea level fluctuations were found to be correlated to local winds which were strong and persistently northeasterly. The observed inshore distribution of freshwater during FLEX, characterized by the presence of a coastal front confined to the coast inside the 25 m isobath, reflects the local autumn discharge subjected to strong and persistent downwelling winds. The freshwater signal found outside the 25 m isobath is suggested to be the previous summers discharge advected northward by the summer winds subsequently returning south forced by the autumn winds.

Physical processes controlling the formation, evolution, and perturbation of the low-salinity front in the inner shelf off the southeastern United States: A modeling study

Physical processes that control the formation, evolution, and perturbation of the low-salinity front over the inner shelf of the South Atlantic Bight have been examined using a fully three-dimensional primitive equation and turbulent closure model. The model was forced by semidiurnal tides (M2, S2, and N2), climatological means of multiple river discharges, and upwelling-favorable wind. This model has provided a reasonable simulation of the fortnightly and monthly variations of semidiurnal tides. Computed amplitudes and phases of tides show good agreement with observational data available at tidal gauge stations along the coast. Spatial structures of buoyancy currents are significantly modified by stratified tidal rectification. As a result, the southward residual current intensifies significantly at the front and reduces or reverses close to the coast. A weak velocity area forms in the downstream region of each river, which blocks the low-salinity water to form multiple “tongue-like” domes in the inner shelf. For a given springtime climatological upwelling-favorable wind, isolated low-salinity lenses can form episodically in two steps. At first, a geometrically controlling wave-like frontal shape develops at the outer edge of the frontal zone as a result of the interaction between tides, multiple river discharges, and upwelling-favorable wind. Then, isolated low-salinity lenses form at the crest when water on the shoreward side of the crest is displaced by relatively high salinity water advected from the upstream trough south of the crest and diffused upward from the deeper region. Wind-induced upwelling is noticeable to compensate for the water loss due to the near-surface offshore Ekman transport, but it does not play a critical role in the formation of isolated lenses under the climatological conditions of river discharges and upwelling-favorable winds over the inner shelf of the South Atlantic Bight.

Summer upwelling on the southeastern continental shelf of the U.S.A. during 1981Hydrographic observations

During the summer of 1981 a large scale sampling effort took place in the shelf waters of the southeastern United States. The goal of this effort was to quantify the effect of the intrusion of deep Gulf Stream water into these shallow, euphotic waters. By making repeated hydrographic measurements over the entire shelf area, the actual volume of the intrusions was determined. Two main intrusion events were observed: one in June and early July and a second in late July and early August. The intruding water entered the shelf in the region south of St Augustine and was transported northward by the mean northward shelf circulation. The subsurface cold water mass was isolated from the Gulf Stream by a ridge of warmer water along the shelf break. The intrusions resulted in increased stability in shelf waters with vertical temperature gradients as high as 10°C m−1. The potential energy in the stratified shelf waters typically ranged from 1000 to 2000 J m−2 that would require 2 to 4 mW m−2 to mix. However, since only 0.1 to 1.0mW m−2 was available from wind and tidal mixing, the water column stayed stratified in most cases. The amount of nitrate transported onto the shelf was determined by two methods: direct observation by synoptic cruises and transport measurements using current meter arrays. Both methods gave similar results with about 18,000 tons nitrogen in the large June/July intrusion. On a seasonal basis, 40,000 tons of nitrogen could be advected into shelf waters which could result in 200,000 to 400,000 tons carbon production.

Transport of salt and suspended sediments in a curving channel of a coastal plain estuary: Satilla River, GA

Abstract This study describes the transport of salt and suspended sediment in a curving reach of a shallow mesotidal coastal plain estuary. Circulation data revealed a subtidal upstream bottom flow during neap tide, indicating the presence of a gravitational circulation mode throughout the channel. During spring tide, landward bottom flow weakened considerably at the upstream end of the channel and changed to seaward in the middle and downstream areas of the reach, suggesting the importance of tidal pumping. Salt flux near-bottom was landward at both ends of the channel during neap tide. At spring, however, the salt flux diverged along the bottom of the thalweg suggesting that tidal pumping caused a transfer of salt vertically and laterally into the intertidal zone. Thus, landward flux of salt is maintained even in the presence of subtidal seaward flow along the bottom at the downstream end of the channel. Landward bottom stress is greater than seaward stress, preferentially transporting suspended sediments upstream. Compared with salt, however, the weight of the suspended sediments causes less upward transfer of sediments into the intertidal zone. Flood flow carried more suspended sediments landward at the upstream end compared with the downstream end. We speculate that secondary flow in the curving channel picks up increasing amounts of suspended sediments along the sides during flood and adds them to the axial flow in the thalweg. Since the landward flow along the bottom of the thalweg weakens and even reverses during spring tide, there appears to be a complex re-circulation system for sediments re-suspended in curving channels that complicates the picture of a net transport of sediments landward.

Shoreward Intrusion of Upper Gulf Stream Water onto the U.S. Southeastern Continental Shelf

Abstract In winter, cooling of the South Atlantic Bight continental shelf water results in higher density in the middle shelf region relative to the shelf-break region where the western flank of the Gulf Stream flows. Shoreward, estuarine-like intrusion of the upper Gulf Stream water in the presence of such a positive onshore density gradient is then possible through advective processes triggered either by the meander of the Stream or onshore Ekman transports by southward wind stresses. Repeated cross-shelf hydrographic transects were conducted from 10 January through 30 January 1986 to more closely study this intrusion process. These observations show many features predicted by a previous numerical model study. A semi-empirical model is proposed here wherein the state of stratification of water on the outer continental shelf region just inshore of the shelf break is used as an indicator of the intrusion process. Model analysis suggests correlating the observed time rate of change of potential energy of t...

The Use of Land and Sea Based Wind Data in a Simple Circulation Model

Abstract The use of land based wind data in nearshore oceanographic work is common, but these winds do not accurately reflect coastal oceanic winds. Ocean winds are often underestimated by a factor of 2 and directional differences are also observed. Wind time series from land and sea regimes in the South Atlantic Bight (SAB) were applied to a reduced form of the momentum equation to estimate the alongshore current. Currents were closely approximated by ocean wind stress, but were consistently underestimated by land data. Further statistical analyses verified this discrepancy in speed and also indicated significant differences between ocean and speed-adjusted land winds. The bottom frictional coefficient required to balance alongshore momentum was unrealistically small when land based wind data were used as input.