Along-shelf Direction Research Articles

The Mid-Atlantic Bight Dissolved Inorganic Carbon System Observed in the March 1996 DOE Ocean Margins Program (OMP)—A Baseline Study

The United States Department of Energy (DOE)’s Ocean Margins Program (OMP) cruise EN279 in March 1996 provides an important baseline for assessing long-term changes in the carbon cycle and biogeochemistry in the Mid-Atlantic Bight (MAB) as climate and anthropogenic changes have been substantial in this region over the past two decades. The distributions of O2, nutrients, and marine inorganic carbon system parameters are influenced by coastal currents, temperature gradients, and biological production and respiration. On the cross-shelf direction, pH decreases seaward, but carbonate saturation state (ΩArag) does not exhibit a clear trend. In contrast, ΩArag increases from north to south, while pH has no clear spatial patterns in the along-shelf direction. In order to distinguish between the effects of physical mixing of various water masses and those of biological activities on the marine inorganic carbon system, we use the potential temperature-salinity diagram to identify water masses, and differences between observations and theoretical mixing concentrations to measure the non-conservative (primarily biological) effects. Our analysis clearly shows the degree to which ocean margin pH and ΩArag are regulated by biological activities in addition to water mass mixing, gas exchange, and temperature. The correlations among anomalies in dissolved inorganic carbon, phosphate, nitrate, and apparent oxygen utilization agree with known biological stoichiometry. Biological uptake is substantial in nearshore waters and in shelf-slope mixing areas. This work provides valuable baseline information to assess the more recent changes in the marine inorganic carbon system and the status of coastal ocean acidification.

Amazon water lenses and the influence of the North Brazil Current on the continental shelf

The exchange processes on the Amazon continental shelf in northern Brazil are subject to complex interactions that involve forcings derived from distinct sources. The Amazon shelf is a unique and highly dynamic environment in which considerable discharge of freshwater enters the Atlantic Ocean, producing extensive Amazon Water Lenses (AWL). In addition to the presence of the AWL, the shelf is influenced by the semidiurnal oscillations of the tides and the strong North Brazil Current (NBC), a boundary current of the western Atlantic. The present study was based primarily on the influence of the freshwater input and the NBC on the shelf and the Amazon Shelf Break (ASB) off the mouth of the Pará River. For this purpose, hydrographic and hydrodynamic data were obtained by moorings of the AMANDES Project (April–July 2008), located on the Amazon shelf and the ASB. Spectral analysis and the continuous wavelet transform were applied to define tidal (high frequency/short period) and subtidal (low frequency/long period) signals. The results indicated that on both the shelf and the break, the semidiurnal tides are responsible for the residual landward transport and are predominantly across-shelf. Low-frequency motions in the synoptic bands and the AWL are related to spatial changes in the velocity field, mainly on the ASB in the along-shelf direction. The flow of the NBC can be interpreted as an along-shelf low-frequency oscillation capable of altering the spatial configuration of the velocity field, although its influence is perceived only in the absence of the AWL.

Gap winds and their effects on regional oceanography Part II: Kodiak Island, Alaska

Frequent gap winds, defined here as offshore-directed flow channeled through mountain gaps, have been observed near Kodiak Island in the Gulf of Alaska (GOA). Gap winds from the Iliamna Lake gap were investigated using QuikSCAT wind data. The influence of these wind events on the regional ocean was examined using satellite and in situ data combined with Regional Ocean Modeling System (ROMS) model runs. Gap winds influence the entire shelf width (> 200km) northeast of Kodiak Island and extend an additional ~150km off-shelf. Due to strong gradients in the along-shelf direction, they can result in vertical velocities in the ocean of over 20md−1 due to Ekman pumping. The wind events also disrupt flow of the Alaska Coastal Current (ACC), resulting in decreased flow down Shelikof Strait and increased velocities on the outer shelf. This disruption of the ACC has implications for freshwater transport into the Bering Sea. The oceanographic response to gap winds may influence the survival of larval fishes as Arrowtooth Flounder recruitment is negatively correlated with the interannual frequency of gap-wind events, and Pacific Cod recruitment is positively correlated. The frequency of offshore directed winds exhibits a strong seasonal cycle averaging ~7 days per month during winter and ~2 days per month during summer. Interannual variability is correlated with the Pacific North America Index and shows a linear trend, increasing by 1.35 days per year. An accompanying paper discusses part I of our study (Ladd and Cheng, 2016) focusing on gap-wind events flowing out of Cross Sound in the eastern GOA.

Assessment of ice flow dynamics in the zone close to the calving front of Antarctic ice shelves

AbstractWe investigate the relationship between four ice-shelf characteristics in the area close to the calving front: ice flow speed, strain rate, ice thickness and shelf width. Data are compiled for these glaciological parameters at the calving fronts of 22 Antarctic ice shelves. Clarification concerning the viscous supply of ice to the calving front is sought following the empirical calving law of Alley and others (2008), derived from a similar but smaller dataset, and the scaling analysis of Hindmarsh (2012). The dataset is analysed and good agreement is observed between the expected theoretical scaling and geophysical data for the flow of ice near the calving front in the case of ice shelves that are laterally confined and have uniform rheology. The lateral confinement ensures flow is aligned in the along-shelf direction, and uniform rheological parameters mean resistance to flow is provided by near-stationary ice in the grounded margins. In other cases, the velocity is greater than predicted, which we attribute to marginal weakening or the presence of ice tongues.

Subinertial and tidal currents on the Abrolhos Bank shelf

Analysis of simultaneous current measurements in the middle (point PA1) and outer (point PA2) Abrolhos Bank shelf showed different characteristics and forcing mechanisms between the two shelf regions. The mean currents at both positions point generally southwestward but are weak due to the high time variability; at PA1, the mean along-shelf currents are almost one order of magnitude larger than the mean cross-shelf currents, while at PA2, both mean current components exhibit the same order of magnitude. At both points, the tidal currents are more important in the cross-shelf direction than they are in the along-shelf direction. The subinertial currents are mainly barotropic and along-shelf at PA1, while at PA2, the baroclinic first mode dominates the subinertial variability, especially in the cross-shelf direction, though the currents are also more energetic in the along-shelf direction. Correlations are significant between the along-shelf subinertial currents and synoptic winds at PA1, showing a local response (almost in phase) to the wind stress. At PA2, the correlations between winds and currents are generally not significant, showing that the subinertial current variability near the shelf break is forced by other mechanisms; the mesoscale variability of the Brazil Current is most likely important at this position. A simple frictional balance model allows a dynamical interpretation of the significant correlations between winds and currents at PA1.

Extratropical storm inundation testbed: Intermodel comparisons in Scituate, Massachusetts

[1] The Integrated Ocean Observing System Super-regional Coastal Modeling Testbed had one objective to evaluate the capabilities of three unstructured-grid fully current-wave coupled ocean models (ADCIRC/SWAN, FVCOM/SWAVE, SELFE/WWM) to simulate extratropical storm-induced inundation in the US northeast coastal region. Scituate Harbor (MA) was chosen as the extratropical storm testbed site, and model simulations were made for the 24–27 May 2005 and 17–20 April 2007 (“Patriot's Day Storm”) nor'easters. For the same unstructured mesh, meteorological forcing, and initial/boundary conditions, intermodel comparisons were made for tidal elevation, surface waves, sea surface elevation, coastal inundation, currents, and volume transport. All three models showed similar accuracy in tidal simulation and consistency in dynamic responses to storm winds in experiments conducted without and with wave-current interaction. The three models also showed that wave-current interaction could (1) change the current direction from the along-shelf direction to the onshore direction over the northern shelf, enlarging the onshore water transport and (2) intensify an anticyclonic eddy in the harbor entrance and a cyclonic eddy in the harbor interior, which could increase the water transport toward the northern peninsula and the southern end and thus enhance flooding in those areas. The testbed intermodel comparisons suggest that major differences in the performance of the three models were caused primarily by (1) the inclusion of wave-current interaction, due to the different discrete algorithms used to solve the three wave models and compute water-current interaction, (2) the criterions used for the wet-dry point treatment of the flooding/drying process simulation, and (3) bottom friction parameterizations.

Sub-inertial characteristics of the surface flow field over the shelf of the central Mid-Atlantic Bight

Observations of surface velocity data from August 2002 to February 2004 were collected by a series of four long-range high-frequency (HF) radars along the coast of New Jersey. The shelf observations of the central Mid-Atlantic Bight (MAB) were compared to historical observations of surface flow characteristics in the area. The time-averaged spatial mean velocity of 4 cm/s in the down-shelf along-shelf direction and 3 cm/s in the offshore across-shelf direction compared very well to historical surface measurements in the study region. However, as the spatial resolution of the data set revealed, this simple measure masked significant spatial variations in the overall and seasonal mean flow structures. Three regions – the south bank of the Hudson Shelf Valley, the southern New Jersey inner shelf (LEO-15) region, and the region offshore of the Delaware Bay mouth (southwest corner) – had mean flows that favor offshore transport of surface water. In terms of temporal variability, maps of the principle axes showed that the across-shelf (minor) axis contribution was not insignificant in the surface layer ranging from 0.3 to 0.9 of along-shelf (major) axis and that there were seasonal differences in orientation and ellipticity. Analysis of the spatial changes in the temporal and spatial correlation scales over the shelf showed that shelf position, in addition to site separation, contributed to the differences in these properties. Furthermore, observations over the Hudson Shelf Valley region suggested that this was a region of transition in which the orientation of along- and across-shelf components begin to change.

Satellite-measured temporal and spatial variability of the Tokachi River plume

River plumes have important effects on marine ecosystems. Variation in the extent and dispersal of river plumes is often associated with river discharge, wind characteristics and ocean circulation. The objectives of this study were to identify the Tokachi River plume by satellite, determine its relationship with river discharge and clarify its temporal and spatial dynamics. SeaWiFS multispectral satellite data (normalized water-leaving radiance: n L w) with 1.1 km spatial resolution were used to determine the spatial and temporal variability of the plume during 1998–2002. Supervised maximum likelihood classification using six channels of n L w at 412, 443, 490, 510, 555 and 670 nm with each band's spectral signature statistic was used to define classes of surface water and to estimate the plume area. Supervised maximum likelihood classification separated three to four classes of coastal water based on optical characteristics as a result of wind stress events. The satellite-observed plume area was correlated with the amount of river discharge from April to October. The plume distribution patterns were influenced by wind direction and magnitude, the occurrences of a near-shore eddy field and surface currents. Empirical orthogonal function (EOF) was used to express the spatial and temporal variability of the plume using anomalies of n L w(555) monthly averaged images. The first mode (44% of variance) showed the turbid plume distribution resulting from re-suspension by strong wind mixing along the coast during winter. This mode also showed the plume was distributed along-shelf direction in spring to early autumn. The second mode (17% of variance) showed spring pattern across-shelf direction. EOF analysis also explained the interannual variability of the plume signature, which might have been affected by the flow of the Oyashio Current and the occurrence of a near-shore eddy field.

Sediment transport on the mid-continental shelf in Onslow Bay, North Carolina during Hurricane Isabel

Hurricane Isabel made landfall on the southeastern U.S. coast along the lower Outer Banks of North Carolina near Cape Lookout, NC on September 18, 2003. An instrumented quadrapod frame moored approximately 43.5 km off the coast of Wilmington, NC in Onslow Bay, at a depth of approximately 30 m, was present on the shelf when the hurricane passed approximately 160 km from Onslow Bay. The attached instrumentation includes a downward-looking Pulse-Coherent Acoustic Doppler Profiler (PC-ADP) and an upward-looking Acoustic Doppler Current Profiler (ADCP). Simultaneous measurements of flow velocities from the surface to the seabed along with acoustic backscatter measurements and seabed elevation were obtained during this storm event. Three days prior to the direct effects of the hurricane, long-period swells (14–17 s) began to impact the area causing sediment transport to occur on the mid-shelf. Shear velocities calculated from a bottom boundary layer model ranged from approximately 6 to 10 cm s −1 over the course of the sediment transport event. Based on shear velocity calculations, bedload and suspended sediment transport were occurring at the site 72 h prior to the passage of the hurricane as long-period swells reached heights of 4 m. The seabed altimeter recorded large fluctuations during the storm but was unable to trace elevation changes due to the presence of a highly concentrated layer of suspended sediments in the lower 10 cm of the bottom boundary layer. Large amounts of suspended sediment transport occurred in the along-shelf direction towards the southwest after tropical storm force winds of 30 m s −1 began to directly affect the area and wind-driven currents were generated. Wind-driven, subtidal currents responded rapidly to local wind forcing and shifted from 15 cm s −1 towards the southwest to 16 cm s −1 towards the northeast. Model calculated velocity profiles compared well with measured current velocity profiles. Boxcores collected pre- and post-storm show a storm layer of approximately 5 cm and a sediment trap that was attached to the frame 30 cm above the bed collected approximately 400 cm 3 of fine to medium sands that were suspended during the storm.

Assemblages of fish larvae and mesozooplankton across the continental shelf and shelf slope of the Andaman Sea (NE Indian Ocean)

MEPS Marine Ecology Progress Series Contact the journal Facebook Twitter RSS Mailing List Subscribe to our mailing list via Mailchimp HomeLatest VolumeAbout the JournalEditorsTheme Sections MEPS 274:87-97 (2004) - doi:10.3354/meps274087 Assemblages of fish larvae and mesozooplankton across the continental shelf and shelf slope of the Andaman Sea (NE Indian Ocean) Peter Munk1,*, Peter Koefoed Bjørnsen2, Pensri Boonruang3, Michael Fryd2, Per Juel Hansen4, Vudhichai Janekarn3, Vitthaya Limtrakulvong3, Torkel Gissel Nielsen2, Ole Schou Hansen2, Suree Satapoomin3, Suchat Sawangarreruks3, Helge Abilhauge Thomsen1, Jette Buch Østergaard5 1Department of Marine Ecology and Aquaculture, Danish Institute for Fisheries Research, Charlottenlund Castle, 2920 Charlottenlund, Denmark 2Department of Marine Ecology, National Environmental Research Institute, Frederiksborgvej 399, PO Box 358, 4000 Roskilde, Denmark 3Phuket Marine Biological Center, PO Box 60, 83000 Phuket, Thailand 4Marine Biological Laboratory, University of Copenhagen, Strandpromenaden 5, 3000 Helsingør, Denmark 5Botanical Institute, University of Copenhagen, Øster Farimagsgade 2D, 1353 Copenhagen, Denmark *Email: pm@dfu.min.dk ABSTRACT: We studied the cross-shelf variation in hydrography and plankton dynamics off west Thailand, focusing on physical-biological linkages. The overall research programme investigated linkages between physics, chemistry and plankton biology; in the present paper we consider the findings based on the sampling of fish larvae and mesozooplankton. Surveys were carried out during 2 monsoon periods in March and August 1996, using 3 cross-bathymetric transects extending to the deeper part of the shelf slope of the Andaman Sea. Station distances were either 5 or 10 n miles apart, and at each station a series of net tows were carried out, targeting different size ranges of organisms. Plankton were identified to order (invertebrates) or family (fish larvae), and their abundances and biomass estimated. The abundance of both mesozooplankton and fish larvae peaked mid-shelf (50 to 65 m bottom depth) coinciding with a hydrographic front generated where the pycnocline meets the sea-bottom. An internal wave of pronounced amplitude interacts with the shelf slope at ca. 300 m bottom depth, and findings indicated another zone of enhanced abundance in this area. Analysis of the relative abundances of fish larvae within families revealed a marked cross-shelf structuring into a number of larval assemblages. Distinct assemblages were identified in nearshore areas, at mid-shelf in the area of the hydrographic front, and off the shelf break in oceanic water. Less pronounced variation was seen in the along-shelf direction and between monsoon periods. KEY WORDS: Fish larval assemblages · Cross-shelf plankton distribution · Frontal hydrography · Monsoon periods Full text in pdf format PreviousNextExport citation RSS - Facebook - Tweet - linkedIn Cited by Published in MEPS Vol. 274. Online publication date: June 24, 2004 Print ISSN: 0171-8630; Online ISSN: 1616-1599 Copyright © 2004 Inter-Research.

Interannual variability of nearbed sediment flux on the Eel River shelf, northern California

A 5-year data record of currents, sea level variations, and suspended-sediment concentration is analyzed to evaluate the interannual variability of currents and suspended-sediment transport on the Eel River continental shelf of northern California. Data were collected within 2 m of the seabed in 60-m water depth at a site located 25 km north of the Eel River mouth. This site is near the landward edge of the Eel River mid-shelf mud deposit that rests between 50 and 110 m depth and extends for approximately 50 km northward along this shelf. Results of this analysis show that sediment resuspension is forced primarily by nearbed oscillatory flows generated during winter months, and the nearbed sediment transport occurs both as suspended load and as fluid mud. In the alongshelf direction, the net annual sediment flux experiences significant northerly and southerly excursions over individual years but exhibits almost no net transport past the study site over the 5-year period. In the across-shelf direction, net annual sediment-flux magnitude varies from year to year but is always directed offshore. Frequency analysis indicates that net sediment flux in the alongshelf direction is primarily driven by winds and very low-frequency oscillations thought to be associated with mesoscale eddies formed on the shelf. Annual variations in the frequency and possibly the spatial scale of this eddy structure correlate to net northerly or southerly alongshelf sediment flux. In the across-shelf direction, sediment flux is driven by annual variations in the across-shelf currents. Mesoscale circulation features are suggested to impact the transport and redistribution of the Eel River mid-shelf sediment deposit on this mid-latitude continental shelf.

Changes in plankton and fish larvae communities across hydrographic fronts off West Greenland

The variability in plankton community structure was studied in Disko Bay and across important fishing banks off the west coast of Greenland. The primary goal of the study was to investigate possible linkages between hydrographical processes and plankton structures, hypothesizing that hydrographic fronts would be present in the area, and that these to a large extent determine plankton distribution, composition and productivity. We sampled along four cross-shelf transects, one covering Disko Bay and Disko Bank, while the other three covered Store Hellefiske Bank, Lille Hellefiske Bank and Sukkertop Bank. The hydrography was examined by CTD profiling, the phytoplankton by fluorescence profiling and water bottle sampling, while mesozooplankton and ichthyoplankton were sampled by vertical or oblique net hauls, respectively. We observed distinct along-shelf flowing currents in the area (e.g. the West Greenland Current, the Polar Current and the Irminger Current), and the physical characteristics indicated frontogenesis at the shelf slope, in regions of 80–100 m water depth. Phytoplankton and ichthyoplankton showed a cross-shelf structuring with apparent linkages to frontal characteristics, while a more diverse pattern was observed for the mesozooplankton which were dominated by Calanus finmarchicus, Calanus glacialis and Calanus hyperboreus. The relationship between hydrographic characteristics and plankton distribution differed among species, and apparently specific plankton communities were established in different areas of the shelf. For example the larvae of Boreogadus saida, Ammodytes sp., Reinhardtius hippoglossoides and Stichaeus punctatus differed markedly in distributional characteristics. In addition to the cross-shelf structuring, marked differences in species composition and total plankton abundance were observed in the along-shelf (north–south) direction. The latitudinal differences in the unicellular plankton communities are interpreted largely within a seasonal successional framework (i.e. an early dominance of diatoms followed by increasing importance of smaller unicellular plankton), while the ichthyo- and zooplankton communities also differed by the respective dominance of species with polar versus temperate origin. Our findings suggest that the flow of major currents and the establishment of hydrographical fronts are of primary importance to the plankton communities in the West Greenland shelf area, influencing the early life of fish and the recruitment to the important fisheries resources.

Prognostic Modeling Studies of the Keweenaw Current in Lake Superior. Part II: Simulation

The Keweenaw Current, observed along the coast of the Keweenaw Peninsula in Lake Superior during July 1973, was simulated using a 3D, nonorthogonal coordinate transformation, primitive equation coastal ocean model. The model domain covered the entire lake with a high resolution of 250–600 m in the cross-shelf direction and 4–6 km in the alongshelf direction along the peninsula. The model was initialized using the monthly averaged temperature field observed in June 1973 and was run prognostically with synoptic wind forcing plus monthly averaged heat flux. Good agreement was found between model-predicted and observed currents at buoy stations near Eagle Harbor. Comparison of the model results with and without inclusion of heat flux suggested that combined wind and heat fluxes played a key role in the intensification of the Keweenaw Current during summer months. The model-predicted relatively strong near-inertial oscillations occurred episodically under conditions of a clockwise-rotating wind. These oscillations intensified at the surface, were weak near the coast, and increased significantly offshore.

Mesoscale Currents on the Inner New Jersey Shelf Driven by the Interaction of Buoyancy and Wind Forcing

Abstract Shipboard hydrographic and acoustic Doppler current profiler surveys conducted in August 1996 on the New Jersey inner shelf revealed a buoyant intrusion advancing southward along the coast. This buoyant intrusion originated from the Hudson estuary more than 100 km upshelf and appeared as a bulge of less saline water with a sharp across-shelf frontal zone at its leading edge. During this time, the study area was also forced by a brief upwelling-favorable wind event opposing the direction of buoyant flow propagation. The interaction of buoyancy and wind forcing generated a spatially variable velocity field. In particular, across-shelf currents were comparable to their alongshelf counterparts. Variability in the alongshelf direction occurred on the scales of the order of the baroclinic Rossby radius. Intensive across-shelf currents reached speeds of 20– 40 cm s−1 and appeared as spatially localized mesoscale flows with a width of O(10 km). They were generated at the leading edge of the buoyant intru...

The wind driven currents on the Middle Atlantic Bight inner shelf

A set of month-long current meter data from two moorings deployed at the 20 and 30 m isobaths to the southeast of Atlantic City, New Jersey are examined to characterize the structure of the wind-induced subtidal currents in the Middle Atlantic Bight under summer time conditions. The wind stress and currents are dominated by variability at the 2–4 d time scales. The majority of the wind stress variance is oriented in the along-shelf direction (35°T), but the subtidal currents at both moorings also show substantial across-shelf variability, with the standard deviation of the across-shelf current component exceeding 50% of that of the along-shelf current component. Furthermore, there is an appreciable reduction in magnitude and a change in orientation of the subtidal current vector with depth. The currents from the mooring located at the 30 m isobath are significantly coherent with the wind stress, with surface current rotating clockwise and bottom current rotating anticlockwise of the wind. With an upwelling favorable wind, there is a significant offshore flow in the upper layer and an onshore flow in the lower layer, consistent with Ekman transport. The situation reverses with a downwelling favorable wind. The depth-averaged current is dominated by variability in the along-shelf direction. Wind stress and along-shelf surface slope are the leading terms in the depth-integrated along-shelf momentum balance, but bottom stress also plays an important role in the balance. The currents at the inner shelf mooring (20 m isobath) are much less coherent with the wind, particularly for the surface current. The reduced linear correlation between the wind and the observed subtidal current there may be caused by the influence of the buoyancy-driven coastal current originating from the Hudson River estuary to the north of the mooring site.

A surface‐trapped intrusion of slope water onto the continental shelf in the Mid‐Atlantic Bight

A high‐resolution hydrographic transect across the shelfbreak south of Nantucket Shoals in August, 1995, revealed an unusual surface‐trapped intrusion of saline Slope water onto the continental shelf. The intrusion was confined to the upper 25 m of the water column and penetrated 15 km shoreward of the 100‐m isobath, the typical position of the shelfbreak front. The maximum salinity within the intrusion was 35.0. Several strong jets were present within the surface layer with maximum velocities between 0.30 and 0.44 m s−1 in the alongshelf direction. Satellite thermal imagery confirms the presence of a warm‐core ring at the offshore edge of the section and shows a warm feature penetrating shoreward of the 100‐m isobath.

The Nantucket Shoals Flux Experiment: 3. The alongshelf transport of volume, heat, salt, and nitrogen

The alongshelf transport of volume, heat, salt, and nitrogen through a section across the continental shelf south of Nantucket Island was examined using moored current and bottom pressure measurements in conjunction with data from 15 hydrographic cruises along the mooring transect. The mean “observed” transport of shelf water, estimated from the moored current observations in the traditional way for the Summer time period (March to September 1979), was westward at 38.9 ± 8.2 × 104 m3 s−1. The “geostrophic” transport was inferred from differences between measured bottom pressures whose mean and trend were geostrophically levelled using the observed currents between each station. The mean geostrophic transports for the Summer and Annual (March 1979 to April 1980) time periods were westward at 31.9 ± 10.0 × 104 m3 s−1, and 29.0 ± 9.1 × 104 m3 s−1, respectively, with no significant seasonal variation. There was excellent agreement between the two methods in estimating transport fluctuations in the wind‐driven (4–20 day) band. The best estimates of the mean advective transport of heat, salt, and dissolved inorganic nitrogen were westward 3.16 ±1.42 × 109 kcal s−1, 1.18 ± 0.53 × 107 kg s−1, and 1.22 ± 3.03 × 109 μg ‐ at N s−1, respectively. Eddy transports were estimated where possible and found to be insignificant relative to advective transports in the alongshelf direction. This new value of mean salt transport is consistent with recently observed values of volume and salt transport into the Gulf of Maine through the Northeast Channel and the overall regional salt budget as computed by the Brown and Beardsley [1979] box model. The observed nitrogen transport can potentially supply between 39 and 53% of the nitrogen demand in the Middle Atlantic Bight.

Diurnal Shelf Waves in the Southern Weddell Sea

Strong evidence is presented for the existence of barotropic shelf waves at diurnal frequencies in the shelf-break region of the southern Weddell Sea. Rotary spectra from long-term current meter records on the slope, at the shelf break and on the shelf show the same rotational sense as that predicted by a barotropic model. Phase speeds of the zero-mode shelf waves calculated from tidal analysis and rotary coherencies of current meter pairs oriented in an along-shelf direction are in the same range as that predicted by barotropic theory except for the O1 constituents. The weak stratisfication of the Weddell Sea in the region of the slope results in nearly barotropic motion at diurnal frequencies. The anomalous behavior of the O1 constituent could not be explained.

Moored wind, temperature, and current observations made during Coastal Ocean Dynamics Experiments 1 and 2 over the Northern California Continental Shelf and upper slope

Moored observations of winds, currents, and temperature made over the northern California shelf and upper slope during the 1981 and 1982 upwelling seasons as part of the Coastal Ocean Dynamics Experiment (CODE) are described. In both field experiments (CODE 1 and CODE 2), the onset of the upwelling season is triggered by the strengthening of the North Pacific high off northern California and signalled by the presence of water colder than 9°C near the bottom over the shelf. Vertical temperature stratification over the shelf is reduced during active upwelling. Local winds and subtidal currents over the shelf are strongly polarized in the alongshelf direction, and the mean wind stress is directed equatorward. Response of alongshelf currents to wind forcing is similar at all depths over the shelf. The response of the cross‐shelf current is more complex. In the surface layer, offshelf flow is significantly and positively correlated with the upwelling‐favorable alongshelf wind stress. Beneath this layer a weak return flow exists. The thickness of the upper mixed layer varies greatly, reaching 60 m during upwelling events. Thus direct wind forcing affects a substantial fraction of the water over the shelf. Low‐frequency wind stress, current, and temperature fields are adequately described by a few energetic empirical orthogonal functions with simple spatial structure. The most energetic mode typically accounts for 75% or more of the variance of wind stress and temperature. Over the middle and inner shelf, the most energetic current mode accounts for 90% or more of the variance, is dominated by he alongshelf component, and veers to the left with depth. The low‐frequency alongshelf flow fluctuations are quasi‐barotropic, with some vertical shear but no reversal with depth; vertical shear in alongshelf current and the cross‐shelf density gradient obey the thermal wind relation. The simple structure of the subdiurnal currents allow estimation of the alongshelf volume transport between the 60 and 130 m isobaths through three cross‐shelf transects in CODE 2. Volume transport over the shelf is often nearly two‐dimensional, but occasional large differences indicate significant mass exchange between the shelf and the adjacent upper slope. Correlation of low‐frequency currents and alongshelf wind stress shows the response of alongshelf current to be maximum near the surface and near midshelf, where a 15 cm−1 alongshelf flow lags a 1 dyn cm−2 alongshelf wind stress by the order of 10 hours. Currents over the upper slope are not significantly correlated with local wind. Alongshelf transport over the shelf is highly correlated with local wind stress, explaining 80% of the variance in the low‐frequency transport during CODE 2. Volume transport is best correlated with wind stress approximately 30 km to the south. Winds in the CODE area during spring and summer are characterized by strong and persistent upwelling‐favorable winds interrupted by shorter periods of weak winds. Regression analysis indicates that in the absence of wind, a poleward barotropic current jet exists over the shelf with a maximum amplitude over the inner shelf in the CODE area. An upwelling‐favorable wind counters this jet over the shelf, so that the alongshore flow is everywhere equator‐ward during strong wind events. Since the maximum response to wind forcing is found at midshelf and near surface, the mean current over the upwelling season is strongly sheared in both the vertical and cross‐shelf directions. Mean flow over the outer shelf is surface intensified and is directed offshelf and equatorward, while the deeper flow at midshelf and through most of the water column over the inner shelf is directed poleward.

A Model of the Summer Progression of the Cold-Pool Temperature in the Middle Atlantic Bight

Abstract A simple one-dimensional model (in the alongshelf direction) that incorporates spatially non-uniform heating, advection and initial temperature field is proposed to explain the summer progression of the cold-pool temperature in the Middle Atlantic Bight. The observed local cooling is shown to require an upstream cold water source at the onset of the heating season. The observed secondary temperature minimum that stagnates at the Hudson Shelf Valley and the observed thermal front in the New York Bight can be explained by the alongshelf variation of the heating rate which is attributed mainly to the varying shelf width and hence the heat capacity of the cold pool. Although the model is highly idealized, a crude simulation using the input functions inferred from data has reproduced all the gross features observed in the thermal field.