Transport Of Shelf Water Research Articles

Substantial kelp detritus exported beyond the continental shelf by dense shelf water transport

Kelp forests may contribute substantially to ocean carbon sequestration, mainly through transporting kelp carbon away from the coast and into the deep sea. However, it is not clear if and how kelp detritus is transported across the continental shelf. Dense shelf water transport (DSWT) is associated with offshore flows along the seabed and provides an effective mechanism for cross-shelf transport. In this study, we determine how effective DSWT is in exporting kelp detritus beyond the continental shelf edge, by considering the transport of simulated sinking kelp detritus from a region of Australia’s Great Southern Reef. We show that DSWT is the main mechanism that transports simulated kelp detritus past the continental shelf edge, and that export is negligible when DSWT does not occur. We find that 51% per year of simulated kelp detritus is transported past the continental shelf edge, or 17–29% when accounting for decomposition while in transit across the shelf. This is substantially more than initial global estimates. Because DSWT occurs in many mid-latitude locations around the world, where kelp forests are also most productive, export of kelp carbon from the coast could be considerably larger than initially expected.

Cross-shelf exchange associated with a shelf-water streamer at the Mid-Atlantic Bight shelf edge

Significant exchanges between the Mid-Atlantic Bight (MAB) continental shelf and the neighboring open ocean can be induced by shelf water streamers, submesoscale filaments of shelf water entrained into the open ocean by Gulf Stream warm-core rings (WCRs) impinging onto the MAB continental shelf. Shelf water streamers have distinctive surface temperature and chlorophyll signals, and are thus visible from space. Satellite-measured sea surface height, temperature and chlorophyll show the evolution of a WCR over its 6-month lifespan in February-August 2019 and the persistent shelf water streamer it generated on its outskirt. In situ measurements from a two-week cruise in July 2019 were analyzed to investigate the physical, biological and biogeochemical characteristics of the shelf water streamer below the surface, and to quantify the associated cross-shelf transport of volume, heat, salt, carbon and oxygen. The analyses demonstrated that offshore transport of shelf water by the streamer, which was presumably balanced by either onshore intrusion of ring water or enhanced transport of shelf water from upstream, represented a major form of exchange between the MAB continental shelf and the open ocean. The streamer caused significant net onshore transport of heat and salt, and a significant net offshore transport of organic carbon and oxygen. Primary productivity in the streamer was higher than the surrounding slope and ring waters on the surface, which likely resulted from subsurface nutrients in the offshore-flowing shelf water being gradually consumed as the overlying water became clearer. WCR-induced shelf water streamers thus enhanced surface biological productivity in the slope sea.Plain Language Summary: Waters of the shallow Mid-Atlantic Bight continental shelf and the neighboring deep slope sea have distinctly different physical, biological and chemical properties. Mixing between them can affect the shelf ecosystem and the dispersal of coastal materials into the deep ocean. One type of cross-shelf-edge mixing process results from strong clockwise-rotating vortices – so-called warm-core rings – formed from meanders of the Gulf Stream. As a warm-core ring intrudes onto the shelf edge, it often draws shelf water offshore, forming a thin filament in the slope sea. This filament is called a shelf-water streamer and has distinctive surface temperature and chlorophyll signals that are visible from space. Warm-core rings can also push offshore water onto the shallow shelf. This study examines the evolution of a warm-core ring over its 6-month lifespan in 2019 and the shelf-water streamer the ring induced in 5 of the 6 months. Interdisciplinary measurements from a field expedition in July 2019 were examined to assess the subsurface patterns of the streamer and to quantify the induced cross-shelf fluxes of heat, salt, organic carbon and oxygen. The analysis showed that the streamer represented a major form of cross-shelf mixing and caused a substantial onshore transport of heat and salt, as well as a substantial offshore transport of organic carbon and oxygen.

Circulation and structure of the Mackenzie River plume in the coastal Arctic Ocean

The Mackenzie River flows into the Arctic Ocean via a complex network of channels, creating a plume that dominates water properties over the southern Beaufort Sea shelf. Observations of temperature, salinity, water level elevations and currents collected in August 2007 and August 2010 are used to investigate the circulation and structure of the river plume, during periods with predominantly easterly/offshore winds. The measurements were collected from instrument platforms in a river channel, in shallow water of 2–6 m depth (10–25 km from the coastline), from a research vessel, and from satellites. The observations indicate that moderate atmospheric forcing, including sea level pressure changes of 1–2 kPa and easterly alongshore wind stress (< 0.1 Nm−2), is sufficient to induce a 0.2–0.3 m set-down of the mean water level and surface currents up to 0.6 ms−1. A theoretical model of the water level at the delta suggests that local wind stress is the primary driver of sea level variability. The observations show that the region is stratified with a shallow river surface plume at the surface, and winds primarily cause offshore transport of the plume up to 30 km per day. Winds also cause separation of the plume from the coast and onshore transport of shelf water near the bottom that induces local changes in temperature of up to 12oC. River water is the primary source of heat, and therefore sea surface temperatures in the coastal ocean are strongly determined by the inflowing freshwater temperatures. The coastal plume responds to atmospheric forcing over timescales of less than 1 day, leading to rapid development of strong currents and large fluctuations in water mass properties in the shallow region across the submarine delta.

Intrusions of Kuroshio and Shelf Waters on Northern Slope of South China Sea in Summer 2015

The northern slope region of the South China Sea (SCS) is a biological hot spot characterized by high primary productivity and biomasses transported by cross-shelf currents, which support the spawning and growth of commercially and ecologically important fish species. To understand the physical and biogeochemical processes that promote the high primary production of this region, we conducted a cruise from June 10 and July 2, 2015. In this study, we used fuzzy cluster analysis and optimum multiparameter analysis methods to analyze the hydrographic data collected during the cruise to determine the compositions of the upper 55-m water masses on the SCS northern slope and thereby elucidate the cross-slope transport of shelf water (SHW) and the intrusions of Kuroshio water (KW). We also analyzed the geostrophic currents derived from acoustic Doppler current profiler measurements and satellite data. The results reveal the surface waters on the northern slope of the SCS to be primarily composed of waters originating from South China Sea water (SCSW), KW, and SHW. The SCSW dominated a majority of the study region at percentages ranging between 60% and 100%. We found a strong cross-slope current with speeds greater than 50 cm s−1 to have carried SHW into and through the surveyed slope area, and KW to have intruded onto the slope via mesoscale eddies, thereby dominating the southwestern section of the study area.

Eastern Venezuela coastal upwelling in context of regional weather and climate variability

Regional factors modulating eastern Venezuela coastal upwelling emerge from this observational study. Trade winds of 3–9 m/s are a persistent feature of the shelf zone, and drive offshore Ekman transport and pumping, cooling SST by 1–3 °C especially in spring. Wavelet spectral analysis of SST anomalies (11ºN, 65ºW) show significant intra-seasonal (9, ∼45, ∼100 days) and inter-annual oscillations (∼3, 9 years). A case study upwelling event of 3–5 May 2014 is distinguished by a low level wind jet on the shelf edge. An ocean eddy propagated westward along the shelf, initially suppressing then enhancing offshore transport. Venezuela SST variability reflects atmospheric control by the North Atlantic anticyclone and associated trade wind-driven shelf water transport and regional evaporation. Eastern Venezuela coastal upwelling involves an atmospheric meridional overturning circulation that affects the frequency of Caribbean hurricanes.

Offshore Transport of Shelf Water by Deep-Ocean Eddies

AbstractAt continental margins, energetic deep-ocean eddies can transport shelf water offshore in filaments that wrap around the eddy. One example is that of Gulf Stream warm-core rings interacting with the Mid-Atlantic Bight shelf. The rate at which shelf water is exported in these filaments is a major unknown in regional budgets of volume, heat, and salt. This unknown transport is constrained using a series of idealized primitive equation numerical experiments wherein a surface-intensified anticyclonic eddy interacts with idealized shelf–slope topography. There is no shelfbreak front in these experiments, and shelf water is tracked using a passive tracer. When anticyclones interact with shelf–slope topography, they suffer apparent intrusions of shelf–slope water, resulting in a subsurface maximum in offshore transport. The simulations help construct an approximate model for the filament of exported water that originates inshore of any given isobath. This model is then used to derive an expression for the total volume of shelf–slope water transported by the eddy across that isobath. The transport scales with water depth, radius, and azimuthal velocity scale of the eddy. The resulting expression can be used with satellite-derived eddy properties to estimate approximate real-world transports ignoring the presence of a shelfbreak front. The expression assumes that the eddy’s edge is at the shelf break, a condition not always satisfied by real eddies.

Offshore Transport of Shelf Waters through Interaction of Vortices with a Shelfbreak Current

Abstract Interactions between vortices and a shelfbreak current are investigated, with particular attention to the exchange of waters between the continental shelf and slope. The nonlinear, three-dimensional interaction between an anticyclonic vortex and the shelfbreak current is studied in the laboratory while varying the ratio ε of the maximum azimuthal velocity in the vortex to the maximum alongshelf velocity in the shelfbreak current. Strong interactions between the shelfbreak current and the vortex are observed when ε &amp;gt; 1; weak interactions are found when ε &amp;lt; 1. When the anticyclonic vortex comes in contact with the shelfbreak front during a strong interaction, a streamer of shelf water is drawn offshore and wraps anticyclonically around the vortex. Measurements of the offshore transport and identification of the particle trajectories in the shelfbreak current drawn offshore from the vortex allow quantification of the fraction of the shelfbreak current that is deflected onto the slope; this fraction increases for increasing values of ε. Experimental results in the laboratory are strikingly similar to results obtained from observations in the Middle Atlantic Bight (MAB); after proper scaling, measurements of offshore transport and offshore displacement of shelf water for vortices in the MAB that span a range of values of ε agree well with laboratory predictions.

The southward transport of sub‐mesoscale lenses of Bass Strait Water in the centre of anti‐cyclonic mesoscale eddies

Dense shelf water from Bass Strait, southeast Australia, is presently understood to travel northward along the continental shelf, and disperse eastward into the Tasman Sea. Here we report the unexpected discovery by autonomous gliders of lenses of shelf water ∼40 km in diameter and 200–300 m tall at depth in the center of three ∼200 km diameter anti‐cyclonic eddies. Reanalysis of 2420 vertical profiles off the continental slope in the western Tasman Sea since 1982 found only 3 distinct patches of Bass Strait Water (BSW), all with positive dynamic height anomalies indicative of anti‐cyclones. Through a yet to be understood process, BSW separates from the continental slope and forms a mid‐depth lens that aligns vertically with the larger anti‐cyclonic mesoscale eddy; and remains at the center of the eddy for 6+ months as it is advected 700 km southward. This pathway subducts shelf water into the ocean interior, and provides a link between mesoscale circulation and shelf water transport. The BSW that is captured in anti‐cyclones advects south past the east coast of Tasmania, with some moving into the eastern Indian Ocean.

Shelf water entrainment by Gulf Stream warm-core rings between 75°W and 50°W during 1978–1999

Previous work concerning Gulf Stream warm-core rings (WCRs) and their associated shelf water entrainments have been based upon single surveys or time series from individual WCRs. To date, estimates of annual shelf water volume entrained into the Slope Sea by WCRs and its interannual variability have not been made. Using a long time series of satellite-derived sea surface temperature (SST) observations of Slope Sea WCRs, we have completed an analysis of 22 years of WCR data (1978–1999) between 75°W and 50°W to understand the interannual variability of WCRs and their role in entraining shelf water. Satellite-derived SST data digitized at Bedford Institute of Oceanography are analyzed using an ellipse-fitting feature model to determine key WCR characteristics including WCR center position, radius and orientation. Key characteristics are then used to compute WCR swirl velocity by finite-differencing WCR orientations ( θ) obtained from the feature model time series. Global mean WCR-edge swirl velocity calculated from all observations is 105.72±10.7 km day −1 (122.36±12.4 cm s −1), and global mean WCR radius is 64.8±6.2 km. Primary and derived WCR data are incorporated into a two-dimensional ring entrainment model (RM) using the quasi-geostrophic approximation of the potential vorticity equation. The RM defines ambient water as entrained by a WCR only if the gradient of relative vorticity term (horizontal shear) dominates the potential vorticity. Proximity of a WCR to the position of the shelf-slope front (SSF) is then used to determine whether the ambient water is entrained from the outer continental shelf. WCR-induced shelf entrainment derived from the RM displays considerable spatial variability, with maximum entrainment occurring offshore of Georges Bank, advecting a mean total annual shelf water volume of 7500 km 3 year −1 from the region. Estimates of shelf water fluxes display significant interannual variability, which may be in part due to the observed covariance between WCR occurrences and the state of the North Atlantic Oscillation (NAO). Increased (decreased) occurrences of WCRs are evidenced during positive (negative) phases of the NAO. The total mean annual shelf-wide WCR-induced shelf water transport is estimated to be 23,700 km 3 year −1 (0.75 Sv), accounting for nearly 25% of the total transport in the Slope Sea region neighboring the outer continental shelf.

Western Arctic Shelfbreak Eddies: Formation and Transport

Abstract The mean structure and time-dependent behavior of the shelfbreak jet along the southern Beaufort Sea, and its ability to transport properties into the basin interior via eddies are explored using high-resolution mooring data and an idealized numerical model. The analysis focuses on springtime, when weakly stratified winter-transformed Pacific water is being advected out of the Chukchi Sea. When winds are weak, the observed jet is bottom trapped with a low potential vorticity core and has maximum mean velocities of O(25 cm s−1) and an eastward transport of 0.42 Sv (1 Sv ≡ 106 m3 s−1). Despite the absence of winds, the current is highly time dependent, with relative vorticity and twisting vorticity often important components of the Ertel potential vorticity. An idealized primitive equation model forced by dense, weakly stratified waters flowing off a shelf produces a mean middepth boundary current similar in structure to that observed at the mooring site. The model boundary current is also highly variable, and produces numerous strong, small anticyclonic eddies that transport the shelf water into the basin interior. Analysis of the energy conversion terms in both the mooring data and the numerical model indicates that the eddies are formed via baroclinic instability of the boundary current. The structure of the eddies in the basin interior compares well with observations from drifting ice platforms. The results suggest that eddies shed from the shelfbreak jet contribute significantly to the offshore flux of heat, salt, and other properties, and are likely important for the ventilation of the halocline in the western Arctic Ocean. Interaction with an anticyclonic basin-scale circulation, meant to represent the Beaufort gyre, enhances the offshore transport of shelf water and results in a loss of mass transport from the shelfbreak jet.

Northward transport of pacific summer water along the Northwind Ridge in the Western Arctic Ocean

The recent sea-ice reduction in the Arctic Ocean is not spatially uniform, but is disproportionally large around the Northwind Ridge and Chukchi Plateau compared to elsewhere in the Canada Basin. In the Northwind Ridge region, Pacific Summer Water (PSW) delivered from the Bering Sea occupies the subsurface layer. The spatial distribution of warm PSW shows a quite similar pattern to the recent ice retreat, suggesting the influence of PSW on the sea-ice reduction. To understand the regionality of the recent ice retreat, we examine the dynamics and timing of the delivery of the PSW into this region. Here, we adopt a two-layer linearized potential vorticity equation to investigate the behavior of Rossby waves in the presence of a topographic discontinuity in the high latitude ocean. The analytical results show a quite different structure from those of mid-latitude basins due to the small value of β. Incident barotropic waves excited by the sea-ice motion with large annual variation can be scattered into both barotropic and baroclinic modes at the discontinuity. Since the scattered baroclinic Rossby wave with annual frequency cannot propagate freely, a strong baroclinic current near the topographic discontinuity is established. The seasonal variation of current near the topographic discontinuity would cause a kind of selective switching system for shelf water transport into the basin. In our simple analytical model, the enhanced northward transport of summer water and reduced northward transport of winter water are well demonstrated. The present study indicates that these basic dynamics imply that a strengthening of the surface forcing during winter in the Canada Basin could cause sea-ice reduction in the Western Arctic through the changes of underlying Pacific Summer Water.

Overrunning of shelf water in the southern Mid-Atlantic Bight

Analyses of two years (1992 and 1993) of high-resolution sea surface temperature satellite images of the southern Mid Atlantic Bight (MAB), showed that unusually extensive overhang of shelf water occurs episodically, and coherently over along shelf distances of several 100 km. These episodes are dubbed overrunning of the Slope Sea by shelf water. The overrunning volume has a “face” and a “back” (southern and northern limit). It transports substantial quantities of shelf water southward, and does not retreat onto the shelf, but eventually joins the western edge of the Gulf Stream in the vicinity of Chesapeake Bay. The combined analyses of satellite imagery and various in situ data further demonstrated that the shelf waters overrunning the Slope Sea were not mere surface features but reached depths between 40 and 60 m. Results confirm previous concepts on shelf circulation, shelf–slope exchange and fate of shelf water. They also shed new light on shelf water budget: overrunning of the Slope Sea and southwest transport by upper slope current constitutes an important conduit for shelf water transport. Winter storms move the shelf–slope front, and with it shelf water, offshore to distances ∼10–40 km. The offshore displacement of shelf water can be related to the onshore veering of the Gulf Stream near Cape Hatteras, producing a blocking effect on the shelf circulation. Such a blocking effect of the southwestward flow of shelf water in the MAB appeared to be the reason for the overrunning of shelf water off New Jersey. In addition, the excess fresh water discharge from the St. Lawerence was also observed to be related to the overflow of shelf water off New Jersey.

Distribution, formation, and seasonal variability of Okhotsk Sea Mode Water

Russian historical data and recently completed conductivity‐temperature‐depth surveys are used to examine the formation and spread in the deep Ohkotsk Sea of dense shelf water (DSW) produced in the Okhotsk Sea polynyas. Isopycnal analysis indicates that all of the main polynyas contribute to the ventilation at σθ &lt; 26.80, including the Okhotsk Sea Mode Water (OSMW), which has densities σθ = 26.7–27.0. At densities greater than 26.9 σθ the northwest polynya is the only contributor to OSMW. (Although Shelikhov Bay polynyas produce the densest water with σθ &gt; 27.1, vigorous tidal mixing leads to outflow of water with a density of only about 26.7 σθ). In the western Okhotsk Sea the East Sakhalin Current rapidly transports modified dense shelf water along the eastern Sakhalin slope to the Kuril Basin, where it is subject to further mixing because of the large anticyclonic eddies and tides. Most of the dense water flows off the shelves in spring. Their average flux does not exceed 0.2 Sv in summer and fall. The shelf water transport and water exchange with the North Pacific cause large seasonal variations of temperature at densities of 26.7–27.0 σθ (depths of 150–500 m) in the Kuril Basin, where the average temperature minimum occurs in April–May, and the average temperature maximum occurs in September, with a range of 0.2°–0.7°C. The average seasonal variations of salinity are quite small and do not exceed 0.05 psu. The Soya Water mixed by winter convection, penetrating to depths greater than 200 m, in the southern Kuril Basin also produces freezing water with density greater than 26.7 σθ. Using a simple isopycnal box model and seasonal observations, the OSMW production rate is seen to increase in summer up to 2.2 ± 1.7 Sv, mainly because of increased North Pacific inflow, and drops in winter to 0.2 ± 0.1 Sv. A compensating decrease in temperature in the Kuril Basin implies a DSW volume transport of 1.4 ± 1.1 Sv from February through May. The residence time of the OSMW in the Kuril Basin is 2 ± 1.7 years.

Flow variability at the tip of the Antarctic Peninsula

Recently ventilated water leaves the landlocked northwestern Weddell Sea near the Antarctic Peninsula and possibly spreads out into the basins of the world oceans at shallow to intermediate depths. To determine the pathways of the water through the complex topography and the flow variability, water-mass circulation and properties are described in the northwestern Weddell Sea and along the boundary of the Powell Basin by means of data from current-meter moorings and hydrographic sections. The mean flow is strongly controlled by the topography. Meso-scale, seasonal and interannual fluctuations are superimposed. The mean northward volume transport of shelf water, which represents the potential source water for intermediate layer ventilation, is estimated for the time interval between May 1996 and March 1998 to be 2.4±1.0 Sv. Water-mass properties suggest that much of this water leaves the Weddell Sea to Bransfield Strait and therefore does not reach the Weddell Scotia Confluence. The water masses are able to serve as the only source of Bransfield Strait deep water since the shelf water properties in the northwestern Weddell Sea vary over time within a range that corresponds to the required source waters. The Scotia Sea is supplied by water from the Powell Basin, which has varied significantly over the past two decades.

Structure and transport of alongshelf currents across the southern flank of Georges Bank during late summer, 1982

An array including seven moorings was in place between 10 August and 25 September 1982 on southern Georges Bank. Ten of the records from the Vector-Averaging Current Meters (VACMs) were complete. Short-term drifters with window-shade drogues at 10 m seaward of the shelfbreak (200 m) documented flow of shelf water across bathymetry. Winds were available for this period from NOAA buoy 44003 at the southwestern corner of Georges Bank near the array. The major structure in the subtidal flow is an alongshelf maximum toward the west (mean speed ∼27 cm s −1 with bursts often >30 cm s −1) at the shelfbreak front (∼100 m): the shelfbreak jet. The first two modes in empirical orthogonal analysis of the cross-bank structure contain 82% of the variance. Mode 1 is associated with the large-scale alongshelf current and is partially correlated with local winds. Mode 2, generally not correlated with wind, may be generated by features initiated by the Gulf Stream. The current record at the northern periphery of the warm eddy or remnant of a warm-core ring has a mean velocity of 5 cm s −1 toward the east for the period, opposite the general circulation over the shelf. Elsewhere the circulation over the shelf seemed minimally affected even with the proximity of the offshore feature. A mean estimate for volume transport of shelf water during the period is 0.83±0.2 Sv with periods of greater than 1 Sv related to NE winds leading by 6 h. While a bias is possible due to undersampling of the velocity field, this mean value and variability during late summer are higher than previously reported estimates for transport of shelf water. This is primarily due to the additional contribution of the shelfbreak jet near the 100 m isobath. Confidence limits for the transport were calculated by two independent methods, geometric weighting and block estimation by universal kriging.

Transport of Middle Atlantic Bight shelf water to the Gulf Stream near Cape Hatteras

Flowing along the U.S. eastern seaboard from Nantucket Shoals to Cape Hatteras is a low‐salinity (&lt;34 practical salinity units) water mass known as Middle Atlantic Bight (MAB) shelf water. It has long been recognized that a large quantity of this water is entrained into the Gulf Stream near Cape Hatteras. We examine this process using moored instrument, hydrographic, and drifter track data acquired in the Cape Hatteras region over 1992–1994. These measurements reveal that the cross section and volume transport of MAB shelf water tend to shrink considerably going southward toward and past Cape Hatteras. In particular, our hydrographic data show shelf water cross section to be reduced by about a factor of 8 over a region stretching 80 km north of Diamond Shoals (between 35° 20′N and 36°10′N). There appear to be two fairly distinct zones of export in this region. Our data suggest that shelf water loss in the northern zone, over 35°40′N–36°10′N, occurs principally seaward of the shelf edge. Seaward movement of shelf water into Gulf Stream meanders and into the circulation of water discharged from the Gulf Stream are identified as important agents effecting this loss. Export of shelf water in the southern zone occurs over middle and outer shelf, as indicated by 5–10 cm s−1 mean off shelf velocities measured by moored current meters. Contributing to this export is a seaward flow of shelf water in a strong current at the edge of the front separating MAB shelf water from the more saline shelf water of the South Atlantic Bight. The offshore transport of shelf water at the edge of this front is principally observed when the front is migrating shoreward across the shelf. This intermittency in the seaward transport of shelf water at the front may be partly responsible for the variability of the presence and transport of shelf water seen at the edge of the Gulf Stream.

Cold anticyclonic eddies formed from cold pool water in the southern Middle Atlantic Bight

AVHRR satellite imagery of the southern Mid-Atlantic Bight during May 1993 revealed a large area of cold water over the shelf break and slope that appeared to spin up into a series of southward propagating anticyclonic eddies. The eddies had diameters of 35–45 km at the surface and moved southward at about 20 cm/sec. A radial TOYO CTD (to 50m) and ADCP velocity (to 400m) transect was conducted across the southern-most of these eddies. The upper 50 meters had minimum temperatures of less than 7°C and salinities of about 33 pss, characteristics similar to cold pool waters usually found over the continental shelf. ADCP velocity data from one of the eddies revealed anticyclonic flow extending to a depth of about 250m. The transport of cold pool water by the eddies was estimated to be 0.1 to 0.2 Sv which is of the same order as the annual mean alongshore transport of shelf water in this region. The origin of the deeper water within the eddy is unlikely to be the continental shelf because the shelf break is less than 100 m. The depth and velocity profiles along the TOYO transect were consistent with the constant potential vorticity eddy model of Flierl (1979) although the source of the eddy kinetic energy is uncertain. The cause for the exodus of cold pool water from the shelf, which extended northward to at least 38°N, is unclear but must involve the establishment of an alongshore baroclinic pressure gradient against the usual southwestward shelf flow. It is possible that the intrusion of Gulf Stream waters onto the shelf near Cape Hatteras was a precursor of this off shelf transport. The southern-most eddy was marked by high biological productivity and very high oxygen supersaturation. The phytoplankton bloom detected within the exported cold pool water, located over the continental slope, suggests a mechanism whereby production fueled by nutrients derived from the shelf can be locally exported into deep water.

Observations of offshore shelf-water transport induced by a warm-core ring

Gulf Stream warm-core rings often are observed to entrain water from the continental shelf and transport filaments of shelf water out over the continental slope. In June 1982 the R. V. Endeavor made a transect through one such shelf-water “streamer” that was seen from satellite thermal imagery to the east of warm-core ring 82B. Acoustic doppler current profiles together with repeated CTD stations were made through the streamer. The CTD system was also equipped with a dissolved oxygen sensor and transmissometer. We were able to make an unprecedented section of velocity and water mass structure of the shelf-water filament. The offshore velocity structure observed was complicated with velocities ranging from 10 to 50 cm s −1, temperatures, in a sub-surface temperature minimum, of less than 8°C, surface salinities less than 33‰, with high levels of suspended particulate matter and manganese, and with shelf-water properties as deep as 100 m in the center of the streamer. Transports calculated in different temperature and salinity classes showed a total volume transport of waters in the filament of 0.8–0.9 × 10 6 m 3 s −1 for water with a salinity <35‰. Thermal imagery shows that the streamer was not “wrapped around” the ring but was advected by the ring and deposited at the inshore edge of the Gulf Stream. Using adjacent stations not in the streamer, but in the Slope Water, as a reference for water that presumably replaced the above amount of shelf water, estimates have been made of the net exchange of heat, salt, particulates, dissolved oxygen and Mn due to the offshore transport of shelf water by 82B. These estimates indicate that warm-core rings are capable of inducing significant exchange across the Shelf/Slope Water front off the northeast coast of the U.S.A.

Gulf Stream water on the shelf and upper slope north of Cape Hatteras

Although intrusions of water from the Gulf Stream have often been observed over the Carolina shelf, there has been no published report of Gulf Stream water near the continental margin north of Cape Hatteras. By examining sea surface temperature distributions and hydrographic data collected over an 11-yr period, we have found that water discharged from the Gulf Stream often appears over the shelf and upper slope north of Cape Hatteras. For example, in the band between 36 and 38°N, roughly 80–300 km north of the tip of Cape Hatteras, Gulf Stream water was detected 13–27% of the time at sites on the upper slope and 3–9% of the time at locations on the shelf. In most instances, sea surface temperature distributions suggest that the Gulf Stream water which appeared near the continental margin was not part of the Gulf Stream's main current but was fluid which had been expelled from the Gulf Stream. The Gulf Stream water found on the shelf did not appear to significantly influence the local circulation or alongshelf density structure. Its subtidal flow was largely driven by the alongshelf wind stress; and in most instances its vertical density profile nearly matched that of adjacent shelf water situated alongshore. In contrast, the discharged Gulf Stream water observed over the upper slope was significantly less dense than abutting fluid of equivalent depth located along the slope to the northeast. The circulation associated with this density contrast often conveyed Middle Atlantic Bight shelf water seaward of the continental margin. The resulting rate of shelf water export is not well resolved by the available data. Some rough estimates put it at 0.1 Sv, comparable with the estimated mean alongshore transport of shelf water over the Middle Atlantic Bight shelf.

Velocity and hydrographic structure of subsurface shelf water at the Gulf Stream's edge

Relatively cool and fresh water, originating from the Middle Atlantic Bight continental shelf, has often been found along the northwestern margin of the Gulf Stream. In this paper we present measurements of temperature, salinity, and velocity within subsurface filaments of entrained shelf water adjacent to the Gulf Stream. The data indicate that vertical mixing within and near these filaments is effected by double‐diffusive processes and shear‐induced turbulence. The measurements also reveal a complex velocity and hydrographic structure within the filaments and show that the character of individual filaments differed considerably. For example, the measured mean flow within two filaments was much slower than the current in the adjacent Gulf Stream, whereas a third filament appeared to be incorporated within the high‐velocity region of the Gulf Stream. The most thoroughly sampled band of entrained shelf water exhibited temperature inversions and contained a slowly moving intrusion of warm water, presumably detached from the Gulf Stream. The estimated transport of shelf water within this band was roughly 7.2×104 m3 s−1, comparable with the rate at which water is transported over the southern Middle Atlantic Bight continental shelf as estimated by a previous study. However, the transport of individual entrained shelf water filaments may vary considerably because of the significant variation in their currents and hydrographic composition.