Cross-shelf Sections Research Articles

Spatial and temporal variations in the micronutrient Fe across the Peruvian shelf from 1984 to 2017

High dissolved iron (dFe) concentrations of the order of 10–100 nmol L−1 are a feature of waters influenced by sedimentary inputs in oxygen minimum zones (OMZ). However, the temporal development of dFe concentrations is poorly defined due to a general reliance on snapshot cross-shelf sections to study marine trace metal dynamics. Multiple cruise campaigns since the 1980s have investigated Fe dynamics over the Peruvian shelf, particularly between 9° S and 17°S where the shelf is broad, extremely productive and known to feature benthic dFe effluxes which are amongst the highest measured globally. This extensive long-term dataset uniquely allows us to study the interannual variability in dFe concentrations and their response to El Niño–Southern Oscillation (ENSO) events. By combining data from 11 cruises during the period 1984–2017 we are able to evaluate dFe dynamics on interannual timescales in a major OMZ. The region where average dFe concentrations are sensitive to variations in ENSO is confined to a subsurface layer at depths between 50 and 150 m, particularly in the narrow coastal region within 50 km of the coastline. Subsurface dFe concentrations were generally low during El Niño events (0.7–15.4 nmol L−1) and relatively high with a wider range of variability during the cold ENSO phase (1.1–52.1 nmol L−1). Inverse relationships between wind speed and surface/subsurface dFe were evident. In the subsurface layer, this may be attributable to enhanced dFe offshore transport along isopycnals when upwelling-favorable winds relax in accordance with previously outlined theories. Surface layer (<40 m) dFe variability was likely associated with a dilution and/or oxidation effect depending on the strength of wind driven water column mixing. Upwelling brings macronutrient-rich water into the euphotic zone, but its intensity had a limited impact on upper layer dFe concentrations possibly due to the influence of an onshore geostrophic flow. Interannual variability in surface chlorophyll-a (Chl-a) was found to correlate with dFe concentration in the offshore zone of northern Peru. This is consistent with bioassay experiments and climatological residual nitrate concentrations which both indicate proximal Fe limitation of phytoplankton growth over and beyond the northern Peruvian shelf. Overall, our work highlights the importance of physical factors driving short-term variations in Fe availability in one of the world’s most economically important fishery regions and suggests that, despite pronounced spatial and temporal variability in dFe concentrations, the ENSO phase has an impact on dFe availability.

Observation of continental shelf wave propagating along the eastern Taiwan Strait during Typhoon Meranti 2016

This study is the first to depict typhoon-induced continental shelf wave (CSW) propagation in the eastern Taiwan Strait (TWS) during the passage of Typhoon Meranti in 2016 using tidal gauge data and along-track satellite altimeter data. The strong amplitude response of sea level oscillations (within the range of 0.30–0.54 m) as a free, barotropic CSW after Meranti, which impacted the TWS, was clearly detected in time and frequency (in bands of 64–81 h) using wavelet and cross-wavelet analyses. The measured group and phase speeds were consistent with the dispersion curves for CSW with the first-mode derived from the cross-shelf sections of the eastern TWS, with the mean speeds reaching 3 and 5.6±0.7 m/s, respectively. Coincidentally, the typhoon-induced sea level anomaly (SLA) was also captured by the satellite altimeter before this CSW entered into the TWS. Using the theoretical cross-shore CSW modes to fit the SLA data, the results indicated that the first three wave modes can interpret this CSW event appeared in the southern TWS very well, with the first mode being the dominant one.

Tides and Turbulent Mixing in the North of Taiwan Island

Microstructure and hydrological profiles were collected along two cross-shelf sections from the deep slope to the shallow water in the north of Taiwan Island in the summer of 2006. While the tidal currents on the shelf were dominated by the barotropic tide with the current ellipse stretched across the shelf, significant internal tides were observed on the slope. The depth-mean turbulent kinetic energy (TKE) dissipation rate on the shelf was 10−6 W kg−1, corresponding to a diapycnal diffusivity of 10−2 m2 s−1. The depth-mean TKE dissipation rate on the slope was 1×10−7 Wkg−1, with diapycnal diffusivity of 3.4×10−4 m2 s−1. The shear instability associated with internal tides largely contributed to the TKE dissipation rate on the slope from the surface to 150 m, while the enhanced turbulence on the shelf was dominated by tidal or residual current dissipations caused by friction in the thick bottom boundary layer (BBL). In the BBL, the Ekman currents associated with the northeastward Taiwan Warm Current were identified, showing a near-bottom velocity spiral, which agreed well with the analytical bottom Ekman solution.

Seasonality in the cross-shelf physical structure of a temperate shelf sea and the implications for nitrate supply

We address a long-standing problem of how nutrients are transported from the shelf edge and from rivers to support regular, seasonal primary production in the interior of a wide, temperate, shelf sea. Cross-shelf sections of hydrography and nutrients, from a series of cruises between March 2014 and August 2015, along with time series of river discharge and river nutrient load are used to assess the seasonality of cross-shelf transports. Riverine nitrogen inputs are estimated to account for 30% of the nitrate available for the spring bloom on the inner shelf, and 10% in the mid- to outer-shelf. In the bottom layer in summer, high salinity, nutrient-rich waters are transported on-shelf as a result of wind-driven Ekman transport, cross-shelf pressure gradients and/or internal tidal wave Stoke’s drift. In the centre of the shelf this advection is responsible for 25% of the increase in bottom water nitrate seen between April and November 2014. The remaining nitrate increase suggests that about 50–62% of the nitrogen fixed into organic material during spring, summer and autumn phytoplankton growth is recycled in the bottom water over the 12 months between March 2014 and March 2015. In winter, when the water column is vertically mixed, there is a weak net off-shelf transport of about 1 m2 s−1, possibly driven by a reversal of the horizontal density gradient caused by excess cooling of shallower shelf waters. Overall, shelf nitrate concentrations are maintained by a combination of riverine supply, recycling of organic material, and summer on-shelf transports. We suggest that the main driver of inter-annual variability in pre-spring nitrate concentrations is variability in the depth of the winter mixed layer over the shelf slope.

Observed sub-inertial current variability and volume transport over the continental shelf in the northern South China Sea

Sub-inertial currents (SICs) over the continental shelf of the northern South China Sea (NSCS) are investigated using the in-situ current observations of acoustic Doppler current profiler (ADCP) mooring arrays off the Pearl River Estuary in 2006 and 2007. The survey was carried out in four separate time periods: summer, winter, spring (before the onset of the southwesterly monsoon), and fall (after the establishment of the northeasterly monsoon). The observations showed that the current directions were generally along the shelf, consistent with the directions of monsoonal winds. The currents were also affected by a few of mesoscale eddy events. In summer 2006, the volume transport was northeastward with a mean magnitude of 1.4 Sv through a cross-shelf section from the site of the depth of 135 m to the coast; in winter 2006/2007, spring 2007 and fall 2007, the volume transports were all southwestward with magnitudes of 2.0, 2.1, and 0.9 Sv, respectively, through a cross-shelf section from the site of the depth of 290 m to the coast. The standard deviations of the SICs were generally smaller than the velocities of the mean currents, and the variability of SICs showed significant correlation with the local sea surface winds. No persistent counter-wind currents were observed in the study area during the fall and winter observational periods.

Alongshore and cross-shore circulations and their response to winter monsoon in the western East China Sea

An array of four bottom-mounted acoustic Doppler current profilers (ADCPs) were deployed during the winter of 2008 (28 December 2008 to 12 March 2009) along a cross-shelf section in the western East China Sea to investigate the winter circulation and its response to wind. During the observation period, the observed subtidal currents exhibit coherent spatial structure and temporal variation in terms of their mean (seasonal), trend (intra-seasonal), and synoptic variability. The subtidal currents are polarized roughly in the alongshore direction parallel to local isobaths, and the weak cross-shore current is closely linked to the alongshore component. The temporal variation of the currents follows the rhythm of wind stress, sea level, and sea level difference at the synoptic scale.The mean currents are basically composed of two anti-parallel currents in the alongshore direction: the East China Sea coastal current (ECSCC) flows southwestward along the inner shelf and the Taiwan warm current (TWC) flows in the opposite direction along the outer-shelf. The strongest current occurs over the mid-shelf as a coastal jet. The intra-seasonal currents exhibit an expansion and intensification of the ECSCC along with shrinking and weakening the alongshore component of the TWC. There is a significant increase in onshore current particularly over the mid-shelf.The fluctuations of synoptic currents show a significant positive correlation with wind stress, and the fluctuations are negatively correlated with sea level and sea level difference. The coherent spatial structure of the currents indicates that the depth-independent column motion is related to the sea level difference through a barotropic pressure gradient. The vertical shear of currents is related to the density-related baroclinic pressure gradient in the whole water column and to the friction within the surface and bottom boundary layers.

Hydrographic data from the GEF Patagonia cruises

Abstract. The hydrographic data reported here were collected within the framework of the Coastal Contamination, Prevention and Marine Management Project (Global Environment Facility (GEF) Patagonia), which was part of the scientific agenda of the United Nations Development Program (UNDP). The project goal was to strengthen efforts to improve sustainable management of marine biodiversity and reduce pollution of the Patagonia marine environment. The observational component of the project included three multidisciplinary oceanographic cruises designed to improve the knowledge base regarding the marine environment and to determine the seasonal variability of physical, biological and chemical properties of highly productive regions in the southwest South Atlantic continental shelf. The cruises were carried out on board R/V ARA Puerto Deseado, in October 2005 and March and September 2006. On each cruise, hydrographic stations were occupied along cross-shelf sections spanning the shelf from nearshore to the western boundary currents between 38° and 55° S. This paper reports the quasi-continuous vertical profiles (conductivity–temperature–depth (CTD) profiles) and underway surface temperature and salinity data collected during the GEF Patagonia cruises. These data sets are available at the National Oceanographic Data Center, NOAA, US, doi:10.7289/V5RN35S0.

A NUMERICAL STUDY OF THE BRAZILIAN SHELF/SLOPE REGION SOUTH OF 13S USING THE OCEAN MODEL ROMS

The oceanic features in the eastern and southeastern brazilian shelf/slope south&#x0D; of 13S is investigated using ROMS (Regional Ocean Model System). The model&#x0D; integration was 9 years and it was forced with: i) 6-hourly synoptic atmospheric&#x0D; data from NCEP; ii) initial and boundary conditions from OCCAM (Ocean Climate&#x0D; Circulation Advanced Modelling) monthly mean climatology and iii) tidal&#x0D; forcing from TPXO 7.1 global data set. The model results were compared with&#x0D; observations, which consisted in thermodynamic MDL (Mixed Layer Depth) climatology,&#x0D; satellite data, measurements from tide gauges along the shelf and currents&#x0D; measurements values from literature. The simulated currents represented well the&#x0D; BC (Brazil Current)-IWBC (IntermediateWestern Boundary Current) System. The&#x0D; BC - IWBC system at 22S cross-shelf section, where the mean alongshelf velocity&#x0D; represents our simulation capability of reproducing the western boundary currents,&#x0D; showed poleward BC and a opposing IWBC. At this section, the BC velocity core&#x0D; is in 50 m with 0.41 m.s−1 and the IWBC core around 800 m with 0.15 m.s−1.

Diurnal heat balance for the northern Monterey Bay inner shelf

[1] In the summer of 2007, physical measurements including velocity from acoustic Doppler current profilers, surface gravity wave heights measured acoustically, and temperature from thermistor chain arrays were collected along- and across- the mid to inner shelf (water depths from 10–60 m) in northern Monterey Bay. The oceanic response to a strong (8–15 m s−1 daily maximum) along-shelf sea breeze is examined by evaluating the diurnal heat budget over a cross-shelf section of the inner shelf. The diurnal heat budget closes to within the 95% confidence level with daily warming and cooling periods explained by two separate, but related processes. During evening/early morning warming period, 77% of the observed temperature increase is due to along-shelf advection of a temperature gradient within the upwelling shadow zone, a process which is arrested during the period of wind-forcing. In contrast, 75% of the afternoon cooling period is explained by the cross-shelf heat flux driven by diurnal along-shelf winds. In this study, diurnal tides are found to contribute less than 10% of the observed temperature variability and surface gravity waves do not show any significant diurnal variability. Richardson number estimates show that, on average, wind-induced shear is not strong enough to erode the strength of water column stratification within the upwelling shadow.

Sources of Nutrients and Seasonal Dynamics of Chlorophyll in the Inner Shelf off Paraná State—South Brazil Bight

Abstract The seasonal dynamics of phytoplankton chlorophyll and nutrients were studied between August 1997 and March 1999 along a cross-shelf section of the inner coastal of Parana State, southern Brazil. Vertical thermohaline stratifications in the rainy summer alternated with winter homogeneities. Nitrate-N varied from 0.17 to 1.13 µM, nitrite-N from 0.04 to 1.10 µM, phosphate-P from 0.37 to 0.73 µM, and silicate-Si from 2.5 to 10.2 µM, all presenting maximum concentrations at the bottom layers at the outer stations. Chlorophyll concentrations varied from <0.02 to 10.8 µg·L−1 throughout the study period, with maxima at subsurface and bottom layers depending on the season. Seasonal differences in the distributional patterns of nutrients and chlorophyll were caused by two main wind-driven physical processes: (i) onshore intrusions of South Atlantic Central Waters and (ii) remote continental runoff from La Plata River. These were considered the main sources of new nutrients in summer and winter periods, re...

On the double-diffusive and cabbeling environment of the Arctic Front, West Spitsbergen

This paper describes the thermohaline characteristics of the Arctic Front to the west of Spitsbergen, in terms of the double-diffusive environment and the potential for densification through cabbeling. The front separates the warm, saline Atlantic water in the West Spitsbergen Current from the cooler, fresher water on the West Spitsbergen Shelf. We have investigated processes at the front that can contribute to, or enhance, mixing and water mass modification in relation to heat transport to the Arctic. Hydrographic data were collected along a cross-shelf section in September 2005. The double-diffusive properties along the section were determined by calculating Turner angles, and cabbeling was investigated with a simple linear mixing scheme. Double diffusion, in the form of diffusive layering, was found to be active within the Arctic Front, with considerable interleaving between water masses. Furthermore, mixing of water masses across the front was found to generate a potential increase in density of more than 0.03 kg m -3 through cabbeling, which would then promote sinking and convergence. Our analyses indicate that the two processes of double diffusion and cabbeling are active at the Arctic Front. We discuss their potential contribution to maintaining the density-compensated nature of the front, and conclude that they will promote isopycnal mixing and subsequent modification of the core water of the West Spitsbergen Current.

Primary production on the continental shelf off Cape Hatteras, North Carolina

Abstract Distributions of primary production and chlorophyll size structure were examined in relationship to hydrographic properties during March and July 1996 in continental shelf waters off Cape Hatteras, North Carolina. Chlorophyll concentrations ranged highest (>10 mg m −3 ) during March. Peak values were associated with subsurface maxima near the shelf edge, which were dominated by larger (>8 μm) phytoplankton. In July, chlorophyll concentrations were generally lower with higher proportions associated with the 2.0 g C m −2 d −1 ) in association with the shelf–slope front. In contrast, highest values in July (>3.0 g C m −2 d −1 ) were associated with an intrusion of high-salinity, high-nutrient water into the southern portion of the study region. Average shelf-wide productivities were 1.3 g C m −2 d −1 in March and 2.1 g C m −2 d −1 in July. Contemporaneous Advanced Very High-Resolution Radiometer sea-surface temperature imagery revealed that regions of high productivity were proximal to Gulf Stream circulation features. In March, cross-shelf sections of hydrographic properties and nutrients provided evidence for onshore displacement of the shelf–slope front, apparently related to the combined effects of the strong winds prior to the cruise and the offshore presence of energetic Gulf Stream processes. We speculate that variable forcing, related to the dynamics of the Gulf Stream and wind, resulted in significant frontal displacement and frontal velocity fluctuations leading to enhanced mixing and injection of nutrients across the frontal boundary. In July, Gulf Stream circulation, in conjunction with upwelling favorable conditions, resulted in a subsurface intrusion of high-nutrient water onto the shelf analogous to similar phenomena observed in the South Atlantic Bight. Localized regions of enhanced productivity appear to be quantitatively important in the overall carbon/nitrogen budget of the southern Mid-Atlantic Bight shelf.

Continuity of the poleward undercurrent along the eastern boundary of the mid-latitude north Pacific

Several recent data sets improve our view of the poleward undercurrent of the California Current System. As part of a triennial National Marine Fisheries Service (NMFS) survey of Pacific whiting, a series of 105 shipboard acoustic Doppler current profiler (ADCP) velocity sections across the shelf break from 33 to 51°N at about 18 km meridional spacing were collected from July to August 1995. Significant (>0.05 m s −1) subsurface poleward flow occurred in 91% of the sections. A mean cross-shelf section using the entire data set has statistical significance, revealing an undercurrent core >0.1 m s −1 from 200–275 m depth 20–25 km off the shelf break. The mean poleward volume transport in a 125–325 m layer is 0.8±0.2×10 6 m 3 s −1 . We focus particular attention on the Cape Blanco to Cape Mendocino region, and the NMFS results are compared with shipboard ADCP three weeks later from a study of coastal upwelling processes near Cape Blanco. ADCP streamfunction maps are derived and strongly suggest that one portion of flow is continuous over the 440 km meridional extent of the analysis region. Other portions of the flow show evidence of offshore turning, separation, and the formation of anti-cyclonic eddies. We also note that isopycnic potential vorticity from alongslope CTD stations during the NMFS survey appears to be a tracer for the poleward flow.

Spectral light absorption and attenuation measurements from a towed undulating vehicle

Measurements from a nine-wavelength light absorption and attenuation instrument mounted on a towed, undulating vehicle (SeaSoar) capable of rapidly profiling the water column – complete up-down cycles in 1.5–12 min depending on the maximum sampling depth – are used to characterize strong spatial variations in the distributions of upper-ocean bio-optical properties. Water sampled from adjacent to the conductivity and temperature sensors located in the nose of the vehicle is pumped through the 25-cm dual optical flow tubes of a Western Environmental Technology Laboratories (WET Labs) ac-9 instrument mounted on top of SeaSoar. A three-stage algorithm for post-processing the optical data to insure high-quality measurements concurrent with conductivity–temperature–depth data is described. After synchronizing the optical, navigational and conductivity–temperature–depth data streams, the method relies on finding the optimal time delay between when a water parcel is sampled first by the temperature and conductivity sensors and then by the absorption and attenuation optics. After applying the calculated time-dependent lag, a correction for the dependence of light absorption on temperature and salinity and a scattering correction to absorption are made. The final processed optical data from SeaSoar compare well with the same parameters sampled by a slowly lowered vertical profiling package deployed from a nearby stationary vessel. A 2-h, 30-km long cross-shelf section consisting of 184 vertical profiles separated by 150–200 m reveals strong horizontal variations on short spatial scales (1 km or less) of the vertical distributions of bio-optical properties.

Secondary circulation associated with a shelfbreak front

Evidence for secondary circulation associated with a shelfbreak front is obtained from a high‐resolution, cross‐shelf section of hydrographic, optical and velocity fields. Convergence in the bottom boundary layer on the inshore side of the front and subsequent upwelling into the interior is evident by a mid‐water region of suspended bottom material emanating from the foot of the front and extending to within 35 m of the surface, 80 m above bottom. Downwelling on the offshore side of the front in the upper water column is inferred from a 20‐m downward bend of the subsurface phytoplankton layer. These observations are in agreement with recent model predictions for secondary circulation near an idealized shelfbreak front. Convergence in measured cross‐shelf velocity at the foot of the front is consistent with upwelling of bottom material detected there. An estimate of 9±2 m day−1 of upwelling on the inshore side of the shelfbreak front is obtained, implying a transit time from the bottom to the surface of 10–16 days.

Seasonal and inter-annual variability in properties of surface water off Santander, Bay of Biscay, 1991–1995

The annual cycle of temperature, salinity and nutrients of surface waters (up to 100 m depth) was studied from June 1991 to December 1995 in a cross-shelf section over the continental shelf waters off Santander (southern Bay of Biscay). The time series showed that the temperature followed the expected seasonal warming and cooling pattern, which determines a seasonal process of stratification and mixing of the water column. The stratification period occurs annually between May and October in a layer of about 50 m depth from the neritic station beyond to the shelf-break. In the period between November and April the water column remained mixed. During spring and summer low salinity values were found in the surface due to continental runoff and advection from oceanic waters. In late autumn and winter, the salinity pattern was governed by an influx of salty water associated with the poleward current. As in other temperate latitudes, nitrates showed the highest values in winter throughout the water column and the lowest values at the surface during the stratified period. Wind-induced upwelling events were observed mainly in summer, which are characterised by low temperatures (< 12°C), high salinity and nutrient concentrations. The inter-annual variability of temperature showed a warming trend in the upper layers but this sign was not found at 100 m depth. In salinity a decreasing trend was observed throughout the water column, and this feature corresponds to the relaxing of the high salinity anomaly detected in the North Atlantic at the beginning of the 1990s. Both trends were coherent in the cross-shelf section from the coast to the slope.

Current-meter observations in the Argentine Continental Shelf

The first moored current measurements carried out on the Argentine Continental Shelf are presented. In order to explore the mean value and the main temporal scales of the velocity field, current meter data were collected in a cross-shelf section near 43°S between September 1991 and August 1992. The records indicate that the mean flow is parallel to the local topography and that its intensity increases towards the shelf-break. The mean flow values seem to be related to the interaction of tidal currents with sea-surface height in the inner shelf and cross-shelf variations in the mass field in the middle shelf. The high frequency component (periods of less than 30 h) accounts for more than 80% of the observed kinetic energy. Close to the coast the semidiurnal component is dominant (possibly due to a resonance process), while in the middle shelf the inertial and diurnal components become equally important. The very low frequency component (periods longer than 10 days) increases its energy seaward, which suggests that it might be forced by ocean circulation. The relation between currents in the band 0.5-0.1 cycles per day and the wind measured on the coast is not clear.

Nutrient inputs into the central Great Barrier Reef (Australia) from subsurface intrusions of Coral Sea waters: a two-dimensional displacement model

In the central Great Barrier Reef (17°–20°S), near-bottom intrusions of cool, nutrient-rich water upwell episodically onto the outer shelf several times each summer (October–April). Near-bottom water temperature at the shelf break was found to be a useful but conservative predictor of the volume of intruded water and inorganic nutrient (N, P) inputs in cross-shelf sections. Large upwelling events that occur at least once per summer may displace up to 1 3 of the water on the outer shelf and import phosphate and nitrate stocks 2–6 times those normally present in outer-shelf waters. The concurrently displaced outer-shelf waters export substantial amounts of organic N and P. Net N and P imports are on the order of 75% and 30% of gross N and P imports, respectively. Interannual variations in nutrient inputs from the Coral Sea are related to the number and intensity of intrusive upwelling events occurring each summer. Comparison with other nutrient fluxes suggests upwelling is a substantial source of “new” N and P to the central Great Barrier Reef.

Hydrographic profiling across the East China sea shelf edge by an underwater sliding vehicle with CTD sensors

Temperature, salinity and density structures were observed on Sept. 23 and 24, 1986 at one vertical section across the East China Sea shelf edge by an advanced type of towed vehicle with CTD sensors which was developed by the Japan Marine Science and Technology Center. The vehicle was towed at a speed of 2.5 m s −1 down to 150 m depth and at intervals of 170–500 m width. The observed profile was 50 km long on Sept. 23 and 70 km long on Sept. 24 along the cross-shelf section. An on-ship acoustic Doppler current profiler was simultaneously used to measure current velocities at depths of 20, 50 and 100 m. Interesting features were noticed. Firstly, there was a vertical displacement of pycnoclines at the lower edge of the surface mixed layer accompanied by vertical inversion of the salinity and temperature in the vicinity of the shelf edge. Pycnoclines were displaced upward by 12 m toward the outer edge on Sept. 23 and by 20 m on Sept. 24. On Sept. 23, the salinity inversion took place in a layer 20 m thick and 8 km wide, whereas the temperature inversion took place in a layer 8 m thick and 1.5 km wide. These vertical inversions were probably generated by vertical shear of tidal currents which was observed by the Doppler current profiler. These results throw light on understanding the vertical mixing process of stratified water on the continental shelf edge. Secondly, an intrusion of the shelf water into the Kuroshio water was observed along pycnoclines below the surface mixed layer 60 to 70 m deep in the Kuroshio region outer break. The measurement was successful in showing a horizontal mixing process of the shelf water and the Kuroshio water which could not be found out by standard CTD observations.

Mixing and bottom water formation in the shelf break region of the southern Weddell Sea

The shelf break region of the southern Weddell Sea was investigated during the period 1977–1980 with sections of closely spaced CTD stations and long-term moored current meters. An intrusion of warmer water onto the shelf from intermediate depths offshore was found in all cross-shelf sections. Evidence that this warmer water intrudes onto the shelf throughout the year was found in the current meter temperature records. Continental shelf waves appear to be the most likely mechanism for bringing this warmer water onto the shelf. Mixing of very cold water on the shelf with the warmer intruded water appears to be promoted by stirring induced by the semidiurnal tides. The mixing also may be directly caused by shear instabilities. Bottom water that forms near the shelf break and flows down the continental slope in the southern Weddell Sea appears to represent a mixture of the cold shelf water with the warmer intruded water and not cold, dense shelf water alone.