Mixing and advection of a cold water cascade over the Wyville Thomson Ridge

The Wyville Thomson Ridge forms part of the barrier to the meridional circulation across which cold Nordic Sea and Arctic water must traverse to reach the Atlantic Ocean. Overflow rates across the ridge are variable (but can be dramatic at times), and may provide a subtle indicator of significant change in the circulation in response to climate change. In spring 2003, a series of CTD sections were conducted during a large overflow event in which Norwegian Sea Deep Water (NSDW) cascaded down the southern side of the ridge into the Rockall Trough at a rate of between 1 and 2 Sv. The NSDW was partially mixed with overlying North Atlantic Water (NAW), and comprised about 1/3rd of the cascading water. The components of NAW and NSDW in the overflow were sufficiently large that there must have been a significant divergence of the inflow through the Faroe-Shetland Channel, and of the outflow through the Faroe Bank Channel.As the plume descended, its temperature near the sea bed warmed by over 3 °C in about a day. Although the slope was quite steep (0.03), the mean speed of the current (typically 0.36 m s⁻¹) was too slow for significant entrainment of NAW to occur (the bulk Richardson number was of order 5). However, very large overturns (up to 50 m) were evident in some CTD profiles, and it is demonstrated from Thorpe scale estimates that the warming of the bottom waters was due to mixing within the plume. It is likely that some of the NSDW had mixed with NAW before it crossed the ridge. The overflow was trapped in a gully, which caused it to descend to great depth (1700 m) at a faster rate, and with less modification due to entrainment, than other overflows in the North Atlantic. The water that flowed into the northern part of the Rockall Trough had a temperature profile that ranged from about 3 to 8 °C. Water with a temperature of >6 °C probably escaped into the Iceland Basin, between the banks that line the north-western part of the Trough. Colder water (< 6 °C) must have travelled down the eastern side of the Rockall Bank, and may have had a volume flux of up to 1.5 Sv.     

Water mass properties and fluxes in the Rockall Trough, 1975–1998

A time series of a standard hydrographic section in the northern Rockall Trough spanning 23 yr is examined for changes in water mass properties and transport levels. The Rockall Trough is situated west of the British Isles and separated from the Iceland Basin by the Hatton and Rockall Banks and from the Nordic Seas by the shallow (500 m) Wyville–Thompson ridge. It is one pathway by which warm North Atlantic upper water reaches the Norwegian Sea and is converted into cold dense overflow water as part of the thermohaline overturning in the northern North Atlantic and Nordic Seas. The upper water column is characterised by poleward moving Eastern North Atlantic Water (ENAW), which is warmer and saltier than the subpolar mode waters of the Iceland Basin, which also contribute to the Nordic Sea inflow. Below 1200 m the deep Labrador Sea Water (LSW) is trapped by the shallowing topography to the north, which prevents through flow but allows recirculation within the basin. The Rockall Trough experiences a strong seasonal signal in temperature and salinity with deep convective winter mixing to typically 600 m or more and the formation of a warm fresh summer surface layer. The time series reveals interannual changes in salinity of ±0.05 in the ENAW and ±0.04 in the LSW. The deep water freshening events are of a magnitude greater than that expected from changes in source characteristics of the LSW, and are shown to represent periodic pulses of newer LSW into a recirculating reservior. The mean poleward transport of ENAW is 3.7 Sv above 1200 dbar (of which 3.0 Sv is carried by the shelf edge current) but shows a high-level interannual variability, ranging from 0 to 8 Sv over the 23 yr period. The shelf edge current is shown to have a changing thermohaline structure and a baroclinic transport that varies from 0 to 8 Sv. The interannual signal in the total transport dominates the observations, and no evidence is found of a seasonal signal.     

Declining nutrient concentrations in the northeast Atlantic as a result of a weakening Subpolar Gyre

Between 1996 and the mid-2000s the upper waters (200–700 m) of the Rockall Trough became warmer (+0.72 °C), saltier (+0.088) and reduced in nitrate and phosphate (−2.00 µM and −0.14 µM respectively). These changes, out-with calculated errors, can be explained by the varying influence of southern versus subpolar water masses in the basin as the Subpolar Gyre weakened and contracted. Upper water properties strongly correlate with a measure of the strength of the Subpolar Gyre (the first principal component of sea surface height over the Subpolar North Atlantic) prior to the mid-2000s. As the gyre weakens, the upper layers of the trough become warmer (r−0.85), more saline (r−0.86) and reduced in nitrate and phosphate (r+0.81 and r+0.87 respectively). Further the proportion of subpolar waters in the basin decreases from around 50% to less than 20% (r+0. 88). Since the mid−2000s the Subpolar Gyre has been particularly weak. During this period temperatures decreased slightly (−0.21 °C), salinities remained near constant (35.410±0.005) and phosphate levels low and stable (0.68±0.02 µM). These relative lack of changes are thought to be related to the maximum proportion of southern water masses within the Rockall Trough having been reached. Thus the upper water properties are no longer controlled by changes in the relative importance of different water masses in the basin (as prior to the mid-2000s), but rather a different process. We suggest that when the gyre is particularly weak the interannual changes in upper water properties in the Rockall Trough reflect changes in the source properties of the southern water masses. Since the early-2000s the Subpolar Gyre has been weaker than observed since 1992, or modelled since 1960–1970. Hence upper waters within the Rockall Trough may be warmer, saltier and more depleted in nitrate and phosphate than at any time in the last half century.     

Wyville Thomson Ridge Overflow Water: Spatial and temporal distribution in the Rockall Trough

Wyville Thomson Ridge Overflow Water (WTOW), which is the only part of the outflow from the Norwegian Sea not to directly enter the Iceland Basin, is shown to be a significant water mass in the northern Rockall Trough. It is found primarily at intermediate depths (600–1200 m) beneath the northward flowing warm Atlantic waters, and above recirculating Mediterranean influenced waters and Labrador Sea Water (LSW). The bottom of the WTOW layer can be identified by a mid-depth inflexion point in potential temperature–salinity plots. An analysis of historical data reveals that WTOW has been present in all but eight of the last 31 years at 57.5°N in the Rockall Trough. A denser component of WTOW below 1500 m has also been present, although it appears to be less persistent (12 out of the 31 years) and limited to the west of the section. The signature of intermediate WTOW was absent in two periods, the mid-1980s and early 1990s, both of which coincided with a freshening, and probable increase in volume, of LSW in the trough. Potential temperature–salinity diagrams from historical observations indicate that WTOW persists at least as far south as 55°N (and as far west as 20°W in the Iceland Basin) although its signature is quickly lost on leaving the Rockall Trough. We suggest that a transport of WTOW down the western side of the trough exists, with WTOW at intermediate depths entering the eastern trough either via a cyclonic recirculation, or as a result of eddy activity. Further, WTOW is seen on the Rockall–Hatton Plateau and in the deep channels connecting with the Iceland Basin, suggesting additional possible WTOW transport pathways. These suggested transport routes remain to be confirmed by further observational or modelling studies.     

Hydrodynamic controls on cold-water coral growth and carbonate-mound development at the SW and SE Rockall Trough Margin,NE Atlantic Ocean

Long-term (⩽1-year) records obtained by seabed observatories (BOBO) and repeated (24-h) CTD casts show the presence of a highly energetic environment in and around two cold-water carbonate-mound provinces, on the Southwest and Southeast Rockall Trough (SW and SE RT) margin. Carbonate mounds, covered with a thriving coral cover, are embedded mainly in the Eastern North Atlantic Water (ENAW) and are observed in a confined bathymetric zone between 600 and 1000 m water depth. Cold-water corals seem to be restricted in their growth by temperature and food availability. The presence of living corals on top of the carbonate mounds appears linked to the presence of internal waves and tidal currents in the water column, and consequently carbonate mound structures are shaped by the local hydrodynamic regime. Mound clusters have an elongated shape perpendicular to the regional contours and corresponding to the direction of the highest current speeds. On the SW RT margin temperature, salinity and current speed reflect a diurnal tidal pattern, causing maximum temperature variations at 900 m depth of more than 3 °C. Current speeds up to 45 cm s⁻¹ occur, and a residual current of 10 cm s⁻¹ is directed along the slope to the southwest. At the SE RT margin the temperature of the bottom water fluctuates more than 1 °C with a semi-diurnal tidal cyclicity. Amplitudes of average and peak current speeds here are comparable with those measured on the southwest margin, but the residual current in this area is directed to the northeast. Tidal currents and internal waves at both margins force the formation of intermediate and bottom nepheloid layers and bring fresh food particles with increased velocity to the mounds. The distribution of corals in both mound areas is considered directly related to the presence of enhanced turbidity. An increase in temperature can be directly related to an increase in the amount of particles in the water column. Current velocity increases when a transition occurs from cold to warm waters. High current velocities prevent local sedimentation but provide sufficient food particles to the corals, so that the corals thrive at the mound summits.     

On the circulation of bottom water in the region of the Vema Channel

The circulation and transport of Antarctic Bottom Water (σ₄<45.87) in the region of the Vema Channel are studied along three WOCE hydrographic lines, the geostrophic velocities referenced to previously published direct current measurements. The primary supply of water to the deep Vema Channel is from the Argentine Basin's deep western boundary current, with no indication of an inflow from the southeast. In the northern Argentine Basin, detachment of lower North Atlantic Deep Water from the continental slope is associated with a deep thermohaline front near 34°S. To the north of this front, the upper part of the AABW bound for the Vema Channel (σ₄<46.01) exhibits a significant NADW influence. Further modification of the throughflow water occurs near 30°30′S, where the channel orientation changes by ∼50°. Southward flow of bottom water on the eastern flank of the Vema Channel, amounting to ∼1.5 Sv, represents a significant countercurrent to the deep channel transport. Inclusion of this countercurrent reduces the net flow of AABW through the Vema Channel from 3.2±0.7 to 1.7±1.1 Sv. Water properties imply that the near-zero net flow over the Santos Plateau results from a near-closed cyclonic circulation fed by the deep Vema Channel throughflow. A disruption of the northward boundary current in the upper AABW (lower circumpolar water) is required by this flow pattern. The extension of the cyclonic circulation on the Santos Plateau enters the Brazil Basin as a ∼1 Sv flow distinct from the outflow in the Vema Channel Extension (6.2 Sv). The high magnitude of the latter suggests a southward recirculation of bottom water near the western boundary to the north of the region of study.     

Late Quaternary siliciclastic/carbonate sedimentation model for the Capricorn Channel,southern Great Barrier Reef province,Australia

A model is presented for hemipelagic siliciclastic and carbonate sedimentation during the last glacial–interglacial cycle in the Capricorn Channel, southern Great Barrier Reef (GBR). Stable isotope ratios, grainsize, carbonate content and mineralogy were analysed for seven cores in a depth transect from 166 to 2892 m below sea level (mbsl). Results show variations in the flux of terrigenous, neritic and pelagic sediments to the continental slope over the last sea level cycle.During the glacial lowstand terrigenous sediment influenced all the cores down to 2000 mbsl. The percentages of quartz and feldspar in the cores decreased with water depth, while the percentage of clay increased. X-ray diffraction analysis of the glacial lowstand clay mineralogy suggests that the siliciclastic sediment was primarily sourced from the Fitzroy River, which debouched directly into the northwest sector of the Capricorn Channel at this time. The cores also show a decrease in pelagic calcite and an increase in aragonite and high magnesium calcite (HMC) during the glacial. The influx of HMC and aragonite is most likely from reworking of coral reefs exposed on the continental shelf during the glacial, and also from HMC ooids precipitated at the head of the Capricorn Channel at this time. Mass accumulation rates (MARs) are high (13.5 g cm^? 3 kyr^? 1) during the glacial and peak at ~ 20 g cm^? 3 kyr^? 1 in the early transgression (16–14 ka BP). MARs then decline with further sea level rise as the Fitzroy River mouth retreats from the edge of the continental shelf after 13.5 ka BP. MARs remain low (4 cm^? 3 kyr^? 1) throughout the Holocene highstand.Data for the Holocene highstand indicate there is a reduction in siliciclastic influx to the Capricorn Channel with little quartz and feldspar below 350 mbsl. However, fine-grained fluvial sediments, presumably from the Fitzroy River, were still accumulating on the mid slope down to 2000 mbsl. The proportion of pelagic calcite in the core tops increases with water depth, while HMC decreases, and is present only in trace amounts in cores below 1500 mbsl. The difference in the percentage of HMC in the deeper cores between the glacial and Holocene may reflect differences in supply or deepening of the HMC lysocline during the glacial.Sediment accumulation rates also vary between cores in the Capricorn Channel and do not show the expected exponential decrease with depth. This may be due to intermediate or deep water currents reworking the sediments. It is also possible that present bathymetry data are too sparse to detect the potential role that submarine channels may play in the distribution and accumulation of sediments.Comparison of the Capricorn Channel MARs with those for other mixed carbonate/siliciclastic provinces from the northeast margin of Australia indicates that peak MARs in the early transgression in the Capricorn Channel precede those from the central GBR and south of Fraser Island. The difference in the timing of the carbonate and siliciclastic MAR peaks along the northeast margin is primarily related to differences in the physiography and climate of the provinces. The only common trend in the MARs from the northeast margin of Australia is the near synchronicity of the carbonate and siliciclastic MAR peaks in individual sediment cores, which supports a coeval sedimentation model.     

Regional circulation around Heard and McDonald Islands and through the Fawn Trough,central Kerguelen Plateau

The fine-scale circulation around the Heard and McDonald Islands and through the Fawn Trough, Kerguelen Plateau, is described using data from three high-resolution CTD sections, Argo floats and satellite maps of chlorophyll a, sea surface temperature (SST) and absolute sea surface height (SSH). We confirm that the Polar Front (PF) is split into two branches over the Kerguelen Plateau, with the NPF crossing the north-eastern limits of our survey carrying 25 Sv to the southeast. The SPF was associated with a strong eastward-flowing jet carrying 12 Sv of baroclinic transport through the deepest part of Fawn Trough (relative to the bottom). As the section was terminated midway through the trough this estimate is very likely to be a lower bound for the total transport. We demonstrate that the SPF contributes to the Fawn Trough Current identified by previous studies. After exiting the Fawn Trough, the SPF crossed Chun Spur and continued as a strong north-westward flowing jet along the eastern flank of the Kerguelen Plateau before turning offshore between 50°S and 51.5°S. Measured bottom water temperatures suggest a deep water connection between the northern and southern parts of the eastern Kerguelen Plateau indicating that the deep western boundary current continues at least as far north as 50.5°S. Analysis of satellite altimetry derived SSH streamlines demonstrates a southward shift of both the northern and southern branches of the Polar Front from 1994 to 2004. In the direct vicinity of the Heard and McDonald islands, cool waters of southern origin flow along the Heard Island slope and through the Eastern Trough bringing cold Winter Water (WW) onto the plateau. Complex topography funnels flow through canyons, deepens the mixed layer and increases productivity, resulting in this area being the preferred foraging region for a number of satellite-tracked land-based predators.     

Numerical investigation of deep water circulation in the Faroese Channels

The overflow of dense water from the Nordic Seas through the Faroese Channels is investigated numerically using the Massachusetts Institute of Technology General Circulation Model. The model is forced by the removal of a barrier that separates different water masses in the bottom layer of the Faroe-Shetland Channel at the north-eastern boundary. An analysis of the output reveals that during its adjustment in the rotating channel the propagating flow is unstable and forms cyclonic and anti-cyclonic eddies in the Faroese Channels. The life-time of the cyclonic eddy is about 10 days, but an anti-cyclonic eddy that is formed upstream of the sill crest of the Faroe Bank Channel has a longer life-time. However, after 50 days it eventually loses its structure below 400 m due to the decay of a counter-rotating current. In the upper 400 m layer this anti-cyclonic eddy remains persistent for longer. Observational evidence of the eddy is confirmed by the tracks of experimental drifters released in the area and by the temperature and salinity fields observed in the Faroese Channels.The pinching of isotherms along the Wyville Thomson Ridge results in the concentration of cold water on the southern side of the Faroese Channels that overflows into the Rockall Trough. The model results demonstrate that the main part of the cold water outflows through the Faroe Bank Channel, rather than across the Wyville Thompson Ridge, due to Earth rotation. The apparent similarity of modelled temperature, salinity and velocity sections to recent measurements in this area adds confidence to these results.     

Volume transport and distribution of deep circulation at 165°W in the North Pacific

We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O₂>4.2 ml l^?1), and lowest in silica (Si<135 μmol kg^?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=10⁶ m³ s^?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.     

An inverse modeling study in Fram Strait. Part II: Water mass distribution and transports

A water-mass analysis is carried out in Fram Strait, between 77.15 and 81.15°N, based on three-dimensional large-scale potential temperature and salinity distributions reconstructed from the MIZEX 84 hydrographic data collected in summer 1984. Combining these distributions with the geostrophic flow field derived from the same data in a companion paper (Schlichtholz and Houssais, 1999), the heat, fresh water and volume transports are estimated for each of the water masses identified in the strait. Twelve water masses are selected based on their different origins. Among them, the Polar Water (PW) enters Fram Strait from the Arctic Ocean both over the Greenland Slope and over the western slope of the Yermak Plateau. In the Atlantic Water (AW) range, four modes with distinct geographical distributions are indentified. In the Deep Water range, the Eurasian Basin Deep Water (EBDW) is confined to the Lena Trough and to the Molloy Deep area where it is involved in a cyclonic circulation. The warm and shallower mode of the Norwegian Sea Deep Water (NSDW), concentrated to the west, is mainly seen as an outflow from the Arctic Ocean while the cold and deeper mode, essentially observed to the east, enters the strait from the Greenland Sea. Apart from the EBDW, there is a tendency for all water masses of polar origin to flow along the Greenland Slope. The two most abundant water masses, the AW and the NSDW, occupy as much as 67% of the total water volume. The southward net transport of PW through Fram Strait is about 1 Sv at 78.9°N. At the same latitude, the net transport of AW is southward and equal to about 1.7 Sv. Only the transport of the warm mode (AWw) is northward, amounting to 0.2 Sv. The overall net outflow of the Deep Waters to the Greenland Sea is about 2.6 Sv. Two upper water masses, the fresh (AWf) and the cold (AWc) mode of the AW, and one deep-water mass, the NSDW, appear to be produced in the strait, with production rates, between 77.6 and 79.9°N, of about 0.2, 1.0 and 1.7 Sv, respectively. A southward net fresh-water transport through the strait of about 2000 km³ yr⁻¹ (relative to a salinity of 34.93) is mainly due to the PW. The net heat transport relative to −0.1°C is northward, but undergoes a rapid northward decrease, suggesting an area-averaged surface heat loss of 50–100 W m⁻² in the strait.     

Vertical and horizontal extension of the oxygen minimum zone in the eastern South Pacific Ocean

Recent hydrographic measurements within the eastern South Pacific (1999–2001) were combined with vertically high-resolution data from the World Ocean Circulation Experiment, high-resolution profiles and bottle casts from the World Ocean Database 2001, and the World Ocean Atlas 2001 in order to evaluate the vertical and horizontal extension of the oxygen minimum zone (<20 μmol kg⁻¹). These new calculations estimate the total area and volume of the oxygen minimum zone to be 9.82±3.60×10⁶ km² and 2.18±0.66×10⁶ km³, respectively. The oxygen minimum zone is thickest (>600 m) off Peru between 5 and 13°S and to about 1000 km offshore. Its upper boundary is shallowest (<150 m) off Peru, shoaling towards the coast and extending well into the euphotic zone in some places. Offshore, the thickness and meridional extent of the oxygen minimum zone decrease until it finally vanishes at 140°W between 2° and 8°S. Moving southward along the coast of South America, the zonal extension of the oxygen minimum zone gradually diminishes from 3000 km (15°S) to 1200 km (20°S) and then to 25 km (30°S); only a thin band is detected at ∼37°S off Concepción, Chile. Simultaneously, the oxygen minimum zone's maximum thickness decreases from 300 m (20°S) to less than 50 m (south of 30°S). The spatial distribution of Ekman suction velocity and oxygen minimum zone thickness correlate well, especially in the core. Off Chile, the eastern South Pacific Intermediate Water mass introduces increased vertical stability into the upper water column, complicating ventilation of the oxygen minimum zone from above. In addition, oxygen-enriched Antarctic Intermediate Water clashes with the oxygen minimum zone at around 30°S, causing a pronounced sub-surface oxygen front. The new estimates of vertical and horizontal oxygen minimum zone distribution in the eastern South Pacific complement the global quantification of naturally hypoxic continental margins by Helly and Levin [2004. Global distribution of naturally occurring marine hypoxia on continental margins. Deep-Sea Research I 51, 1159–1168] and provide new baseline data useful for studies on the role of oxygen in the degradation of organic matter in the water column and the related implications for biogeochemical cycles. Coastal upwelling zones along the eastern Pacific combine with general circulation to provide a mechanism that allows renewal of upper Pacific Deep Water, the most oxygen-poor and oldest water mass of the world oceans.     

Determining the weights of two types of artificial reefs required to resist wave action in different water depths and bottom slopes

The interaction between waves and artificial reefs (ARs; a hollow cube weighing 8.24 kN (0.84 t) and a water pipe weighing 1.27 kN (0.13 t)) in shallow waters was investigated with respect to variations in design weight, orientation (for cube; 45° and 90° angles, for pipe; 0°, 90°, and 180° angles to flow), depth (1–20 m), and bottom slope (10^?1, 30^?1, and 50^?1). Physics equations and FLUENT software were used to estimate resisting and mobilising forces, and drag coefficients. Drag coefficients for the hollow cube were 0.76 and 0.85 at 45° and 90° angles to the current, respectively, and 0.97, 0.38, and 1.42 for the water pipe at 0°, 90°, and 180° angles to the current, respectively. Deepwater offshore wave conditions at six stations were transformed into shallow nearshore waters representative of the artificial reef site. Waters deeper than 12 and 16 m are safe to deploy blocks with angles of 45° and 90°, respectively. However, water pipes constructed at angles of 90° and 180° to the current were estimated as being unstable for 365 out of 720 cases at all stations (only one station was stable for all cases). Water pipes angled at 0° were found to be stable in all 360 cases. Slope had a significant effect on weight and depth. Results from this study provide an important reference for engineers performing projects aiming to increase the performance and service life of ARs.     

Cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel,North Atlantic Ocean

Boundary currents and internal waves determine cross-slope zonation of erosion and deposition in the Faeroe-Shetland Channel. Currents were measured at 8 and 34–50 m above the bottom at three mooring sites (502, 595 and 708 m depth) for 14 days. The structure of the water column was evaluated from CTD sections, and included nepheloid layers and particulate matter concentrations. Indicators for recent deposition in the sediment (organic carbon, phytopigments, ²¹⁰Pb) were measured at eight stations across the slope. Strong near-bottom currents at the upper slope sustain down-slope particle transport in a benthic nepheloid layer, which is eroded under the influence of critically reflecting M₂ internal tidal waves at 350–550 m, where the major pycnocline meets the sloping bottom. Beam attenuation profiles confirmed the presence of intermediate nepheloid layers intruding into the Channel along the major pycnocline, and elevated concentrations of particulate matter and chlorophyll-a were measured at this depth. Near-bottom currents decreased with depth, thus allowing particle deposition down the slope. Inventories of excess ²¹⁰Pb activity in the sediment deeper than 600 m were higher than what was expected on the basis of atmospheric input of ²¹⁰Pb and production in the water column, thus indicating additional lateral inputs. Simple calculations showed that off-slope input of particles from areas shallower than 600 m may be responsible for the enhanced deposition at greater depths.     

Frontal structure and Antarctic Bottom Water flow through the Princess Elizabeth Trough,Antarctica

Hydrographic, current meter and ADCP data collected during two recent cruises in the South Indian Ocean (RRS Discovery cruise 200 in February 1993 and RRS Discovery cruise 207 in February 1994) are used to investigate the current structure within the Princess Elizabeth Trough (PET), near the Antarctic continent at 85°E, 63–66°S. This gap in topography between the Kerguelen Plateau and the Antarctic continent, with sill depth 3750 m, provides a route for the exchange of Antarctic Bottom Water between the Australian–Antarctic Basin and the Weddell–Enderby Basin. Shears derived from ADCP and hydrographic data are used to deduce the barotropic component of the velocity field, and thus the volume transports of the water masses. Both the Southern Antarctic Circumpolar Current Front (SACCF) and the Southern Boundary of the Antarctic Circumpolar Current (SB) pass through the northern PET (latitudes 63 to 64.5°S) associated with eastward transports. These are deep-reaching fronts with associated bottom velocities of several cm s^-1. Antarctic Bottom water (AABW) from the Weddell–Enderby Basin is transported eastwards in the jets associated with these fronts. The transport of water with potential temperatures less than 0°C is 3 (±1) Sv. The SB is shown to meander in the PET, caused by the cyclonic gyre immediately west of the PET in Prydz Bay. The AABW therefore also meanders before continuing eastwards. In the southern PET (latitudes 64.5 to 66°S) a bottom intensified flow of AABW is observed flowing west. This AABW has most likely formed not far from the PET, along the Antarctic continental shelf and slope to the east. Current meters show that speeds in this flow have an annual scalar mean of 10 cm s^-1. The transport of water with potential temperatures less than 0°C is 20 (±3) Sv. The southern PET features westward flow throughout the water column, since the shallower depths are dominated by the flow associated with the Antarctic Slope Front. Including the westward flow of bottom water, the total westward transport of the whole water column in the southern PET is 45 (±6) Sv.     

Scale of temperature variability in the maritime Antarctic intertidal zone

The Antarctic Peninsula is currently considered as one of the fastest changing regions on Earth yet temperature variability in some of its environments and habitats is not well-documented. Given the increased glacier retreat, summer melts, sea level rise and ozone losses the intertidal zone is likely to be one of the most rapidly altering of environments but also one of the least investigated in polar waters. This study aims to quantify summer temperature variability in some habitats of the intertidal zone at King George Island. Three transects were selected across tidal flat. Four temperature loggers were deployed at each of them from extreme low water spring tide level to extreme high water spring tide level between 07.12.2010 and 18.03.2011. All the loggers were deployed at the rocky substratum. The temperature range across the study tidal flat was between − 2.26 °C and + 21.18 °C. The average (summer) temperature obtained from 12 loggers varied from + 1.89 to + 3.26 °C. In all the three transects average temperature increased with tidal height. Much higher temperature variability was recorded at higher than at lower tide locations. Differences in temperature between the three study transects existed. Results obtained from the studied tidal flat show that several factors combined altogether, including: water movement by tidal forces, wave action, air temperature, sun light intensity, shore lithology and the presence of ice and snow in the area, seem to influence its temperature.     

Hydrography of chromophoric dissolved organic matter in the North Atlantic

The distribution and optical absorption characteristics of chromophoric dissolved organic matter (CDOM) were systematically investigated along three meridional transects in the North Atlantic Ocean and Caribbean Sea conducted as part of the 2003 US CLIVAR/CO₂ Repeat Hydrography survey. Hydrographic transects covered in aggregate a latitudinal range of 5° to 62° north along longitudes 20°W (line A16N, Leg 1), 52°W (A20), and 66°W (A22). Absorption spectra of filtered seawater samples were collected and analyzed for depths ranging from the surface to ∼6000 m, sampling all the ocean water masses in the western basin of the subtropical North Atlantic and several stations on the North and South American continental slopes. The lowest surface abundances of CDOM (< 0.1 m⁻¹ absorption coefficient at 325 nm) were found in the central subtropical gyres while the highest surface abundances (∼0.7 m⁻¹) were found along the continental shelves and within the subpolar gyre, confirming recent satellite-based assessments of surface CDOM distribution. Within the ocean interior, CDOM abundances were relatively high (0.1–0.2 m⁻¹ absorption coefficient at 325 nm) except in the subtropical mode water, where a local minimum exists due to the subduction of low CDOM surface waters during mode water formation. In the subthermocline water masses of the western basin, changes in CDOM abundance are not correlated with increasing ventilation age as assessed using chlorofluorocarbon (CFC) concentrations and the atmospheric CFC history. But dissolved organic carbon (DOC) mass-specific absorption coefficients of CDOM increase with increasing ventilation age in the deep sea, indicating that CDOM is a refractory component of the DOC pool. The overall CDOM distribution in the North Atlantic reflects the rapid advection and mixing processes of the basin and demonstrates that remineralization in the ocean interior is not a significant sink for CDOM. This supports the potential of CDOM as a tracer of ocean circulation processes for subducted water masses.     

Observations of the Storfjorden overflow

The mixing and spreading of the Storfjorden overflow were investigated with density and horizontal velocity profiles collected at closely spaced stations. The dense bottom water generated by strong winter cooling, enhanced ice formation and the consequent brine rejection drains into and fills the depression of the fjord and upon reaching a 120-m deep sill, descends like a gravity current following the bathymetry towards the shelf edge. The observations covered an approximate 37-km path of the plume starting from about 68 km downstream of the sill. The plume is identified as two layers: a dense layer 1 with relatively uniform vertical structure underlying a thicker layer 2 with larger vertical density gradients. Layer 1, probably remnants from earlier overflows, almost maintains its temperature–salinity characteristics and spreads to a width of about 6 km over its path, comparable to spread resulting from Ekman veering. Layer 2, on the other hand, is a mixing layer and widens to about 16 km. The overflow, in its core, is observed to have salinities greater than 34.9, temperatures close to the freezing point, and light transmissivity typically 5% less than that of the ambient waters. The overall properties of the observed part of the plume suggest dynamical stability with weak entrainment. However local mixing is observed through profiles of the gradient Richardson number, the non-dimensional ratio of density gradient over velocity gradient, which show portions with supercritical values in the vicinity of the plume–ambient water interface. The net volume transport associated with the overflow is estimated to be 0.06 Sv (Sv≡10⁶ m³ s⁻¹) out of a section closest to the sill and almost double that as it leaves the section furthest downstream. The weak entrainment is estimated to account for the doubling of the volume transport between the two sections. A simple model proposed by Killworth (J. Geophys. Res. 106 (2001) 22267), giving the path of the overflow from a constant rate of vertical descent along the slope, compares well with our observations.     

Tropical instability wave interactions within the Galápagos Archipelago

The effects of tropical instability waves (TIW) within the eastern equatorial Pacific during the boreal fall of 2005 were observed in multiple data sets. The TIW cause oscillations of the sea surface temperature (SST), meridional currents (V), and 20 °C isotherm (thermocline). A particularly strong 3-wave packet of ～15-day period TIW passed through the Galápagos Archipelago in Sep and Oct 2005 and their effects were recorded by moored near-surface sensors. Repeat Argo profiles in the archipelago showed that the large temperature (>5 °C) oscillations that occurred were associated with a vertical adjustment within the water column. Numerical simulations report strong oscillations and upwelling magnitudes of ～5.0 m d^?1 near the Tropical Atmosphere Ocean (TAO) buoy at 0°, 95°W and in the Archipelago at 92°W and 90°W. A significant biological response to the TIW passage was observed within the archipelago. Chlorophyll a measured by the Sea-viewing Wide Field-of-view Sensor (SeaWiFS) increased by >30% above 1998–2007 mean concentrations within the central archipelago. The increases coincide with coldest temperatures and the much larger increases within the archipelago as compared to those of 95°W indicate that TIW induced upwelling over the island platform itself brought more iron-enriched upwelling waters into the euphotic zone.     

Spring distribution of larger (>64 μm) protozoans in the Atlantic sector of the Southern Ocean

Distribution of larger protozoans (armoured dinoflagellates, tintinnids, heliozoans, radiolarians and foraminiferans >64 μm) is presented for three major water masses of the Southern Ocean: the Polar Front region (PFr), the southern Antarctic Circumpolar Current (southern ACC) and the northern Weddell Gyre. Sampling took place during the SO-JGOFS cruise ANT X/6 of R/V Polarstern (October–November 1992) along a meridional transect at 6°W between 48°00′S and 59°30′S. Multinet samples (64 μm mesh size) were taken at six stations from the surface down to 500 m depth at five different depth intervals. In the upper 100 m of the water column abundances of larger protozoans varied between 94 and 10,930 ind. m^–3, with highest abundances in the PFr, where phytoplankton blooms occurred, and lowest values in the Antarctic Circumpolar Current–Weddell Gyre Boundary (AWB). Foraminiferans and polycystine and smaller (<300 μm) phaeodarian radiolarians dominated larger protozoan assemblages in the PFr. In open water of the southern ACC, tintinnids, armoured dinoflagellates, foraminiferans and smaller (<300 μm) phaeodarian radiolarians were equally important. The heliozoans Sticholonche spp. and nassellarian radiolarians dominated assemblages in the Weddell Gyre and AWB. Larger protozoan biomasses ranged between 2 and 674 μg C m⁻³ and were always dominated by larger (>300 μm) phaeodarians. Highest biomasses were found in the AWB between 200 and 500 m depth. Standing stocks of larger protozoans constituted a negligible fraction of zooplankton biomass in the upper 200 m of the water column. In deeper layers of the ice-covered Weddell Gyre and AWB their biomasses, dominated by larger (>300 μm) phaeodarians, was significant contributing up to 45% to total larger protozoan and metazoan biomass. Analysis of correlation between distribution patterns and environmental conditions at the stations sampled indicate that spring distribution patterns of heterotrophic armoured dinoflagellates, polycystine radiolarians and foraminiferans follow productivity in the water column. Of the protozoan groups studied the smaller (<300 μm) phaeodarian radiolarians also showed a significant correlation with productivity during spring, however, results from previous studies do not suggest a consistent pattern. Spring distribution patterns of other larger protozoans were not related to differences in productivity in the water column, and effects such as ice-cover, grazing or silica limitation might be determining. Dead radiolarian skeletons constituted on average 27, 8 and 11% of the population of nassellarians, spumellarians and smaller (<300 μm) phaeodarians, respectively. The contribution of dead radiolarian skeletons to total radiolarian stocks varied with depth and water mass. Differences between live and skeleton assemblages composition were observed. These differences should be taken into consideration when interpreting the geological record.