Seasonal Cycle of Vertical Structure and Deep Water Renewal in the Clyde Sea

Two strings of moored current meters deployed between March 1993 and May 1994, together with monthly CTD surveys, provide the first comprehensive set of observations over the seasonal cycle in the Clyde Sea. In the summer, a strong thermal stratification maintained a partial isolation of the deep waters. In winter, the stratification was weaker, and a 1 °C temperature inversion was persistent from November to the end of March. Rapid inflow of dense water from the North Channel of the Irish Sea served to re-establish the strong stratification in the spring. The mean rate of exchange was estimated from the salinity (practical salinity scale) and mass budgets to be 1·1×10⁴ m³ s⁻¹, indicating an average flushing time for the Clyde Sea of 3–4 months.Episodic increases in deep water salinity indicated that bottom water renewal occurred throughout the winter. Intense renewal events were observed in March 1993 and February 1994, when the North Channel density was near its seasonal maximum, and were coincident with periods of high wind stress. In the month prior to these rapid spring inflows, the basin bottom salinity reached its seasonal minimum, indicating that the effects of mixing dominated over renewal at this time. A marked inflow in the summer was inferred from the salinity budget, and observed as a salinity increase at a depth of 90 m. A 2-layer flow was observed in the Arran Deep basin throughout the year, the surface flow forming part of a clockwise circulation about Arran, with an opposing bottom layer circulation. This surface circulation prevents freshwater from entering the Kilbrannan Sound, leaving this area relatively susceptible to deep water mixing by the wind.At a station in the north of the basin, the internal tidal current was observed to have an amplitude of 2–3 cm s⁻¹, which is half the amplitude of the barotropic tide. The energy available to mix the water column mixing associated with the internal tide at this position is estimated to be 0·01 mWm⁻², which is 2 orders of magnitude less than wind mixing. The kinetic energy density in the Clyde Sea was found to be predominantly in low frequency oscillations (<1·0 cycles per day), the seasonal variation exhibiting some correlation with the wind.     

Circulation and mixing at the New England shelfbreak front: Results of purposeful tracer experiments

We present the results of six dye tracer experiments that measured the mixing and circulation at the shelfbreak front on the New England Shelf. The last three were conducted during the New England Shelfbreak Productivity Experiment (NESPEX) with concurrent isopycnal float deployments. The results are consistent with the Chapman and Lentz [Chapman, D.C., and Lentz, S.J. (1994). Trapping of a coastal density front by the bottom boundary layer. Journal of Physical Oceanography, 24, 1465–1479.] model prediction of the separation and upwelling along the shelfbreak front of bottom boundary layer (BBL) water forced by an Ekman buoyancy flux, but show considerable variability. Cross-shelf velocities at the detachment point are 2–3 × 10⁻² m/s. But seaward, over the slope region, dye tagged water was sheared from the main patch into small filaments that upwelled along the front with cross-shelf speeds up to 0.1 m/s. Cross-shelf diffusion was of order 10 m²/s in the mixed bottom layer and 1 m²/s in the interior along the front. Within the stratified front, the mean vertical diffusivity was K_z  4 × 10⁻⁶ m²/s. The dispersion of shelfwater in the slope region is effected by turbulent flow with advective speeds exceeding the small scale diffusive mixing. The mean flux of the detached BBL water is sufficient to account for the net loss of shelf water during its transit from Cape Cod to Cape Hatteras.     

Mean flow and variability in the southwestern East Sea

The Ulleung Basin is one of three deep basins that are contained within the East/Japan Sea. Current meter moorings have been maintained in this basin beginning in 1996. The data from these moorings are used to investigate the mean circulation pattern, variability of deep flows, and volume transports of major water masses in the Ulleung Basin with supporting hydrographic data and help from a high-resolution numerical model. The bottom water within the Ulleung Basin, which must enter through a constricted passage from the north, is found to circulate cyclonically—a pattern that seems prevalent throughout the East Sea. A strong current of about 6 cms⁻¹ on average flows southward over the continental slope off the Korean coast underlying the northward East Korean Warm Current as part of the mean abyssal cyclonic circulation. Volume transports of the northward East Korean Warm Current, and southward flowing East Sea Intermediate Water and East Sea Proper Water are estimated to be 1.4 Sv (1 Sv=10⁻⁶ m³ s⁻¹), 0.8 Sv, and 3.0–4.0 Sv, respectively. Deep flow variability involves a wide range of time scales with no apparent seasonal variations, whereas the deep currents in the northern East Sea are known to be strongly seasonal.     

南极半岛周边海域水团及水交换的研究

利用中国第34次南极考察于2018年1–2月在南极半岛周边海域获得的温盐、海流现场观测数据,分析了调查区域主要水团及水交换特征。结果表明,观测区域内主要存在南极表层水、绕极深层水、暖深层水、南极底层水、布兰斯菲尔德海峡底层水。威德尔海的暖深层水、威德尔海深层水通过南奥克尼海台东侧的奥克尼通道、布鲁斯通道和南奥克尼海台西侧的埃斯佩里兹通道进入斯科舍海,其中奥克尼通道的深层海流最强,流速最大可达0.25 m/s,密度较大的威德尔海深层水可以通过此通道进入斯科舍海;布鲁斯通道海流流速约为0.13 m/s,通过此通道的暖深层水位势温度较高;埃斯佩里兹通道海流流速约为0.10 m/s,通过此通道的暖深层水位势温度最低,威德尔海深层水密度最小。在南奥克尼海台东西两侧均观测到南向和北向的海流,但整体上来看,向北的海流和水交换更强。水体进入斯科舍海后,沿着南斯科舍海岭的北侧向西北方向流动,流速约为0.21 m/s。德雷克海峡中的南极绕极流仅有一部分向东进入斯科舍海南部海域,且受到向西流动的暖深层水、威德尔海深层水的影响,斯科舍海南部海域的绕极深层水明显比德雷克海峡中绕极深层水的高温高盐性质弱;受到南极绕极流的影响,南斯科舍海岭北侧的威德尔海深层水比南侧暖。南斯科舍海岭上的水体可能受到北侧绕极深层水、暖深层水,西侧陆架水,东侧冬季水的影响,因此海岭上水体结构较为复杂。     

Long-term bottom water warming in the north Ross Sea

We measured potential temperature, salinity, and dissolved oxygen profiles from the surface to the bottom at two locations in the north Ross Sea (65.2°S, 174.2°E and 67.2°S, 172.7°W) in December 2004. Comparison of our data with previous results from the same region reveals an increase in potential temperature and decreases in salinity and dissolved oxygen concentration in the bottom layer (deeper than 3000 m) over the past four decades. The changes were significantly different from the analytical precisions. Detailed investigation of the temperature, salinity, dissolved oxygen and σ ₃ value distributions and the bottom water flow in the north Ross Sea suggests a long-term change in water mass mixing balance. That is to say, it is speculated that the influence of cool, saline, high-oxygen bottom water (high-salinity Ross Sea Bottom Water) formed in the southwestern Ross Sea has possibly been decreased, while the influences of relatively warmer and fresher bottom water (low-salinity Ross Sea Bottom Water) and the Adélie Land Bottom Water coming from the Australia-Antarctic Basin have increased. The possible impact of global warming on ocean circulation needs much more investigation.     

Annual dynamics of pelagic primary production and respiration in a shallow coastal basin

The Wadden Sea (North Sea, Europe) is a shallow coastal sea with high benthic and pelagic primary production rates. To date, no studies have been carried out in the Wadden Sea that were specifically designed to study the relation between pelagic respiration and production by comparable methods. Because previous studies have suggested that the import of primary-produced pelagic organic matter is important for benthic Wadden Sea carbon budgets, we hypothesised that on an annual average the northern Wadden Sea water column is autotrophic. To test this hypothesis, we studied annual dynamics of primary production and respiration at a pelagic station in a shallow tidal basin (List Tidal Basin, northern Wadden Sea). Since water depth strongly influences production estimates, we calculated primary production rates per unit area in two ways: on the basis of the mean water depth (2.7 m) and on the basis of 1 m depth intervals and their respective spatial extent in the List Tidal Basin. The latter more precise estimate yielded an annual primary production of 146 g C m^− 2 y^− 1. Estimates based on the mean water depth resulted in a 40% higher annual rate of 204 g C m^− 2 y^− 1. The total annual pelagic respiration was 50 g C m^− 2 y^− 1. The P/R ratio varied between seasons: from February to October the water column was autotrophic, with the highest P/R ratio of 4–5 during the diatom spring bloom in April/May. In autumn and winter the water column was heterotrophic. On an annual average, the water column of the List Tidal Basin was autotrophic (P/R 3). We suggest that a large fraction of the pelagic produced organic matter was respired locally in the sediment.     

Modeling the pathways and ages of inflowing salt- and freshwater in the Baltic Sea

A three-dimensional, eddy-permitting ocean circulation model with implemented bottom boundary layer model and flux-corrected transport scheme is used to calculate the pathways and ages of various water masses in the Baltic Sea. The agreement between simulated and observed temperature and salinity profiles of the period 1980–2004 is satisfactory. Especially the renewal of the deep water in the Baltic proper by gravity-driven dense bottom flows is better simulated than in previous versions of the model. Based upon these model results details of the mean circulation are analyzed. For instance, it is found that after the major Baltic inflow in January 2003 saline water passing the Słupsk Furrow flows directly towards northeast along the eastern slope of the Hoburg Channel. However, after the baroclinic summer inflow in August/September 2002 the deep water flow spreads along the southwestern slope of the Gdansk Basin. Further, the model results show that the patterns of mean vertical advective fluxes across the halocline that close the large-scale vertical circulation are rather patchy. Mainly within distinct areas are particles of the saline inflow water advected vertically from the deep water into the surface layer. To analyze the time scales of the circulation mean ages of various water masses are calculated. It is found that at the sea surface of the Bornholm Basin, Gotland Basin, Bothnian Sea, and Bothnian Bay the mean ages associated to inflowing water from Kattegat amount to 26–30, 28–34, 34–38, and 38–42 years, respectively. Largest mean sea surface ages of more than 30 years associated to the freshwater of the rivers are found in the central Gotland Basin and Belt Sea. At the bottom the mean ages are largest in the western Gotland Basin and amount to more than 36 years. In the Baltic proper vertical gradients of ages associated to the freshwater inflow are smaller than in the case of inflowing saltwater from Kattegat indicating an efficient recirculation of freshwater in the Baltic Sea.     

Long-term trends and interannual variability of temperature in Drake Passage

Temperature data collected over the last 36 years (1969–2004) in Drake Passage are used to examine interannual temperature variation and long-term trends in the upper ocean. To reduce the effect of variation from different sampling locations and temporal variability introduced by meridional shifts in the Polar Front (PF), the data were divided into two sub-regions north (3800 temperature profiles) and south (3400) of the PF. Temperature anomalies were formed by removing a temporal mean field for each profile in each sub-region at 100 m depth intervals from the surface to 700 m. North of the PF, statistically significant warming trends of 0.02 °C yr⁻¹ were observed that were largely depth-independent between 100 and 700 m. A statistically significant cooling trend of −0.07 °C yr⁻¹ was observed at the surface south of the PF, which was smaller (−0.04 °C yr⁻¹) but still statistically significant when possible seasonal sampling biases were accounted for. The observed cooling at the surface and warming at depth is largely consistent with a poleward shift of the PF due to enhancement of westerly winds in the Southern Ocean, as recently suggested by models and observations. The observed annual temperature anomalies in the upper 400 m north of the PF and in the upper 100 m south of the PF are highly correlated to variability in sea ice, and also to climate indices of the Antarctic Oscillation and the El Niño Southern Oscillation. Variability in sea ice and temperature anomalies lag El Niño variability in the Pacific, with a phasing consistent with the observed cyclical patterns of sea ice and sea surface temperature associated with the Antarctic Circumpolar Wave or Antarctic Dipole Mode in the Southern Ocean. In contrast, the sea ice variability and temperature anomalies at all depths north of the PF and at 0–100 m depth south of the PF were primarily coincident with, or led the Antarctic Oscillation Index. No significant correlations were found with the large-scale climate variability indices in southern Drake Passage below 100 m depth, which is occupied by upper Circumpolar Deep Water (uCDW). This water mass is not formed locally, is largely isolated from the surface, and exhibits vertical and lateral homogeneity. Hence changes may be difficult to detect in the available measurements, and climate variation in the source water regions of uCDW may take a long time to reach Drake Passage.     

Temporal variability in oxygen and nutrient concentrations in the southern Aegean Sea and the Straits of the Cretan Arc

Nutrient and oxygen data collected in the southern Aegean Sea (Cretan Sea) and the straits of the Cretan Arc, during the four seasonal PELAGOS cruises in 1994–1995, are investigated and compared with data collected from 1987 to 1992 within the same area. During the cruises of the PELAGOS Project, nutrient enrichment of the intermediate layers of the Cretan Sea was observed, as a result of intrusion of ‘nutrient-rich, oxygen-poor’ Transition Mediterranean Water (TMW) compensating the Cretan Deep Water (CDW) outflow. TMW occupied the intermediate layers of the entire Cretan Sea. The concentrations of nutrients within this layer were often two times higher than those observed in the same area during previous studies undertaken before 1992 (increase 2.5 μmol/l of nitrate, 0.05 μmol/l of phosphate and 2.5μmol/l of silicate). The decrease of oxygen in this layer is about 0.8ml/l (35 μmol/l). Outflow of CDW occurs principally through the Antikithira and Kassos Straits (the two deeper straits of the Cretan Arc); it results in an increase of oxygen content but a decrease in the nutrient content of water in the deep and bottom layers outside the Cretan Sea. The major mesoscale features in the area have a major influence of the distributions and exchanges of nutrients and oxygen through the straits of the Cretan Arc. The surface and the intermediate layers were richer in nutrients and poorer in oxygen in spring (March 1994), than in autumn (September 1994).     

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.     

Hydrography and circulation of the West Antarctic Peninsula Continental Shelf

The water mass structure and circulation of the continental shelf waters west of the Antarctic Peninsula are described from hydrographic observations made in March–May 1993. The observations cover an area that extends 900 km alongshore and 200 km offshore and represent the most extensive hydrographic data set currently available for this region. Waters above 100–150 m are composed of Antarctic Surface Water and its end member Winter Water. Below the permanent pycnocline is a modified version of Circumpolar Deep Water, which is a cooled and freshened version of Upper Circumpolar Deep Water. The distinctive signature of cold and salty water from the Bransfield Strait is found at some inshore locations, but there is little indication of significant exchange between Bransfield Strait and the west Antarctic Peninsula shelf. Dynamic topography at 200 m relative to 400 m indicates that the baroclinic circulation on the shelf is composed of a large, weak, cyclonic gyre, with sub-gyres at the northeastern and southwestern ends of the shelf. The total transport of the shelf gyre is 0.15 Sv, with geostrophic currents of order 0.01 m s^-1. A simple model that balances across-shelf diffusion of heat and salt from offshore Upper Circumpolar Deep Water with vertical diffusion of heat and salt across the permanent pycnocline into Winter Water is used to explain the formation of the modified Circumpolar Deep Water that is found on the shelf. Model results show that the observed thermohaline distributions across the shelf can be maintained with a coefficient of vertical diffusion of 10^-4 m² s^-1 and horizontal diffusion coefficients for heat and salt of 200 and 1200 m² s^-1, respectively. When the effects of double diffusion are included in the model, the required horizontal diffusion coefficients for heat and salt are 200 and 400 m² s^-1, respectively.     

Seismic and oceanographic evidence of present-day bottom-water dynamics in the Lousy Bank—Hatton Bank area,NE Atlantic

High-resolution multi-channel seismic records from the eastern margin of the Iceland Basin, Northeast Atlantic, are used to infer regional patterns of bottom water dynamics from seafloor morphology and distribution of the most recent geological units deposited along the margin. This information is combined with results from oceanographic and hydrodynamic measurements made in the area. The study area is located between Lousy Bank and Hatton Bank, where deep-water currents are forced around structural highs, leaving a complex pattern of topographically controlled sediment transport pathways. At the top and upper flank of Lousy Bank, George Bligh Bank and Hatton Bank topographic forcing leads to considerable acceleration of the northward flowing North Atlantic Current. At greater water depth, seismic facies indicative of bottom current action are found to be widespread. In addition, seafloor morphology displays moat features extending over large distances. The occurrence of these moats is confined to specific water depths in the range from 700–1400 m and from 1800 to 2200 m depth range. These depth ranges correspond to the basal depth stratum of the North Atlantic Current and the depth range of the Deep Northern Boundary Current, respectively. Geological evidence suggests maximum near-bottom current speed of about 0.5 m/s for these moat areas. Using oceanographic data we suggest that formation of the moats is not likely to be associated with the presence of a persistent high-speed contour current core, but probably originates from the occurrence of solibores and thus may be related to the internal wave field of the Iceland Basin possible linked to atmospheric pressure variation.     

Dissolved organic carbon, nitrogen and phosphorus in the N-E Atlantic and the N-W Mediterranean with particular reference to non-refractory fractions and degradation

Dissolved organic carbon (DOC), dissolved organic nitrogen (DON) and dissolved organic phosphorus (DOP) measured in deep profiles in the N-E Atlantic and in the N-W Mediterranean in the period 1984–2002 are described. After accurate validation, they show close agreement with those previously published.Classic profiles were obtained, with concentrations decreasing in deep waters. In the Mediterranean and in the Atlantic comparable concentrations were found in the 1500–2000 m waters, 44–46 μmol l⁻¹ DOC, 2.6–2.8 μmol l⁻¹ DON and 0.02–0.03 μmol l⁻¹ DOP. In the surface layers, DOC concentrations were higher, but DON and DOP concentrations lower, in the Mediterranean than in the Atlantic, leading to higher element ratios in the Mediterranean. In autumn, values were, respectively, DOC:DON 17 vs. 14, DOC:DOP 950 vs. 500 and DON:DOP 55 vs. 35. The data suggest an increase in DOC and DON in the North Atlantic Central Water over 15 years, which may be linked to the North Atlantic climatic oscillations.Refractory DOM found in the 1500–2000 m layer exhibited C:N:P ratios of 1570:100:1. The labile+semi-labile (=non-refractory) DOM (nrDOM) pool was computed as DOM in excess of the refractory pool. Its contribution to total DOM above the thermocline in the open sea amounted to 25–35% of DOC, 30–35% of DON, and 60–80% of DOP. Element ratios of the nrDOM varied among stations and were lower than those of refractory DOM, except for C:N in the Mediterranean: nrDOC:nrDON 10–19, nrDOC:nrDOP 160–530 and nrDON:nrDOP 15–38. The specific stoichiometry of DOM in the Mediterranean led us to postulate that overconsumption of carbon is probably a main process in that oligotrophic sea.By coupling non-refractory DOM stoichiometry and relationships between the main DOM elements in the water column, the relative mineralization of C, N and P from DOM was studied. Below the thermocline, the preferential removal of phosphorus with regard to carbon from the semi-labile DOM can be confirmed, but not the preferential removal of nitrogen. In the ocean surface layers, processes depend on the oceanic area and can differ from deep waters, so preferential carbon removal seems more frequent. Bacterial growth efficiency data indicate that bacteria are directly responsible for mineralization of a high proportion of DON and DOP in the deep water.     

Dianeutral mixing,transformation and transport of the deep water of the Indian Ocean

The realization of North Atlantic Deep Water (NADW) replacement in the deep northern Indian Ocean is crucial to the “conveyor belt” scheme. This was investigated with the updated 1994 Levitus climatological atlas. The study was performed on four selected neutral surfaces, encompassing the Indian deep water from 2000 to 3500 m. The Indian deep water comprises three major water masses: NADW, Circumpolar Deep Water (CDW) and North Indian Deep Water (NIDW). Since NADW flowing into the southwest Indian Ocean is largely blocked by the ridges (the Madagascar Ridge in the east and Davie Ridge in the north in the Mozambique Channel) and NIDW is the only source in the northern Indian Ocean that cannot provide a large amount of volume transport, CDW has to be a major source for the Indian deep circulation and ventilation in the north. Thus the question of NADW replacement becomes that of how the advective flows of CDW from the south are changed to be upwelled flows in the north—a water-mass transformation scenario. This study considered various processes causing motion across neutral surfaces. It is found that dianeutral mixing is vital to achieve CDW transformation. Basin-wide uniform dianeutral upwelling is detected in the entire Indian deep water north of 32°S, somewhat concentrated in the eastern Indian Ocean on the lowest surface. However, the integrated dianeutral transport is quite low, about a net of 0.2 Sv (1 Sv=10⁶ m³ s^-1) across the lowermost neutral surface upward and 0.4 Sv across the uppermost surface upward north of 32°S with an error band of about 10–20% when an uncertainty of half-order change in diffusivities is assumed. Given about 10–15% of rough ridge area where dianeutral diffusivity could be about one order of magnitude higher (10^-4 m² s^-1) due to internal-wave breaking, the additional amount of increased net dianeutral transport across the lowest neutral surface is still within that error band. The averaged net upward transport in the north is matched with a net downward transport of 0.3 Sv integrated in the Southern Ocean south of 45°S across the lowermost surface. With the previous works of You (1996. Deep Sea Research 43, 291–320) in the thermocline and You (Journal of Geophysical Research) in the intermediate water combined, a schematic dianeutral circulation of the Indian Ocean emerges. The integrated net dianeutral upwelling transport shows a steady increase from the deep water to the upper thermocline (from 0.2 to 4.6) north of 32°S. The dianeutral upwelling transport is accumulated upward as the northward advective transport provided from the Southern Ocean increases. As a result, the dianeutral upwelling transport north of 32°S can provide at least 4.6 Sv to south of 32°S from the upper main thermocline, most likely to the Agulhas Current system. This amount of dianeutral upwelling transport does not include the top 150–200 m, which may contribute much more volume transport to the south.     

Northwestern Weddell Sea deep outflow into the Scotia Sea during the austral summers of 2000 and 2001 estimated by inverse methods

The northward outflow of cold, dense water from the Weddell Sea into the world ocean basins plays a key role in balancing the global heat budget. We estimate the geostrophic flow patterns in the northwestern Weddell Sea using box inverse methods applied to quasi-synoptic hydrographic data collected during the Brazilian DOVETAIL 2000 and 2001 austral summer cruises. The analysis is focused on the variations of the deep Weddell Sea outflow into the Scotia Sea within boxes that bound the main deep gaps over the South Scotia Ridge. To determine the geostrophic volume transports in each box, mass, salt, and heat are conserved within neutral density layers that are not in contact with the atmosphere. Implementing the inverse model and using property anomaly equations weighted by the flow estimate uncertainty our results are consistent with those reported in the literature. A bottom triangle extrapolation method is introduced, which improves the estimated property fluxes through hydrographic sections. In the austral summer of 2000 the transports of Weddell Sea Deep Water (WSDW) through the Philip Passage, Orkney Passage, and southwestern Bruce Passage are 0.01±0.01, 1.15±0.33, and 1.03±0.23 Sv (1 Sv=10⁶ m³ s⁻¹, >0 is northward), respectively. After extrapolation within bottom triangles these transports increase to 0.12±0.03, 3.48±1.81, and 1.20±2.16 Sv. Analysis of the hydrographic data reveal distinct oceanographic conditions over the Philip Passage region, with evidence of mesoscale meanders, warmer and saltier Warm Deep Water (WDW) and colder WSDW observed in 2001 than in 2000. Despite these differences the WSDW transport does not present a significant variation between 2000 and 2001. The WSDW transports through the Philip Passage in 2001 are 0.012±0.001 and 0.113±0.001 Sv after extrapolation within bottom triangles. The circulation derived from the inversion in the austral summer of 2001 suggests a sharp weakening of the barotropic cyclonic flow in the Powell Basin, which may be due to northerly and northeasterly winds associated with an atmospheric low-pressure center located west of the Antarctic Peninsula. We suggest that similar variations in atmospheric forcing may explain changes in the intensity of the cyclonic flow observed in the northwestern Weddell Sea and Powell Basin.     

Review of recent findings on the water masses and circulation in the East Sea (Sea of Japan)

Recent findings on water masses, biogeochemical tracers, deep currents and basin-scale circulation in the East/Japan Sea, and numerical modeling of its circulation are reviewed. Warming continues up to 2007 despite an episode of bottom water formation in the winter of 2000–2001. Water masses have definitely changed since the 1970s and further changes are expected due to the continuation of warming. Accumulation of current data in deep waters of the East/Japan Sea reveals that the circulation in the East/Japan Sea is primarily cyclonic with sub-basin scale cyclonic and anticyclonic cells in the Ulleung Basin (Tsushima Basin). Our understanding of the circulation of intermediate water masses has been deepened through high-resolution numerical studies, and the implementation of data assimilation has had initial success. However, the East/Japan Sea is unique in terms of the fine vertical structures of physical and biogeochemical properties of cold water mass measured at the highest precision and their rapid change with the global warming, so that full understanding of the structures and their change requires in-depth process studies with continuous monitoring programs.     

Contourite identification along Italian margins: The case of the Portofino drift (Ligurian Sea)

A brief review of the published evidence of current deposits around Italy is the occasion to test the robustness of matching bottom current velocity models and seafloor morphologies to identify contourite drifts not yet documented. We present the result of the regional hydrodynamic model MARS3D in the Northern Tyrrhenian and Ligurian Sea with horizontal resolution of 1.2 km and 60 levels with focus on bottom current: data are integrated over summer and winter 2013 as representative of low and high intensity current conditions.The Eastern Ligurian margin is impacted by the Levantine Intermediate Water (LIW) with modeled mean velocity of bottom current up to 20 cm s⁻¹ in winter 2013 and calculated bottom shear stress exceeding 0.2 N m⁻² in water depth of 400–800 m. By crossing this information with seafloor morphology and geometry of seismic reflections, we identify a sediment drift formerly overlooked at ca 1000 m water depth. The Portofino separated mounded drift has a maximum thickness of at least 150 m and occurs in an area of mean current velocity minimum. Independent evidence to support the interpretation include bottom current modelling, seafloor morphology, seismic reflection geometry and sediment core facies. The adjacent areas impacted by stronger bottom currents present features likely resulted from bottom current erosion such as a marine terrace and elongated pockmarks.Compared to former interpretation of seafloor morphology in the study area, our results have an impact on the assessment of marine geohazards: submarine landslides offshore Portofino are small in size and coexist with sediment erosion and preferential accumulation features (sediment drifts) originated by current-dominated sedimentary processes. Furthermore, our results propel a more general discussion about contourite identification in the Italian seas and possible implications.     

Variability of physical and chemical characteristics along the 70-m isobath of the southeastern Bering Sea

From observations of ice cover, temperature, salinity, currents and nitrate, it is evident that along-shelf variability was significant over the middle shelf of the eastern Bering Sea, but less distinct than that observed in the cross-shelf domains. Along the 70-m isobath, three zones were evident in the summer: the southeastern cold pool (centered at 57°N); an intermediate zone, consisting of warmer water, with weaker stratification; and the northern cold pool, extending northward from 58°N. Small-scale (20 km) horizontal features that persisted for months were common. Nutrient concentrations were related to salinity and were replenished more uniformly over the southern shelf, than north of the Pribilof Islands. Although mean currents were weak (1 cm s⁻¹), short energetic advective events impacted the temperature and salinity structure.     

Vertical and horizontal distribution of fluorescent dissolved organic matter in the Southern Ocean

The vertical and horizontal distribution of fluorescent dissolved organic matter (FDOM), determined by fluorescence intensity at 320 nm excitation and 420 nm emission, were clarified in nine stations on two transects at the Southern Ocean, including a subtropical, subantarctic, polar frontal and Antarctic zone. All vertical profiles of fluorescence intensity showed that levels were lowest in the surface waters, increased with increasing the depth in mid-depth waters ( 2000 m), and then stayed within a relatively narrow range from there to the bottom. Such vertical profiles of FDOM were similar to those of nutrients, but were adverse to dissolved oxygen. In water columns below the temperature-minimum subsurface water (dichothermal waters) in the Antarctic zone and below the winter mixed layer in the other zones, we determined the relationships of fluorescence intensity to concentrations of nutrients and apparent oxygen utilization (AOU) over the entire area of the present study, and found significant linear correlations between the levels of fluorescence intensity and nutrient concentrations (r =  0.70 and 0.71 for phosphate and nitrate + nitrite, respectively) and AOU (r = 0.91). From the strong correlation coefficient between fluorescence intensity and AOU, we concluded that FDOM in the Southern Ocean is formed in situ via the biological oxidation of organic matter. The regeneration of the nutrients/consumption of the oxygen/formation of FDOM was active in mid-depth waters. However, the correlations between fluorescence intensities and nutrients and AOU were different in the mid-depth water masses, Subantarctic Mode Water (SAMW), and Antarctic Intermediate Water (AAIW), indicating that the sources of organic matter responsible for FDOM formation were different. A considerable amount of FDOM in the SAMW is thought to be produced by the remineralization of DOM in addition to sinking particulate organic matter, while DOM is less responsible for FDOM formation in the AAIW.