Hydrographic Sections Research Articles

Using Argo data to investigate the Meridional Overturning Circulation in the North Atlantic

Using a variety of oceanographic data, including direct volume transports in the Florida Strait, and Argo float profiles and drift velocities at 24°N and 36°N in the North Atlantic, inverse calculations are presented in which the net meridional transport, down to a depth of approximately 1600 m, is estimated at both latitudes for a 5-year period 2003–2007. The upper ocean is divided into seven layers using neutral density, and mass conservation constraints have been applied to a closed box bounded by these latitudes, including the Florida Strait. Ekman layer transports have been included in the top-most layer, and the inverse calculation has solved for changes from the initial reference velocities, Ekman and Florida Strait transports, given a priori estimates on the accuracy of each of these quantities. Solutions with and without transformations due to Mediterranean Water (MW) formation are made. Our results indicate that (1) time-averaged transport estimates derived from Argo have significant less eddy noise than individual hydrographic sections, (2) Argo drift velocities provide information to the inverse solution for the ocean interior, and (3) comparison of the total integrated interior mass transports in the thermocline waters for the period 2003–2007 with the previous estimates based on trans-ocean hydrographic sections shows that, within the errors of our estimation, the upper limb of the Atlantic Meridional Overturning Circulation has not significantly changed since 1957.

Recent changes in the Greenland–Scotland overflow‐derived water transport inferred from hydrographic observations in the southern Irminger Sea

Recent decadal changes (1955–2007) in the baroclinic transport (TBC) of the Deep Western Boundary Current (DWBC) carrying the Greenland–Scotland overflow‐derived waters along the East Greenland slope are quantified from a set of hydrographic sections in vicinity of Cape Farewell. The updated historical record of TBC shows clear decadal variability (±2–2.5 Sv) with the transport minima in the 1950s and mid‐1990s, maximum in the early 1980s and moderate‐to‐high transport in the 2000s. Since the mid‐1990s, the DWBC TBC has increased by ∼2 Sv (significant at the 99.9% level), which constitute ∼20% of the mean absolute transport (9.0 Sv) as obtained from three cruises in 2002–2006. The DWBC TBC anomalies negatively correlate (R = –0.80) with thickness anomalies of the Labrador Sea Water (LSW) at its origin implying a close association, albeit not necessarily causative, between the DWBC transport east of Greenland and the LSW production.

A New Climatology of Air–Sea Density Fluxes and Surface Water Mass Transformation Rates Constrained by WOCE

Abstract Global air–sea heat and freshwater flux data, constrained by World Ocean Circulation Experiment (WOCE) hydrographic section transports, is used to construct a new global density flux climatology. Global transformations calculated using this density flux dataset show two regimes: surface waters with density less than ∼1023.3 kg m−3 are transformed to lighter density classes with a maximum rate of 130 Sv (1 Sv ≡ 106 m3 s−1) at σ ∼ 1021.6 kg m−3, and surface waters with density greater than 1023.3 kg m−3 are transformed to denser density classes with a maximum rate of 100 Sv at σ = 1025.4 kg m−3. At higher density (σ = 1027 kg m−3) the net transformation rates vanish, reflecting heat loss in the Northern Hemisphere balanced by Southern Hemisphere freshening. This results in a kink in the global transformation rate, which is attributed to the presence of Drake Passage. Further analysis of the control run of the third Hadley Centre global climate model, HadCM3, suggests this feature to be robust and to reflect the “channel” geometry of the Southern Ocean and the “basin” geometry of the Northern Hemisphere.

Variability and propagation of Labrador Sea Water in the southern subpolar North Atlantic

The variability of two modes of Labrador Sea Water (LSW) (upper and deep Labrador Sea Water) and their respective spreading in the interior North Atlantic Ocean are investigated by means of repeated ship surveys carried out along the zonal WOCE line A2/AR19 located at 43–48°N (1993–2007) and along the GOOS line at about 48–51°N (1997–2002). Hydrographic section data are complemented by temperature, salinity, and velocity time series recorded by two moorings. They have been deployed at the western flank of the Mid-Atlantic Ridge (MAR) in the Newfoundland Basin during 1996–2004. The analysis of hydrographic anomalies at various longitudes points to a gradual eastward propagation of LSW-related signals, which happens on time scales of 3–6 years from the formation region towards the MAR. Interactions of the North Atlantic Current (NAC) with the Deep Western Boundary Current (DWBC) close to Flemish Cap point to the NAC being the main distributor of the different types of LSW into the interior of the Newfoundland Basin. Comparisons between the ship data and the mooring records revealed that the mooring sites are located in a region affected by highly variable flow. The mooring time series demonstrate an elevated level of variability with eddy activity and variability associated with the NAC considerably influencing the LSW signals in this region. Hydrographic data taken from Argo profiles from the vicinity of the mooring sites turned out to mimic quite well the temporal evolution captured by the moorings. There is some indication of occasional southward flow in the LSW layer near the MAR. If this can be considered as a hint to an interior LSW-route, it is at least of minor importance in comparison to the DWBC. It acts as an important supplier for the interior North Atlantic, distributing older and recently formed LSW modes southward along the MAR.

Red Sea Intermediate Water in the source regions of the Agulhas Current

Red Sea Intermediate Water (RSIW) has been shown to move down the Agulhas Current as distinct lenses. It has been assumed that this intermittency is the result of variable input. To clarify and quantify the nature of RSIW contributions from the source regions of the Agulhas Current observations at 15 hydrographic sections were examined using a multi-parameter analysis. In the northern Mozambique Channel RSIW is found to be layer-like, but with patches of distinctly different contributions. In the southern part of the channel the layer-like distribution disappears with RSIW mostly confined within anticyclonic and cyclonic eddies exhibiting varying maximum contributions ranging from 15–20% to 25–30% purity. Net transports across the channel ranged from −0.45 to −0.7 Sv. At the southern tip of Madagascar RSIW contributions exhibited similar purity variability ranging from 10–15% to 15–20%. The net southward transport of RSIW in the East Madagascar Current displayed an even greater variability due to changes in the flux of the undercurrent ranging from negligible to −0.3 Sv. Indications therefore were that the transport of RSIW to the Agulhas Current occurs in both cyclones and anti-cyclones through the Mozambique Channel whilst from the East Madagascar Current it is mostly confined to anti-cyclones. This variability in the inflow was also reflected in the northern part of the Agulhas Current proper. The maximum contributions of RSIW range here from 10–15% to 20–25% purity and net transports from −0.75 to −1.39 Sv off Durban. As it was east of Madagascar RSIW was mostly confined to the slope.

Changes in water properties and transports along 24°N in the North Pacific between 1985 and 2005

We conducted a trans‐Pacific hydrographic section along 24°N in 2005 to investigate the ocean structure and its changes from previous observations in 1985. We detected significant basin‐average water property changes from 1985 to 2005. Apparent oxygen utilization increased below the thermocline by up to 6 μmol kg−1 around the density of the central mode water (around 600 m). It appeared that the North Pacific intermediate water (around 800 m) was less dense in 2005 than in 1985 because of warming. From the decrease of the zonal gradient of the temperature and salinity around the North Pacific deep water (2500–4000 m) and lower circumpolar deep water (<4000 m), we suggest that northward bottom water and southward deep water transports became weaker from 1985 to 2005, consistent with the speculation from the observed temperature increase of the bottom water along its main path in previous studies. Although these water property changes suggest a slowdown of the meridional overturn in the North Pacific and large transport changes in the deep layers (below 4000 m) are estimated from an inverse method, significant heat transport changes were not detected. The estimated temperature transport change of 0.1 PW between the two sections was mainly due to shallow overturn changes, especially changes in the Kuroshio. To describe variability due to the Kuroshio changes, we estimated mass and heat transport changes from long‐term observations of the Kuroshio in the Okinawa Trough, and we determined decadal variability of temperature transports, which was consistent with the variability estimated from sea‐surface flux data sets.

Pathways of Nordic Overflows from climate model scale and eddy resolving simulations

The overflows of cold, heavy waters from the Nordic Seas across the Greenland–Iceland–Scotland Ridges are simulated using the Hybrid Coordinate Model in a North Atlantic configuration. Results at three different horizontal model resolutions are compared to each other, to recent hydrographic sections and moored observations. Simulations in the finest grid employed, 1/12° resolution, show realistic overflow pathways, reasonable overflow and Deep Western Boundary Current mean velocities and transports, and overall reasonable North Atlantic three-dimensional temperature and salinity fields, namely the Atlantic Meridional Overturning Circulation (AMOC). In contrast, simulations at coarser grids of 1/3° and 1° resolution exhibit a range of significant problems owing to unresolved, dynamically vital features in the seafloor topography. This lack of resolution, for example of the Faroe Bank Channel, leads to unrealistic overflow pathways between Iceland and Scotland in the 1/3° and 1° cases. Accordingly, overflow mass transports are also unrealistic in this area. In the Denmark Strait Overflow the underlying topographical scales are larger, and pathways are reasonable even at coarse resolution. However, overflow speeds are too small in the 1/3° and 1° cases. Underestimated velocities in the 1° simulations are compensated by an overestimated sill cross-section, whereas it is too small in 1/3°. As such, the 1/3° and 1° simulations show both large under- and overestimations of volume transport at several locations. No significant improvement in modeled overflows takes place when the grid spacing is decreased from 1° to 1/3°. An experiment conducted with hand-tuned topography shows improved volume transports near the regions of modification, but somewhat increased errors in other parts of the deep circulation, indicating the complex response of the system to perturbations in bathymetry. These results demonstrate the importance of an accurate representation of the domain geometry, in particular the channels of the complex Iceland–Scotland ridge system, in order to reproduce the pathways of the deep AMOC.

Currents in the Drake Passage based on observations in 2007

Currents in the Drake Passage are studied using the data of a hydrographic section along the Shackleton Transverse Ridge observed in November 2007. The distribution of the velocity component normal to the section was computed by two methods: (a) directly on the basis of the lowered acoustic Doppler current profiler (LADCP) data; (b) by the calculation of geostrophic currents based on the CTD data with estimating the depth averaged velocity on the basis of the LADCP data. The main results of the research are the relatively low estimated value of the Antarctic Circumpolar Current (ACC) transport, which accounts for 65–70% of the ACC transports in December 2003 and November 2005, and the confirmation of the existence of several abyssal currents confined to the deep passages of the bottom topography.

Warming and Freshening in the Abyssal Southeastern Indian Ocean*

Abstract Warming and freshening of abyssal waters in the eastern Indian Ocean between 1994/95 and 2007 are quantified using data from two closely sampled high-quality occupations of a hydrographic section extending from Antarctica northward to the equator. These changes are limited to abyssal waters in the Princess Elizabeth Trough and the Australian–Antarctic Basin, with little abyssal change evident north of the Southeast Indian Ridge. As in previous studies, significant cooling and freshening is observed in the bottom potential temperature–salinity relations in these two southern basins. In addition, analysis on pressure surfaces shows abyssal warming of about 0.05°C and freshening of about 0.01 Practical Salinity Scale 1978 (PSS-78) in the Princess Elizabeth Trough, and warming of 0.1°C with freshening of about 0.005 in the abyssal Australian–Antarctic Basin. These 12-yr differences are statistically significant from zero at 95% confidence intervals over the bottom few to several hundred decibars of the water column in both deep basins. Both warming and freshening reduce the density of seawater, contributing to the vertical expansion of the water column. The changes below 3000 dbar in these basins suggest local contributions approaching 1 and 4 cm of sea level rise, respectively. Transient tracer data from the 2007 occupation qualitatively suggest that the abyssal waters in the two southern basins exhibiting changes have significant components that have been exposed to the ocean surface within the last few decades, whereas north of the Southeast Indian Ridge, where changes are not found, the component of abyssal waters that have undergone such ventilation is much reduced.

On the oceanographic variability of the North-West European Shelf to the West of Scotland

Temperature, salinity and flow variability on the north-west European Shelf west of Scotland are examined. Analyses are derived from a range of marine and atmospheric data sources which together comprise a “proto-observatory” of the European Shelf to the west of the Scottish mainland. The main time series are from a current meter mooring which has been maintained on the northwest European continental shelf since June 1981, and a hydrographic section repeated at least annually across the shelf at ~ 56°N since 1975. With the exception of occasional episodes the water column at the mooring site is well mixed or weakly stratified throughout the year. The mean water temperature was 10.1 °C and the dominant mode of variance in the temperature record is the seasonal cycle (SA), with amplitude of 3.2 °C. The currents are constrained by a passage and are dominated by the semi-diurnal tidal species. There is also significant energy in SA in the along channel velocity but not in the across channel velocity. The along channel residual has mean value of 10.8 cm s − 1 directed towards the north, a clear manifestation of the coastal current, with very few, short duration periods of flow reversal. The monthly temperature anomaly time series are compared with the NE Atlantic upper layer heat content anomalies. Both anomaly time series show highs in the late 1980s and 1990s and lows in the early 1980s and mid 1990s. The overall trend is of warming at a rate of + 0.57 °C per decade, with the timing of maximum annual temperature receding by 12 days per decade through the time series. A simple barotropic dynamic balance for the along channel flow in the passage demonstrates that the near surface wind dominates the variability of the northward flow, and that both the north/south gradient in sea level slope and atmospheric pressure are significant in driving the coastal current northwards. This balance reveals a bias is previous summer-only estimates of the flow rate of the coastal current, and a revised figure approximately double that of previous estimates is suggested. Residual winter flows correlate significantly with the North Atlantic Oscillation index ( r = 0.59). In contradiction to previous published analyses, shelf salinities exhibit only weak seasonality. No single determining factor is found for the longitudinal excursion of surface isohalines across the shelf, however prolonged periods of high NAO index do coincide with a raised salinity of shelf waters.

Bifurcation of the Subtropical South Equatorial Current against New Caledonia in December 2004 from a Hydrographic Inverse Box Model*

Abstract The South Equatorial Current (SEC), the westward branch of the South Pacific subtropical gyre, extends from the equator to 30°S at depth. Linear ocean dynamics predict that the SEC forms boundary currents on the eastern coasts of the South Pacific islands it encounters. Those currents would then detach at the northern and southern tips of the islands, and cross the Coral Sea in the form of jets. The Fiji Islands, the Vanuatu archipelago, and New Caledonia are the major topographic obstacles on the SEC pathway to the Australian coast. Large-scale numerical studies, as well as climatologies, suggest the formation of three jets in their lee: the north Vanuatu jet (NVJ), the north Caledonian jet (NCJ), and the south Caledonian jet (SCJ), implying a bifurcation against the east coast of each island. The flow observed during the SECALIS-2 cruise in December 2004 between Vanuatu and New Caledonia is presented herein. An inverse box model is used to provide quantitative transport estimates with uncertainties and to infer the pathways and boundary current formation. For that particular month, the 0–2000-m SEC inflow was found to be 20 ± 4 Sv (1 Sv ≡ 106 m3 s−1) between Vanuatu and New Caledonia. Of that, 6 ± 2 Sv bifurcated to the south in a boundary current against the New Caledonia coast (the Vauban Current), and the remainder exited north of New Caledonia, feeding the NCJ. The flow is comparable both above and below the thermocline, while complex topography, associated with oceanic eddy generation, introduces several recirculation features. To the north, the NCJ, which extends down to 1500 m, was fed not only by the SEC inflow, but also by waters coming from the north, which have possibly been recirculated. To the south, a westward current rounds the tip of New Caledonia. A numerical simulation suggests a partial continuity with the deep extension of the Vauban Current (this current would then be the SCJ) while the hydrographic sections are too distant to confirm such continuity.

The solubility and deposition of aerosol Fe and other trace elements in the North Atlantic Ocean: Observations from the A16N CLIVAR/CO 2 repeat hydrography section

Aerosol and precipitation sampling as part of the 2003 Climate Variability and Predictability (CLIVAR)-CO 2 Repeat Hydrography trace element sampling program has produced an aerosol chemistry dataset for a region of the central Atlantic Ocean between 65°N and 5°S. This dataset includes analyses of aerosol particle chemistry as well as Fe and Al solubility (measured using a rapid, flow-through leaching technique). Several factors thought to influence aerosol Fe solubility including chemical weathering and aerosol source are evaluated as well. Air mass back-trajectories were used to characterize the atmospheric regime of each aerosol sample. Aerosol concentrations varied greatly with the highest concentrations observed between 23°N and 8.7°N. Aerosol Fe solubility was 9% ± 5% in seawater and 15% ± 8% in ultrapure deionized water. The concentration of soluble aerosol Fe in seawater was estimated with reasonable accuracy from the concentration of soluble aerosol Fe in deionized water by the relationship logFe SW = (0.85 ± 0.039) logFe DI + log (1 ± 1.2), ( r 2 = 0.93).

Climate variability in the North Pacific thermocline diagnosed from oxygen measurements: An update based on the U.S. CLIVAR/CO2 Repeat Hydrography cruises

New observations of oxygen variability in the North Pacific Ocean are reported on the basis of comparison of the U.S. Climate Variability and Predictability and Carbon (CLIVAR/CO2) Repeat Hydrography sections conducted along 30°N (2004) and 152°W (2006) with the earlier World Ocean Circulation Experiment (WOCE) data and other cruises along these sections. The largest changes in apparent oxygen utilization (AOU) continue to occur, as found in earlier North Pacific repeat section analyses, within the thermocline on σθ = 26.6 kg m−3, which is the densest isopycnal to outcrop in the open North Pacific in climatological data. In the northeastern North Pacific along 152°W, where a total of five cruises (1980, 1984, 1991, 1997, and 2006) spanning a period of 26 years are available, the AOU changes correspond to an overall increase in AOU on σθ = 26.6 kg m−3 from the 1980s/early 1990s to 2006. However, from 1997 to 2006 a decrease in AOU is observed within the boundary region between the subtropical and subpolar gyres at 40°–45°N. Along the center axis of the subtropical gyre at 30°N, where two cruises are available (1994 and 2004), AOU has also substantially increased on σθ = 26.6 kg m−3 from 1994 to 2004 in the eastern part of the section. The repeat section data along 152°W and 30°N are consistent with a pattern of decadal‐scale ventilation anomalies that originate in the northwestern Pacific, possibly through variability (including cessation) of the σθ = 26.6 kg m−3 outcrop, travel eastward along the subtropical‐subpolar gyre boundary, and enter the northern portion of the subtropical gyre along the way. For the 152°W AOU data within the gyre boundary region (40°–45°N), good agreement exists with the close‐by time series data from Ocean Station P (50°N, 145°W) where a bidecadal cycle in AOU has been observed. In contrast, a sensible correlation with the Pacific Decadal Oscillation could not be found.

Observations of the southern East Madagascar Current and undercurrent and countercurrent system

In April 2001 four hydrographic sections perpendicular to the southern East Madagascar Current were surveyed as part of the Agulhas Current Sources Experiment. Observations with a vessel mounted and a lowered ADCP produced information on the current field while temperature, salinity, oxygen and nutrient data obtained with a CTD‐Rosette system, gave information on the water mass structure of the currents southeast of Madagascar. The peak velocity in the pole‐ward East Madagascar Current through these four sections had a typical magnitude of ∼110 cm/s, while the width of this current was of the order of 120 km. The mean pole‐ward volume transport rate of this current during the survey above the 5°C isotherm was estimated to be 37 ± 10 Sv. On all four sections an undercurrent was observed at intermediate depths below the East Madagascar Current. Its equator‐ward transport rate amounted to 2.8 ± 1.4 Sv. Offshore of the East Madagascar Current the shallow South Indian Ocean Countercurrent was observed. This eastward frontal jet coincided with the barotropic and thermohaline front that separates the saline Subtropical Surface Water from the fresher Tropical Surface Water in the East Madagascar Current. The near‐surface geostrophic flow of the East Madagascar Current, derived from satellite altimetry data from 1992 to 2005, suggests a strong variability of this transport due to eddy variability and interannual changes. The long‐term pole‐ward mean transport of the East Madagascar Current, roughly estimated from those altimetry data amounts to 32 Sv. The upper‐ocean water mass of the East Madagascar Current was very saline in 2001, compared to WOCE surveys from 1995. Comparison of our undercurrent data with those of the WOCE surveys in 1995 confirms that the undercurrent is a recurrent feature. Its water mass properties are relatively saline, due to the presence of water originating from the Red Sea outflow at intermediate levels. The saline water was advected from the Mozambique Channel to the eastern slope of Madagascar.

A coastal current in winter: Autonomous underwater vehicle observations of the coastal current east of Cape Cod

Evolution of the coastal current structure on the shallow continental shelf east of Cape Cod was studied using autonomous underwater vehicle (AUV) surveys and moored observations during the winters of 2005 and 2006. A coastally bounded plume of relatively fresh water, characteristic of a coastal current, persisted throughout both winters despite strong mixing. Nondimensional parameter analysis classified the plume as a bottom‐trapped gravity current over a moderately steep slope, placing it in the context of other buoyant coastal currents. The range of water properties within the coastal current, its spatial extent and temporal variability were characterized on the basis of the data from repeat hydrographic sections. Along‐shore freshwater transport was dominated by highly variable barotropic flow driven by local wind and basin‐wide pressure gradients. It eventually contributed substantially to the average southward along‐shore freshwater transport, estimated at 1.1 ± 0.3 × 103 m3 s−1 in February and 1.8 ± 0.4 × 103 m3 s−1 in the first half of March 2006. The contribution of baroclinic buoyancy‐driven freshwater transport was typically an order of magnitude lower during both winters. Despite the relative weakness of the baroclinic freshwater transport, the coastal current potentially had a major impact on water mass modification during the winter. Continual presence of the low‐salinity plume prevented the formation of cold dense water near the coast and its export offshore. The coastal current effectively isolated the inner‐shelf zone, reducing its potential role in ventilation of the intermediate layers of the Wilkinson Basin of the Gulf of Maine.

A hydrographic section from South Africa to the southern limit of the Antarctic Circumpolar Current at the Greenwich meridian

The properties and circulation of water masses leaving and entering the Atlantic Ocean south of Africa are examined using data from a hydrographic and Lowered acoustic Doppler current profiler section from South Africa to the southern limit of the Antarctic Circumpolar Current (ACC). At the upper levels, the ACC fronts are well determined using either classical water mass definitions or gradient-based criteria. While the locations of the Subantarctic Front (SAF), Polar Front (PF), and Southern ACC Front (SACCF) seem controlled by the neighbouring ridges, that of the Subtropical Front (STF) is much influenced in this region (∼10°E) by northwestward propagating Agulhas rings. No large amount of Subantarctic Mode Water (SAMW) is observed, but two varieties of vertically homogeneous water are found in eddies detached from the Agulhas retroflection: one is remotely (Indian Ocean) formed SAMW, and the other a local variety formed through winter convection in some eddies. The deep front imprints allow one to recognize lower circumpolar deep water (LCDW) from the Drake Passage (south of the SACCF), a mix of LCDW and North Atlantic Deep Water (NADW) injected in the ACC in the Argentine Basin (between the PF and STF), and diluted NADW from a southeastward pathway along the African continental slope (north of the STF in these data). The abyssal circulation, much controlled by ridges transverse to the ACC, shows a westward entry of diluted WSDW into the Cape Basin below the PF and two cyclonic patterns in the southern and central Cape Basin superimposed on a wider eastward trend. Transport estimates are given for the ACC, its fronts, and the abyssal circulation. The baroclinic and total ACC transports are ∼136 and ∼153 Sv, respectively. Bottom-intensified westward flows O(20 Sv) have only a limited effect on the ACC net transport, being laterally compensated. They, however, affect the frontal structure: the ∼15 Sv entry of diluted WSDW seems related to a branching of the PF, and its eastward recirculation widens the full-depth transport signature of the SAF.

Recirculation of Atlantic Water in the western Barents Sea

In this article, the Atlantic inflow with the North Cape Current to the Barents Sea, and potential recirculation of this water in the Bear Island Trough is investigated. This is resolved by combining continuous current meter and repeated hydrographic section data in the Barents Sea Opening. The estimated annual mean net fluxes, when excluding the contribution from the Norwegian Coastal Current, are 1.1 Sv and 39 TW (estimated relative to a reference temperature of 0°C). The estimated heat flux associated with the outflow in the Bear Island Trough is relatively large (−12 TW). Long‐term current meter records do not show a close relationship between variations in the inflow and the outflow. One‐year low‐pass filtered variations in temperature and salinity of the outflowing waters in the Bear Island Trough are positively correlated with variations of the inflowing Atlantic Water with a lag of about seven months. On the basis of a simple mixing model, the outflow water consists of about 80% Atlantic Water that is additionally cooled by 2.0–2.5°C by heat loss to the atmosphere. By considering the relative weak mean currents in this region, it can be inferred that the fraction of the Atlantic Water that recirculates follows a relatively short pathway in the Bear Island Trough before returning to the Norwegian Sea.

On the source of Gulf Stream nutrients

Along density surfaces, nutrient concentrations in the Gulf Stream are elevated relative to concentrations to either side of the current. We assess the source of these elevated nutrient concentrations in the western boundary current using historical hydrographic data. The analysis is extended to the separated Gulf Stream with four hydrographic sections recently occupied as part of the Climate Variability and Predictability Program (CLIVAR) Mode Water Dynamics Experiment. The results of this analysis suggest that imported, extrasubtropical waters are the primary source of the elevated nutrient concentrations. Because the high nutrient signature is likely imported, diapycnal mixing need not be invoked to explain the Gulf Stream's high nutrient concentrations, as had been proposed in the past. Moreover, nutrients do not increase along the length of the stream, further suggesting that the stream's high nutrient signature is imported rather than manufactured by processes within the current. The imported nutrients are likely advected into the North Atlantic within the low‐salinity water masses that contribute to the shallow limb of the meridional overturning circulation. Thus the availability of nutrients in the North Atlantic may be linked to upstream processes in the tropics and possibly the Southern Hemisphere as well as to variability in the volume of imported water and its distribution in density space.

Fjord–shelf exchanges controlled by ice and brine production: The interannual variation of Atlantic Water in Isfjorden, Svalbard

Exchanges between oceanic and coastal waters are fundamental in setting the hydrography of arctic shelves and fjords. In West Spitsbergen, Atlantic Water from the West Spitsbergen Current exchanges with the seasonally ice-covered waters of the coast and fjords causing a major annual shift in hydrographic conditions. The extent to which Atlantic Water dominates the fjord systems shows significant interannual variability. Hydrographic sections taken between 1999 and 2005 from Isfjorden and the adjacent shelf have been analyzed to identify the causes of the variability in Atlantic Water occupation of the fjord system. By treating the fjord system as a coastal polynya and running a polynya model to quantify the salt release each winter, we conclude that the critical parameter controlling fjord–shelf exchange is the density difference between the fjord water masses and the Atlantic Water. We provide a full dynamical mechanism for the interaction between water masses at the fjord entrance to rationalize the interannual variability.

Transports of Nordic Seas water masses and excess SF 6 through Fram Strait to the Arctic Ocean

To determine the exchanges between the Nordic Seas and the Arctic Ocean through Fram Strait is one of the most important aspects, and one of the major challenges, in describing the circulation in the Arctic Mediterranean Sea. Especially the northward transport of Arctic Intermediate Water (AIW) from the Nordic Seas into the Arctic Ocean is little known. In the two-ship study of the circulation in the Nordic Seas, Arctic Ocean – 2002, the Swedish icebreaker Oden operated in the ice-covered areas in and north of Fram Strait and in the western margins of Greenland and Iceland seas, while RV Knorr of Woods Hole worked in the ice free part of the Nordic Seas. Here two hydrographic sections obtained by Oden, augmented by tracer and velocity measurements with Lowered Acoustic Doppler Current Profiler (LADCP), are examined. The first section, reaching from the Svalbard shelf across the Yermak Plateau, covers the region north of Svalbard where inflow to the Arctic Ocean takes place. The second, western, section spans the outflow area extending from west of the Yermak Plateau onto the Greenland shelf. Geostrophic and LADCP derived velocities are both used to estimate the exchanges of water masses between the Nordic Seas and the Arctic Ocean. The geostrophic computations indicate a total flow of 3.6 Sv entering the Arctic on the eastern section. The southward flow on the western section is found to be 5.1 Sv. The total inflow to the Arctic Ocean obtained using the LADCP derived velocities is much larger, 13.6 Sv, and the southward transport on the western section is 13.7 Sv, equal to the northward transport north of Svalbard. Sulphur hexafluoride (SF 6) originating from a tracer release experiment in the Greenland Sea in 1996 has become a marker for the circulation of AIW. From the geostrophic velocities we obtain 0.5 Sv and from the LADCP derived velocities 2.8 Sv of AIW flowing into the Arctic. The annual transport of SF 6 into the Arctic Ocean derived from geostrophy is 5 kg/year, which is of the same magnitude as the observed total annual transport into the North Atlantic, while the LADCP measurements (19 kg/year) imply that it is substantially larger. Little SF 6 was found on the western section, confirming the dominance of the Arctic Ocean water masses and indicating that the major recirculation in Fram Strait takes place farther to the south.