The lower limb of the Atlantic overturning circulation is renewed by dense waters from the Southern Ocean, a substantial portion of which flow through the Scotia Sea. We report dense bottom layers here, with gradients in temperature and salinity comparable to those seen near the surface of the Southern Ocean. These are overlain by layers with much weaker stratification, and are caused by episodic overflows of dense waters across the South Scotia Ridge, and topographic trapping within deep trenches. One such layer was found to be at least 3-4 years older than the water immediately above. The estimated vertical diffusivity to which this layer was subject is substantially less than the strong basin-average deep mixing reported previously. We conjecture that (a) vertical mixing in the Scotia Sea is strongly spatially inhomogeneous, and (b) the flushing of these layers, like their formation, is related to overflow events, and hence also strongly episodic. © 2013. American Geophysical Union. All Rights Reserved.

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters. Some of this deficit is made up by intense near-bottom mixing occurring in restricted 'hot-spots' associated with rough ocean-floor topography, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows. Here we show that diapycnal mixing of mid-depth (∼1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1 Sv = 10 6 m 3 s -1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography. © 2013 Macmillan Publishers Limited. All rights reserved.

In summer 1996, a tracer release experiment using sulphur hexafluoride (SF6) was launched in the intermediate-depth waters of the central Greenland Sea (GS), to study the mixing and ventilation processes in the region and its role in the northern limb of the Atlantic overturning circulation. Here we describe the hydrographic context of the experiment, the methods adopted and the results from the monitoring of the horizontal tracer spread for the 1996-2002 period documented by ∼10 shipboard surveys. The tracer marked "Greenland Sea Arctic Intermediate Water" (GSAIW). This was redistributed in the gyre by variable winter convection penetrating only to mid-depths, reaching at most 1800 m depth during the strongest event observed in 2002. For the first 18 months, the tracer remained mainly in the Greenland Sea. Vigorous horizontal mixing within the Greenland Sea gyre and a tight circulation of the gyre interacting slowly with the other basins under strong topographic influences were identified. We use the tracer distributions to derive the horizontal shear at the scale of the Greenland Sea gyre, and rates of horizontal mixing at ∼10 and ∼300 km scales. Mixing rates at small scale are high, several times those observed at comparable depths at lower latitudes. Horizontal stirring at the sub-gyre scale is mediated by numerous and vigorous eddies. Evidence obtained during the tracer release suggests that these play an important role in mixing water masses to form the intermediate waters of the central Greenland Sea. By year two, the tracer had entered the surrounding current systems at intermediate depths and small concentrations were in proximity to the overflows into the North Atlantic. After 3 years, the tracer had spread over the Nordic Seas basins. Finally by year six, an intensive large survey provided an overall synoptic documentation of the spreading of the tagged GSAIW in the Nordic Seas. A circulation scheme of the tagged water originating from the centre of the GS is deduced from the horizontal spread of the tracer. We present this circulation and evaluate the transport budgets of the tracer between the GS and the surroundings basins. The overall residence time for the tagged GSAIW in the Greenland Sea was about 2.5 years. We infer an export of intermediate water of GSAIW from the GS of 1 to 1.85 Sv (1 Sv = 106 m3 s-1) for the period from September 1998 to June 2002 based on the evolution of the amount of tracer leaving the GS gyre. There is strong exchange between the Greenland Sea and Arctic Ocean via Fram Strait, but the contribution of the Greenland Sea to the Denmark Strait and Iceland Scotland overflows is modest, probably not exceeding 6% during the period under study. © 2008 Elsevier Ltd.

The Greenland Sea is one of a few sites in the world ocean where convection to great depths occurs-a process that forms some of the densest waters in the ocean. But the role of deep convective eddies, which result from surface cooling and mixing across density surfaces followed by geostrophic adjustment, has not been fully taken into account in the description of the initiation and growth of convection. Here we present tracer, float and hydrographic observations of long-lived ( approximately 1 year) and compact ( approximately 5 km core diameter) vortices that reach down to depths of 2 km. The eddies form in winter, near the rim of the Greenland Sea central gyre, and rotate clockwise with periods of a few days. The cores of the observed eddies are constituted from a mixture of modified Atlantic water that is warm and salty with polar water that is cold and fresh. We infer that these submesoscale coherent eddies contribute substantially to the input of Atlantic and polar waters to depths greater than 500 m in the central Greenland Sea.

Convective vertical mixing in restricted areas of the subpolar oceans, such as the Greenland Sea, is thought to be the process responsible for forming much of the dense water of the ocean interior. Deep-water formation varies substantially on annual and decadal timescales, and responds to regional climate signals such as the North Atlantic Oscillation; its variations may therefore give early warning of changes in the thermohaline circulation that may accompany climate change. Here we report direct measurements of vertical mixing, by convection and by turbulence, from a sulphur hexafluoride tracer-release experiment in the central Greenland Sea gyre. In summer, we found rapid turbulent vertical mixing of about 1.1 cm2s-1. In the following late winter, part of the water column was mixed more vigorously by convection, indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre. At the same time, mixing outside the gyre centre was only slightly greater than in summer. The results suggest that about 10% of the water in the gyre centre was vertically transported in convective plumes, which reached from the surface to, at their deepest, 1,200-1,400 m. Convection was limited to a very restricted area, however, and smaller volumes of water were transported to depth than previously estimated. Our results imply that it may be the rapid year-round turbulent mixing, rather than convection, that dominates vertical mixing in the region as a whole.

Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatorial Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deeper maximum around 3800 m at level of the Lower North Atlantic Deep Water (LNADW). These Observations suggest a bifurcation at the western boundary, some of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core (σ1.5 = 34.70 kgm-3) was observed eastward as far as 5°W. The deep CFM maximum (σ4 = 45.87 kgm-3), associated with an oxygen maximum, decreased dramatically at the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water 'ages' computed from the F-11/F-12 ratio are estimated. Near the bottom, no enrichment in CFMs is detected at the entrance of the fracture zones in the cold water mass originating from the Antarctic Bottom Water flow.

Abstract
. Small-scale turbulent mixing drives the upwelling of deep water masses in the abyssal ocean as part of the global overturning circulation (Wunsch & Ferrari 2004). However, the processes leading to mixing and the pathways through which this upwelling occurs remain insufficiently understood. Recent observational and theoretical work suggests that deep water upwelling may be focused in bottom boundary layers on the ocean’s sloping seafloor; however, direct evidence of this is lacking (Ledwell et al. 2000, St. Laurent et al. 2001, Ferrari et al. 2016, de Lavergne et al. 2016). Here, we present observations from a near-bottom dye release within a canyon on the North Atlantic continental slope showing upwelling across density surfaces at a rate of 250 +/- 75 m/day over three days, ∼10,000 times higher than the global average value required to account for ∼30 Sv of upwelling globally (Munk 1966). The vigourous upwelling is coupled with adiabatic exchange of near-boundary and interior fluid. These results provide direct evidence of strong, bottom-focused diapycnal upwelling in the deep ocean, supporting previous suggestions that mixing at topographic features, such as canyons, leads to upwelling.

Abstract. We explore historical variability in the volume of Labrador Sea Water (LSW) using ECCO, an ocean state estimate configuration of the Massachusetts Institute of Technology general circulation model (MITgcm). The model’s adjoint, a linearization of the MITgcm, is set up to output the lagged sensitivity of the watermass volume to surface boundary conditions. This allows us to reconstruct the evolution of LSW volume over recent decades using historical surface wind stress, heat, and freshwater fluxes. Each of these boundary conditions contributes significantly to the LSW variability that we recover, but these impacts are associated with different geographical fingerprints and arise over a range of time lags. We show that the volume of LSW accumulated in the Labrador Sea exhibits a delayed response to surface wind stress and buoyancy forcing outside the convective interior of the Labrador Sea, at key locations in the North Atlantic Ocean. In particular, winds and surface density anomalies affect the North Atlantic Current’s (NAC) transport of warm and saline subtropical water masses that are precursors for the formation of LSW. This propensity for a delayed response of LSW to remote forcing allows us to predict a substantial fraction of LSW variability at least a year into the future. Our analysis also enables us to attribute LSW variability to different boundary conditions and to gain insight into the major mechanisms that drive volume anomalies in this deep watermass. We point out the important role of buoyancy loss and preconditioning along the NAC pathway, in the Iceland Basin, the Irminger Sea, and the Nordic Seas, processes which facilitate the formation of LSW both in the Irminger and in the Labrador Sea.
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We provide new estimates of the air-sea flux of CCl 4 using simulations from a global ocean biogeochemistry model (NEMO-PlankTOM) in combination with depth-resolved CCl 4 observations from global oceanic databases. Estimates of global oceanic CCl 4 uptake are derived from a range of model analyses, including prescribed parameterizations using reported values on hydrolysis and degradation, and analyses optimized using the global observational databases. We evaluate the sensitivity of our results to uncertainties in air-sea gas exchange parameterization, estimation period, and circulation processes. Our best constrained estimate of ocean CCl 4 uptake for the period 1996–2000 is 20.1 Gg/year (range 16.6–22.7), corresponding to estimates of the partial atmospheric lifetime with respect to ocean uptake of 124 (110–150) years. This new oceanic lifetime implies higher emissions of CCl 4 than currently estimated and therefore a larger missing atmospheric source of CCl 4.

Since the advent of the industrial revolution, atmospheric CO2 has increased from 275 ppm to over 400 ppm, enhancing the associated Greenhouse effect and being suggested as the main cause of recent climate change. The global ocean sequesters around a third of the CO2 emitted by human activity, mitigating climate impacts, with the highest anthropogenic CO2 (Cant) storage per unit area occurring in the North Atlantic. However, ocean Cant cannot be measured directly, but it is calculated with published uncertainties that range between ±10 % and ±20 %. Here, we assess five methods used to estimate Cant, named ∆C*, ΦCT0, TrOCA, TTD, and eMLR, by using the outputs of four climate models (CCSM, CM2Mc, OCCAM, and GFDL-ESM2M) between 1860 and 2100, the most recent observation database (e.g. GLODAPv2) between 1980 and 2013, and the repeated time series collected along the 24.5◦N Atlantic transect between 1992 and 2016. We focus on the North Atlantic upper 1000 m, where the Mode waters store the largest Cant amount. In this layer, the TTD and ∆C. estimates confine the probable range of Cant concentrations, therefore we focus on these two methods. For both, we quantify a total (analytical precisions + methodological assumptions) uncertainty of ±34 %, which is higher than previously suggested. However, the Cant uncertainties depend on timeframes and regions: between 1992 and 2010, observations enable us to reliably decrease these uncertainties to ±13 % (TTD) and ±14 % (∆C*) in the upper 1000 m of the subtropical North Atlantic (20-30◦N). Here, we estimate with a quasi Monte Carlo approach that the Mode waters Cant pool increases by 0.5 (TTD) and 0.8 (∆C*) ± 0.2 μmol kg−1 yr−1, thus the estimates diverge over time. We associate the divergence to unsteady CO2 disequilibrium between the atmosphere and ocean (0.3 (∆C*) and 0.5 (TTD) ± 0.3 μmol kg−1 yr−1), and biogeochemical changes, as suggested by the increasing (0.3 ± 0.1 μmol kg−1 yr−1) dissolved inorganic carbon from remineralised soft tissue: these alterations are unequally captured by the TTD and ∆C. techniques. Changes in ocean biogeochemistry are further explored using the output of a CM2Mc pre-industrial ‘control’ simulation over two millennia. Here, the statistically significant drivers of the enhancement in remineralised soft-tissue carbon are increasing mean residence time (R2 = 0.86) and acidification (R2 = 0.68). Feedback mechanisms have the potential to shift the oceanic carbon cycle towards new equilibria, significantly influencing the future North Atlantic carbon uptake.

In this work, we use realistic isopycnal velocities with a 3-D eddy diffusivity to advect and diffuse a tracer in the Antarctic Circumpolar Current, beginning in the Southeast Pacific and progressing through Drake Passage. We prescribe a diapycnal diffusivity which takes one value in the SE Pacific west of 67°W and another value in Drake Passage east of that longitude, and optimize the diffusivities using a cost function to give a best fit to experimental data from the DIMES (Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean) tracer, released near the boundary between the Upper and Lower Circumpolar Deep Water. We find that diapycnal diffusivity is enhanced 20-fold in Drake Passage compared with the SE Pacific, consistent with previous estimates obtained using a simpler advection-diffusion model with constant, but different, zonal velocities east and west of 67°W. Our result shows that diapycnal mixing in the ACC plays a significant role in transferring buoyancy within the Meridional Overturning Circulation.

© 2015. The Authors. The waters of the Weddell-Scotia Confluence (WSC) lie above the rugged topography of the South Scotia Ridge in the Southern Ocean. Meridional exchanges across the WSC transfer water and tracers between the Antarctic Circumpolar Current (ACC) to the north and the subpolar Weddell Gyre to the south. Here, we examine the role of topographic interactions in mediating these exchanges, and in modifying the waters transferred. A case study is presented using data from a free-drifting, intermediate-depth float, which circulated anticyclonically over Discovery Bank on the South Scotia Ridge for close to 4 years. Dimensional analysis indicates that the local conditions are conducive to the formation of Taylor columns. Contemporaneous ship-derived transient tracer data enable estimation of the rate of isopycnal mixing associated with this column, with values of O(1000 m 2 /s) obtained. Although necessarily coarse, this is of the same order as the rate of isopycnal mixing induced by transient mesoscale eddies within the ACC. A picture emerges of the Taylor column acting as a slow, steady blender, retaining the waters in the vicinity of the WSC for lengthy periods during which they can be subject to significant modification. A full regional float data set, bathymetric data, and a Southern Ocean state estimate are used to identify other potential sites for Taylor column formation. We find that they are likely to be sufficiently widespread to exert a significant influence on water mass modification and meridional fluxes across the southern edge of the ACC in this sector of the Southern Ocean. Key Points: Taylor columns form above peaks in the South Scotia Ridge They markedly affect circulation and mixing in the Weddell-Scotia Confluence Conditions enable their formation elsewhere around the Scotia Sea and beyond

© 2015. The Authors. The waters of the Weddell-Scotia Confluence (WSC) lie above the rugged topography of the South Scotia Ridge in the Southern Ocean. Meridional exchanges across the WSC transfer water and tracers between the Antarctic Circumpolar Current (ACC) to the north and the subpolar Weddell Gyre to the south. Here, we examine the role of topographic interactions in mediating these exchanges, and in modifying the waters transferred. A case study is presented using data from a free-drifting, intermediate-depth float, which circulated anticyclonically over Discovery Bank on the South Scotia Ridge for close to 4 years. Dimensional analysis indicates that the local conditions are conducive to the formation of Taylor columns. Contemporaneous ship-derived transient tracer data enable estimation of the rate of isopycnal mixing associated with this column, with values of O(1000 m 2 /s) obtained. Although necessarily coarse, this is of the same order as the rate of isopycnal mixing induced by transient mesoscale eddies within the ACC. A picture emerges of the Taylor column acting as a slow, steady blender, retaining the waters in the vicinity of the WSC for lengthy periods during which they can be subject to significant modification. A full regional float data set, bathymetric data, and a Southern Ocean state estimate are used to identify other potential sites for Taylor column formation. We find that they are likely to be sufficiently widespread to exert a significant influence on water mass modification and meridional fluxes across the southern edge of the ACC in this sector of the Southern Ocean.

The variability in the storage of the oceanic anthropogenic CO2 (Cant) on decadal timescales is evaluated within the main water masses of the Subtropical North Atlantic along 24.5°N. Inorganic carbon measurements on five cruises of the A05 section are used to assess the changes in Cant between 1992 and 2011, using four methods (δC*, TrOCA, ϕCT0, TTD). We find good agreement between the Cant distribution and storage obtained using chlorofluorocarbons and CO2 measurements in both the vertical and horizontal scales. Cant distribution shows higher concentrations and greater decadal storage rates in the upper layers with both values decreasing with depth. The greatest enrichment is obserbed in the central water masses, with their upper limb showing a mean annual accumulation of about 1μmolkg-1yr-1 and the lower limb showing, on average, half that value. We detect zonal gradients in the accumulation of Cant. This finding is less clear in the upper waters, where greater variability exists between methods. In accordance with data from time series stations, greater accumulation of Cant is observed in the upper waters of the western basin of the North Atlantic Subtropical Gyre. In intermediate and deep layers, the zonal gradient in the storage of Cant is more robust between methods. The much lower mean storage rates found along the section (

The first direct estimate of the rate at which geostrophic turbulence mixes tracers across the Antarctic Circumpolar Current is presented. The estimate is computed from the spreading of a tracer released upstream of Drake Passage as part of the Diapycnal and Isopycnal Mixing Experiment in the Southern Ocean (DIMES). The meridional eddy diffusivity, a measure of the rate at which the area of the tracer spreads along an isopycnal across the Antarctic Circumpolar Current, is 710 ± 260 m2 s-1 at 1500-m depth. The estimate is based on an extrapolation of the tracer-based diffusivity using output from numerical tracers released in a one-twentieth of a degree model simulation of the circulation and turbulence in the Drake Passage region. The model is shown to reproduce the observed spreading rate of the DIMES tracer and suggests that the meridional eddy diffusivity is weak in the upper kilometer of the water column with values below 500 m2 s-1 and peaks at the steering level, near 2 km, where the eddy phase speed is equal to the mean flow speed. These vertical variations are not captured by ocean models presently used for climate studies, but they significantly affect the ventilation of different water masses.

The variability of nitrate (N), phosphate (P), silicate (Si) and Apparent Oxygen Utilization (AOU) due to water mass mixing was objectively separated from the variability due to mineralization of biogenic materials in the western and eastern South Atlantic Ocean on basis of the constrained Optimum MultiParameter (OMP) analysis implemented in the companion manuscript. Using a consensus linear regression model, AOU/N/P/Si mineralization ratios and the corresponding oxygen utilisation rates (OURs) were obtained for the realm of each water mass defined after the OMP analysis. Combining these results with a stoichiometric model, the organic carbon to nitrogen (C/N) ratios and the biochemical composition (carbohydrates+lipids, proteins and phosphorus compounds) of the mineralized material, were derived. The vertical variability of the AOU/N, AOU/P and AOU/C mineralization ratios pointed to a significant fractionation during the mineralization of sinking organic matter. This fractionation was confirmed by preferential consumption of organic phosphorous compounds and proteins in shallower levels, which produced an increase of the C/N ratio of the mineralised materials of 0.5±0.2molCmolN-1 every 1000dbar. OURs in the twilight zone decreased quadratically with the C/N molar ratio of the mineralised material and exponentially with pressure (p, in 103dbar) according to the following regression equation: Ln (OUR)=6.2(±1.2)-2.0(±0.7)*Ln (C/N)-0.6(±0.2)*p (r2=0.87, p

The horizontal and vertical circulation of the Weddell Gyre is diagnosed using a box inverse model constructed with recent hydrographic sections and including mobile sea ice and eddy transports. The gyre is found to convey 42±8 Sv (1 Sv=106 m3 s-1) across the central Weddell Sea and to intensify to 54±15 Sv further offshore. This circulation injects 36±13 TW of heat from the Antarctic Circumpolar Current to the gyre, and exports 51±23 mSv of freshwater, including 13±1 mSv as sea ice to the midlatitude Southern Ocean. The gyre's overturning circulation has an asymmetric double-cell structure, in which 13±4 Sv of Circumpolar Deep Water (CDW) and relatively light Antarctic Bottom Water (AABW) are transformed into upper-ocean water masses by midgyre upwelling (at a rate of 2±2 Sv) and into denser AABW by downwelling focussed at the western boundary (8±2 Sv). The gyre circulation exhibits a substantial throughflow component, by which CDW and AABW enter the gyre from the Indian sector, undergo ventilation and densification within the gyre, and are exported to the South Atlantic across the gyre's northern rim. The relatively modest net production of AABW in the Weddell Gyre (6±2 Sv) suggests that the gyre's prominence in the closure of the lower limb of global oceanic overturning stems largely from the recycling and equatorward export of Indian-sourced AABW. Key Points the Weddell Sea hosts a 13 Sv doubled-cell overturning the net production of Antarctic Bottom Water in the Weddell Sea is 6 Sv the Weddell gyre recycles and exports bottom water formed outside gyre © 2014. American Geophysical Union. All Rights Reserved.

The lower limb of the Atlantic overturning circulation is renewed by dense waters from the Southern Ocean, a substantial portion of which flow through the Scotia Sea. We report dense bottom layers here, with gradients in temperature and salinity comparable to those seen near the surface of the Southern Ocean. These are overlain by layers with much weaker stratification, and are caused by episodic overflows of dense waters across the South Scotia Ridge, and topographic trapping within deep trenches. One such layer was found to be at least 3-4 years older than the water immediately above. The estimated vertical diffusivity to which this layer was subject is substantially less than the strong basin-average deep mixing reported previously. We conjecture that (a) vertical mixing in the Scotia Sea is strongly spatially inhomogeneous, and (b) the flushing of these layers, like their formation, is related to overflow events, and hence also strongly episodic. © 2013. American Geophysical Union. All Rights Reserved.

Diapycnal mixing (across density surfaces) is an important process in the global ocean overturning circulation. Mixing in the interior of most of the ocean, however, is thought to have a magnitude just one-tenth of that required to close the global circulation by the downward mixing of less dense waters. Some of this deficit is made up by intense near-bottom mixing occurring in restricted 'hot-spots' associated with rough ocean-floor topography, but it is not clear whether the waters at mid-depth, 1,000 to 3,000 metres, are returned to the surface by cross-density mixing or by along-density flows. Here we show that diapycnal mixing of mid-depth (∼1,500 metres) waters undergoes a sustained 20-fold increase as the Antarctic Circumpolar Current flows through the Drake Passage, between the southern tip of South America and Antarctica. Our results are based on an open-ocean tracer release of trifluoromethyl sulphur pentafluoride. We ascribe the increased mixing to turbulence generated by the deep-reaching Antarctic Circumpolar Current as it flows over rough bottom topography in the Drake Passage. Scaled to the entire circumpolar current, the mixing we observe is compatible with there being a southern component to the global overturning in which about 20 sverdrups (1 Sv = 10 6 m 3 s -1) upwell in the Southern Ocean, with cross-density mixing contributing a significant fraction (20 to 30 per cent) of this total, and the remainder upwelling along constant-density surfaces. The great majority of the diapycnal flux is the result of interaction with restricted regions of rough ocean-floor topography. © 2013 Macmillan Publishers Limited. All rights reserved.

A section across the Atlantic at 24S recorded in March 2009, sampled a tracer plume released in the deep Brazil Basin 13 years earlier. The 1-D diffusion equation was used to model the vertical spread of the tracer, yielding a mean diapycnal diffusivity estimate of approximately 3 × 10 -4 m2/s at 4 km depth. This estimate is similar to that found by surveys of the tracer plume made between 1996 and 2000, within four years of the tracer release and therefore provides strong evidence for the long-term stability of that result. Copyright 2012 by the American Geophysical Union.

This study evaluates the presence of intermediate water from the Greenland Sea in the Iceland Basin deduced from the observed excess of the tracer sulphur hexafiuoride (SF6), released in the central Greenland Sea in 1996. The large tracer release experiment has served a unique opportunity to follow the spread of Greenland Sea intermediate water to the adjacent basins of the Nordic Seas and to the areas bordering this region. In the present study, using data from May-June 2001, the released tracer was detected at the sill in the Faroe Bank Channel and at several locations in the Iceland Basin of the North Atlantic, just downstream the sill and southeast of Iceland. The estimated excess of the released tracer at the Icelandic slope combined with reported values of the volume flow at this location suggest an annual transport rate of approximately 1.4 kg excess SF6. The results suggest an upper transit time from the central Greenland Sea to the area southeast of Iceland of approximately 4 years. Copyright 2009 by the American Geophysical Union.

In summer 1996, a tracer release experiment using sulphur hexafluoride (SF6) was launched in the intermediate-depth waters of the central Greenland Sea (GS), to study the mixing and ventilation processes in the region and its role in the northern limb of the Atlantic overturning circulation. Here we describe the hydrographic context of the experiment, the methods adopted and the results from the monitoring of the horizontal tracer spread for the 1996-2002 period documented by ∼10 shipboard surveys. The tracer marked "Greenland Sea Arctic Intermediate Water" (GSAIW). This was redistributed in the gyre by variable winter convection penetrating only to mid-depths, reaching at most 1800 m depth during the strongest event observed in 2002. For the first 18 months, the tracer remained mainly in the Greenland Sea. Vigorous horizontal mixing within the Greenland Sea gyre and a tight circulation of the gyre interacting slowly with the other basins under strong topographic influences were identified. We use the tracer distributions to derive the horizontal shear at the scale of the Greenland Sea gyre, and rates of horizontal mixing at ∼10 and ∼300 km scales. Mixing rates at small scale are high, several times those observed at comparable depths at lower latitudes. Horizontal stirring at the sub-gyre scale is mediated by numerous and vigorous eddies. Evidence obtained during the tracer release suggests that these play an important role in mixing water masses to form the intermediate waters of the central Greenland Sea. By year two, the tracer had entered the surrounding current systems at intermediate depths and small concentrations were in proximity to the overflows into the North Atlantic. After 3 years, the tracer had spread over the Nordic Seas basins. Finally by year six, an intensive large survey provided an overall synoptic documentation of the spreading of the tagged GSAIW in the Nordic Seas. A circulation scheme of the tagged water originating from the centre of the GS is deduced from the horizontal spread of the tracer. We present this circulation and evaluate the transport budgets of the tracer between the GS and the surroundings basins. The overall residence time for the tagged GSAIW in the Greenland Sea was about 2.5 years. We infer an export of intermediate water of GSAIW from the GS of 1 to 1.85 Sv (1 Sv = 106 m3 s-1) for the period from September 1998 to June 2002 based on the evolution of the amount of tracer leaving the GS gyre. There is strong exchange between the Greenland Sea and Arctic Ocean via Fram Strait, but the contribution of the Greenland Sea to the Denmark Strait and Iceland Scotland overflows is modest, probably not exceeding 6% during the period under study. © 2008 Elsevier Ltd.

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 (SF6) 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 SF6 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 SF6 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. © 2008 Elsevier Ltd. All rights reserved.

Submesoscale Coherent Vortices (SCVs) have been observed earlier in the Greenland Sea, but their overall characteristics, the formation and the dissolution mechanisms, and the effects on the large-scale hydrodynamics were not well understood. In order to improve the understanding of these features, a simultaneous investigation of hydrography, chemical tracers, and full-depth velocity profiles in a SCV was employed in September 2003. The observed eddy had a homogeneous cold core from 500 m to 2500 m depth with a radius of 8∼15 km. The velocity field of the eddy was higher than in the previous years, and the eddy was in strong anticyclonic rotation in the intermediate layer (1000∼2000 m). The high velocity field led to the estimate of eddy vorticity twice as high as previous observations, and this was accounted for the eddy migration while the earlier observed eddies were rather stationary around 75°N 0°E. The eddy migrated northeast ward with a speed of 3 km/day driven by the background mean flow under the strong effects of the background shear, which tilted the rotation axis in the upper layer. The concentrations of sulphur hexafluoride (SF6) and chlorofluorocarbons (CFCs) in the eddy provided firm information about the source water end-members. The Greenland Sea Arctic Intermediate Water and winter cold surface water were determined as the principal eddy source waters. This differs from the earlier conception of eddies being sourced from intermediate waters at the periphery of the Greenland Basin. Copyright 2006 by the American Geophysical Union.

The Faroe Bank Channel is the deepest passage for dense water leaving the Nordic Seas into the North Atlantic. The contribution to this part of the Greenland-Scotland Overflow by intermediate water from the Greenland Sea is investigated by the tracer sulphur hexafluoride (SF6) that was released into the central Greenland Sea in summer 1996. Continuous monitoring has since traced it around the Nordic Seas and into the connecting areas. It was observed for the first time close to the Faroe Islands in early 1999, indicating a transport time from the Greenland Sea of around 2.5 years. This study estimates that approximately 16 kg of SF6 had passed the Faroe Bank Channel by the end of 2002, that is 5% of the total amount released. Both the arrival time and the amount of exported SF6 deduced from the observations are consistent with the results from a numerical ocean model simulating the tracer release and spreading. © 2004 Elsevier Ltd. All rights reserved.

The physical and biogeochemical components of nutrients and inorganic carbon distributions along WOCE line A14 are objectively separated by means of a constrained least-squares regression analysis of the mixing of eastern South Atlantic water masses. Contrary to previous approaches, essentially devoted to the intricate South Atlantic circulation, this work is focused on the effects of circulation on nutrients and carbon biogeochemistry, with special emphasis on the stoichiometry and the rate of mineralization processes. Combination of nutrient and apparent CFC-age anomalies, derived from the mixing analysis, indicate faster mineralization rates in the equatorial (12 × 10-2 μmol P kg-1 yr-1) and subequatorial (5.3 × 10-2 μmol P kg-1 yr-1) than in the subtropical (4.3 × 10-2 μmol P kg-1 yr-1) regime at the South Atlantic Central Water (SACW) depth range. Lower rates are obtained in the Antarctic Intermediate Water (AAIW) domain (3.0 × 10-2 μmol P kg-1 yr-1). Significant variation with depth of O2/C/N/P anomalies indicates preferential mineralization of proteins in thermocline waters, as compared with the reference Redfield composition. Copyright 2004 by the American Geophysical Union.

The distribution of turbulent diapycnal mixing in the Nordic seas is mapped from observations of internal wave density and velocity fine structure. The uppermost 500-1500 m host two distinct mixing regimes. In the eastern basins, the diapycnal diffusivity (Kρ) straddles 10-5 m2 s-1, whereas in the weakly stratified Greenland and Boreas basins it is raised by an order of magnitude. Below ∼2000 m, low stratification is associated with intense turbulent mixing across the Nordic seas, with diffusivities in the range 3 × 10-4 - 10-2 m2 s-1. These mixing rates agree within uncertainties with three tracer-based diffusivity estimates in the region and are associated with turbulent dissipation rates (ε) that are at most moderately enhanced above typical open ocean values. A minimum in both ε and Kρ is commonly found at ∼1500 m, a depth level that is most efficiently sheltered from shallow and bottom energy sources for the mixing. Available evidence points to wind work on upper ocean inertial motions as a shallow source, with semidiurnal internal tides generated at different levels of the topography contributing to both shallow and deep turbulence. While the closure of the North Atlantic meridional overturning circulation in the Nordic seas appears to be primarily driven by air-sea interaction, turbulent mixing has the potential to play a critical role in shaping the stratification and ventilation of the region via a range of complex interactions with convection. Copyright 2004 by the American Geophysical Union.

The Greenland Sea is one of a few sites in the world ocean where convection to great depths occurs-a process that forms some of the densest waters in the ocean. But the role of deep convective eddies, which result from surface cooling and mixing across density surfaces followed by geostrophic adjustment, has not been fully taken into account in the description of the initiation and growth of convection. Here we present tracer, float and hydrographic observations of long-lived ( approximately 1 year) and compact ( approximately 5 km core diameter) vortices that reach down to depths of 2 km. The eddies form in winter, near the rim of the Greenland Sea central gyre, and rotate clockwise with periods of a few days. The cores of the observed eddies are constituted from a mixture of modified Atlantic water that is warm and salty with polar water that is cold and fresh. We infer that these submesoscale coherent eddies contribute substantially to the input of Atlantic and polar waters to depths greater than 500 m in the central Greenland Sea.

A quasi-meridional hydrographic section located offshore from South America from 50°S to 10°N, and three shorter transverse lines to the continental slope, are used for a descriptive study of the water masses along the western boundary of the South and Equatorial Atlantic. At the upper and intermediate levels, the tracer analysis provides geographical limits of the wind-driven circulation regimes, and a comparison of the tracer values at the continental slope and along the meridional section shows where the boundary currents originate. At depths shallower than about 200 m, the subdivision of the subtropical gyre into two cells separated by the Subtropical Countercurrent near 28°S, that was pointed out in a previous study, is corroborated. South of this front, a warm variety (18°C) of Subtropical Mode Water in the inner recirculation of the Brazil Current appears, despite its limited extent, as a southern counterpart of the North Atlantic 18°C water. At the deep levels, the Upper Circumpolar Water and Upper North Atlantic Deep Water enter the South Atlantic in a significantly overlapping density range. The ensuing lateral encounter of both water masses occurs at 26°S near the western boundary, where most of the boundary flow of the latter water is stopped and deflected seaward by the base of the subtropical gyre. Other tracer anomalies signal significant eastward escapes of North Atlantic Deep Water: within two jets at about two degrees of latitude on either side of the equator, in another narrow current at 10°S, and at 34°S. The latter latitude marks the confluence, and eastward deflection, of the opposite boundary currents of Lower North Atlantic Deep Water and Lower Circumpolar Water. Near the bottom of the Argentine Basin, the Weddell Sea Deep Water that flows westward north of the Zapiola Ridge is more recently ventilated than the water carried by the boundary current near the Falkland Escarpment. While a part of it flows anticyclonically around the ridge, another part turns equatorward and enhances the southern property signatures of the water farther north. © 2000 Elsevier Science Ltd.

Convective vertical mixing in restricted areas of the subpolar oceans, such as the Greenland Sea, is thought to be the process responsible for forming much of the dense water of the ocean interior. Deep-water formation varies substantially on annual and decadal timescales, and responds to regional climate signals such as the North Atlantic Oscillation; its variations may therefore give early warning of changes in the thermohaline circulation that may accompany climate change. Here we report direct measurements of vertical mixing, by convection and by turbulence, from a sulphur hexafluoride tracer-release experiment in the central Greenland Sea gyre. In summer, we found rapid turbulent vertical mixing of about 1.1 cm2s-1. In the following late winter, part of the water column was mixed more vigorously by convection, indicated by the rising and vertical redistribution of the tracer patch in the centre of the gyre. At the same time, mixing outside the gyre centre was only slightly greater than in summer. The results suggest that about 10% of the water in the gyre centre was vertically transported in convective plumes, which reached from the surface to, at their deepest, 1,200-1,400 m. Convection was limited to a very restricted area, however, and smaller volumes of water were transported to depth than previously estimated. Our results imply that it may be the rapid year-round turbulent mixing, rather than convection, that dominates vertical mixing in the region as a whole.

Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatorial Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deeper maximum around 3800 m at level of the Lower North Atlantic Deep Water (LNADW). These Observations suggest a bifurcation at the western boundary, some of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core (σ1.5 = 34.70 kgm-3) was observed eastward as far as 5°W. The deep CFM maximum (σ4 = 45.87 kgm-3), associated with an oxygen maximum, decreased dramatically at the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water 'ages' computed from the F-11/F-12 ratio are estimated. Near the bottom, no enrichment in CFMs is detected at the entrance of the fracture zones in the cold water mass originating from the Antarctic Bottom Water flow.

Chlorofluoromethanes were sampled along two zonal sections, at 4°30 S and 7°30 N between the African and American continents (A7 and A6 WOCE sections) and two meridional sections, at 35°W and 3°50W, during the CITHER 1 cruise (part of the French program CITHER (Circulation THERmohaline) during January-March 1993. The results reported here deal primarily with the North Atlantic Deep Water, just ten years after the first CFM snapshot of the tropical Atlantic ocean obtained during the Transient Tracers in the Ocean Program (TTO) (Weiss et al. 1985. Nature 314, 608-610). The data provide evidence for the eastward bifurcation of the deep flow near the equator, on both UNADW and LNADW levels. The distributions clearly show the CFM signal corresponding to the UNADW penetrating into the eastern basin: at 3°50W the CFM core extends from 4°S to 3°N with a maximum around 2°S. On both UNADW and LNADW levels, the bifurcation does not occur exactly on the equator but a few degrees south and seems to be partly induced by topographic effects. Previously published circulation schemes for UNADW and LNADW levels are compared to the CITHER 1 CFM data. Great variability is revealed and new patterns from the data are highlighted. The 'young' deep component of the AABW flow seems to be stopped by the topography just north of the equatorial channel. TTO and CITHER 1 data set comparison and 'apparent' ages lead to minimal values of the propagation rate of the CFC signal at low latitudes.

This paper describes the design and operation of a sampling system that makes it possible to collect and store seawater samples in a simple copper tube, for post-cruise CFC measurements. The sampler was designed to attach to a standard hydrographic bottle. Preliminary testing was accomplished by analysing CFCs in the Tropical Atlantic at 15°N, from the surface to 2200 m depth. The data indicate that this technique can produce reliable CFC measurements, even after several months storage. © 1994.

Focuses on conservative tracers (specifically 3H, 3He, CFCs, 39Ar, 85Kr) with the objective of reviewing their specific contribution to our understanding of the ocean circulation and of discussing their possible future. -from Authors