Deep Western Boundary Current Research Articles

Slowdown of Circumpolar Deepwater flow during the Late Neogene: Evidence from a mudwave field at the Argentine continental slope

Geochemical evidence from boreholes suggests enhanced transport of Northern Component Water (NCW) to southern latitudes from about 6 Ma onwards. However, information on how this change in transport influenced the intensity and position of current systems is sparse. Here we use seismic reflection profiles interpreted together with bathymetric data to investigate current derived deposits at the central Argentine Margin. Upslope migrating mudwaves overlying a late Miocene erosional unconformity provide evidence that Circumpolar Deepwater (CDW) flow slowed down with the onset of NCW inflow. During the last ~3 Ma changes in dimensions and migration rates of the waves are small indicating continuous bottom current flow conditions similar to today with only minor variations in flow speed, suggesting that the Deep Western Boundary Current (DWBC) in the western south Atlantic as observed today, has been a pervasive feature of the global thermohaline circulation system during the Plio-/Pleistocene.

Age through tandem correlation of Quaternary relative paleointensity (RPI) and oxygen isotope data at IODP Site U1306 (Eirik Drift, SW Greenland)

Planktic oxygen isotope (δ18O) and relative paleointensity (RPI) data are used in tandem to generate an age model for the last 1 Myr from Integrated Ocean Drilling Program (IODP) Site U1306 drilled on the crest of the Eirik Drift (SW Greenland) in 2272 m water depth. For the 1–1.5 Ma interval, the age model is based on RPI alone due to insufficient foraminifera for isotope analyses. Utilizing RPI and δ18O in tandem allows recognition of low-δ18O “events” prior to glacial Terminations I, III, IV, V, VII, VIII, IX and X, that are independently supported by radiocarbon dates through the last deglaciation, and are attributed to local or regional surface-water effects. At Site U1306, Quaternary sedimentation rates (mean ∼15 cm/ka) are elevated during peak glacials and glacial onsets, and are reduced during interglacials, in contrast to the pattern at Site U1305 in 3460 m water depth at the distal toe of the drift, 191 km SW of Site U1306. The contrasting sedimentation-rate pattern appears to hold for the entire ∼1.5 Myr record. The slackening and/or shoaling (due to lowered salinity) of the Deep Western Boundary Current (DWBC) during glacial intervals coincided with greater sediment supply to Site U1306 whereas the deepening, and possibly increased vigor, of the DWBC during interglacial intervals boosted sediment supply to Site U1305.

Variability in the Internal Wave Field Induced by the Atlantic Deep Western Boundary Current at 16°N

Abstract Five years of continuous mooring data combined with conductivity–temperature–depth (CTD)/lowered acoustic Doppler current profiler (LADCP) measurements from five cruises are used to investigate the influence of the deep western boundary current (DWBC) on the internal wave field and associated vertical mixing at the continental slope at 16°N in the western Atlantic. The mooring data include 2-hourly rotor current-meter measurements and temperature/conductivity time series with a high temporal resolution of 5–20 min. Thus, the data resolve time scales ranging from the low-frequency variability of the large-scale DWBC that generates internal waves due to interactions with the topography to frequencies greater than that of internal waves that are associated with vertical mixing. Estimates of the vertical mixing induced by the breaking of the observed internal waves show elevated diapycnal diffusivities of up to 10−3 ± 0.4 × 10−3 m2 s−1 in the bottommost 1500 m during times of a strong DWBC (maximum velocities at the mooring site up to 50 cm s−1) whereas vertical mixing rates are about an order of magnitude lower (1.6 × 10−4 ± 0.6 × 10−4 m2 s−1) during weak flow. During periods of a strong DWBC, spectra of horizontal velocity and internal wave available potential energy change substantially at depths below 1200 m and show a strong increase in variance particularly in the near-inertial frequency band. Low-frequency, near-inertial waves generated by topography/DWBC interaction on the slope to the west of the moorings can potentially cause this observed wave intensification; ray paths estimated for these waves agree well with the observed spectral changes at different depths. Variability in the high-frequency range, considered as a proxy for turbulent mixing, is significantly correlated with the DWBC strength above the continental slope.

Deep Ocean Changes near the Western Boundary of the South Pacific Ocean

Abstract Repeated occupations of two hydrographic sections in the southwest Pacific basin from the 1990s to 2000s track property changes of Antarctic Bottom Water (AABW). The largest property changes—warming, freshening, increase in total carbon, and decrease in oxygen—are found near the basin’s deep western boundary between 50° and 20°S. The magnitude of the property changes decreases with increasing distance from the western boundary. At the deep western boundary, analysis of the relative importance of AABW (γn > 28.1 kg m−3) freshening, heating, or isopycnal heave suggests that the deep ocean stratification change is the result of both warming and freshening processes. The consistent deep ocean changes near the western boundary of the southwest Pacific basin dispel the notion that the deep ocean is quiescent. High-latitude climate variability is being directly transmitted into the deep southwest Pacific basin and the global deep ocean through dynamic deep western boundary currents.

Reconciling tracer and float observations of the export pathways of Labrador Sea Water

For more than fifty years, it has been generally accepted by oceanographers that the Deep Western Boundary Current (DWBC) is the principal conduit of recently‐convected Labrador Sea Water (LSW) exported from the high‐latitude North Atlantic to the equator. Supporting this supposition is observational evidence that the waters of the DWBC have consistently greater equatorward velocities, higher concentrations of passive tracers, and younger ages compared to ocean interior waters. However, recent observations and simulations of floats launched in the DWBC in the Labrador Sea show that most water parcels are quickly ejected from the DWBC and follow instead interior pathways to the subtropics. Here, we show that tracer observations from the last three decades are compatible with the existence of both DWBC and basin‐interior export pathways. From analyses of observational data and model output, we find that equatorward transport in the basin interior is consistent with the large‐scale vorticity balance at mid‐depth. Furthermore, from the modeling analysis we show that despite higher, localized concentrations of tracer and particles in the DWBC, only 5% of particles released in the Labrador Sea are transported from the subpolar to subtropical gyre via a continuous DWBC pathway. Thus, the interior pathway is a significant contributor to LSW export.

On the Cascading of Dense Shelf Waters in the Irminger Sea

Abstract Hydrographic data collected in the Irminger Sea in the 1990s–2000s indicate that dense shelf waters carried by the East Greenland Current south of the Denmark Strait intermittently descend (cascade) down the continental slope and merge with the deep waters originating from the Nordic Seas overflows. Repeat measurements on the East Greenland shelf at ~200 km south of the Denmark Strait (65°–66°N) reveal that East Greenland shelf waters in the Irminger Sea are occasionally as dense (σ0 > 27.80) as the overflow-derived deep waters carried by the Deep Western Boundary Current (DWBC). Clear hydrographic traces of upstream cascading of dense shelf waters are found over the continental slope at 64.3°N, where the densest plumes (σ0 > 27.80) originating from the shelf are identified as distinct low-salinity anomalies in the DWBC. Downstream observations suggest that dense fresh waters descending from the shelf in the northern Irminger Sea can be distinguished in the DWBC up to the latitude of Cape Farewell (~60°N) and that these waters make a significant contribution to the DWBC transport.

Variability in the Deep Western Boundary Current: Local versus remote forcing

Horizontal velocity, temperature and salinity measurements from the Line W array for the period 2004–2008 show large changes in the water mass structure and circulation of the Deep Western Boundary Current (DWBC). Fluctuations in the flow with periods from 10 to 60 days are bottom intensified: signals most likely associated with topographic Rossby waves (TRW). A fraction (∼15%) of the DWBC transport variability is caused by Gulf Stream rings and meanders. These flow anomalies are surface intensified and fluctuate at frequencies lower than the TRW. Interannual variability in the velocity field appears to be related to changes in the hydrographic properties. The dominant mode of variability is characterized by an overall freshening, cooling, a potential vorticity (PV) increase in the deep Labrador Sea Water (dLSW) and a PV decrease in the Overflow Water (OW). The variability in the flow associated with these property changes is not spatially homogeneous. Offshore (water depths larger than 3500 m) changes in the velocity are in phase with PV changes in the OW: a decrease in the OW PV is accompanied by an increase in the southward (negative) transport. Conversely, variations of the inshore flow are in phase with changes in the dLSW PV (increasing PV and decreasing transport). This trend, true for most of the record, reverses after the winter of 2007–2008. A sudden decrease of the dLSW PV is observed, with a corresponding intensification of the flow in the inner DWBC as well as a northward shift in the Gulf Stream axis.

Deep Western Boundary Current transport variability in the South Atlantic: preliminary results from a pilot array at 34.5° S

Abstract. The first direct estimates of the temporal variability of the absolute transport in the Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic Ocean are obtained using just under one year of data from a line of four pressure-equipped inverted echo sounders. Hydrographic sections collected in 2009 and 2010 confirm, based on neutral density, temperature, salinity, and oxygen values, the presence of the DWBC, one of the main deep pathways of the Meridional Overturning Circulation. Both data sets indicate that the DWBC reconstitutes itself after breaking into eddies in the western sub-tropical Atlantic near 8° S. The amplitude and spectral character of the DWBC transport variability are comparable with those observed in the North Atlantic, where longer records exist, with the DWBC at 34.5° S exhibiting a transport standard deviation of 25 Sv and variations of ∼ 40 Sv occurring within periods as short as a few days. There is little indication of an annual cycle in the DWBC transports, although the observational records are too short to be definitive. A Monte Carlo-style analysis using 27 yr of model output from the same location as the observations indicates that about 48–60 months of data will be required to fully assess the deep transport variability. The model suggests the presence of an annual cycle in DWBC transport, however its statistical significance with even 27 yr of model output is low, suggesting that seasonal variations in the model are weak.

Direct measurements of deep current at 162°E south of the equator in the Melanesian Basin: a trial to detect a cross-equatorial deep western boundary current

We conducted 1-year-long mooring observations four times below 2000 m, slightly south of the equator (2°39′ to 4°35′S) at 162°E in the Melanesian Basin in order to detect the southward deep western boundary return current crossing the equator. Contrary to our initial expectation of the deep flow scheme in the equatorial western boundary region, the observed results indicated a fairly complicated flow configuration. We analyzed the results with the help of a high-resolution model simulation. The ensemble average of the horizontal flow at each level near the deep western boundary indicates a significant westward flow at 2000 and 2250 m, with an insignificant southward component at 2500 and 2750 m. The annual mean meridional transports are very small (>1 Sv) and insignificant, with an ensemble-averaged value of 0.3 Sv (southward) ±0.4 Sv at most. Combining this with high-resolution model results, it is deduced that the southward transport of the deep western boundary current (DWBC) leaving the equator may be smaller than those obtained by low-resolution models, because of trapping of its fairly large fraction in the equatorial zone. Annual-scale flow patterns are classified into several categories, mainly based on the meridional-flow dominating or the zonal-flow dominating pattern. A case of the meridional-flow dominating patterns may possibly capture an annual-scale variability of DWBC, because its meridional transport variation, though somewhat weak, is consistent with that simulated. The zonal-flow dominating regime includes two types: long-lasting, almost steady westward flows and long-term zonal flow oscillations. The former seems to comprise well-known zonally elongated and meridionally narrow structures of the zonal flow beneath the thermocline in the equatorial region. The ensemble-averaged flow mentioned above is dominated by this type at the upper two levels 2000 and 2250 m, with total westward transport of 1.6 ± 0.7 Sv. The latter type seems to be a manifestation of the vertically propagating equatorial annual Rossby waves.

Structure and evolution of the abyssal jet in the Vema Channel of the South Atlantic

The Vema Channel represents the only major conduit through which the deepest and coldest (<0.2°C potential temperature) Antarctic Bottom Water (AABW) flows from the Argentine into the Brazil Basin. From 2003 to 2007 two current meter moorings were present on each side of the Vema Sill, close to the narrowest spot of the Vema Channel. The data from the moorings are compared with earlier current and temperature observations. On average the maximum current core lies ∼100m above the bottom of the sill with a mean northward speed of 0.3ms−1. Farther up in the water column where Lower Circumpolar Deep Water and North Atlantic Deep Water prevail, one finds a level of sluggish currents with a southward tendency in the sub-centimeter-per-second range. The lower boundary of a layer of ‘no’ motion was observed at ∼3700m depth where the mean potential temperature amounts to 1.5°C.The evolution of the abyssal warming phenomenon over the last decades with notable fluctuations at the choke point between the Argentine and the Brazil Basin differs from the more stable attitude of deep horizontal currents. Starting with CTD observations in 1972 we find a steady increase of temperatures of the coldest AABW in the Vema Channel. This general trend of rising abyssal potential temperatures of almost 2mKelvin per year is based on mostly annual CTD observations. The overall warming trend is fully compatible with our three-year moored temperature series in agreement with earlier records with high temporal resolution.Distinct frequently fluctuating horizontal current shear between the western and eastern sides of the Vema Sill may be explained by two different catchment areas for AABW at the mouth of the Vema Channel. One pathway originates at the American continental rise and advects bottom water in form of the deep western boundary current. A second pathway is supplied by an eastern boundary current along the Mid Atlantic Ridge in the Argentine Basin. Both source waters merge at the channel entrance, mix, and their respective strengths can alternate within the sill area.

Variability of the Deep Western Boundary Current at 26.5°N during 2004–2009

Five years of data from a line of dynamic height moorings (DHM), bottom-pressure recorders (BPR), and pressure-equipped inverted echo sounders (PIES) near the Atlantic Ocean western boundary at 26.5°N are used to evaluate the structure and variability of the Deep Western Boundary Current (DWBC) during 2004–2009. Comparisons made between transports estimated from the DHM+BPR and those made by the PIES demonstrate that the two systems are collecting equivalent volume transport information (correlation coefficient r=0.96, root-mean-square difference=6Sv; 1Sv=106m3s−1). Integrated to ∼450km off from the continental shelf and between 800 and 4800dbar, the DWBC has a mean transport of approximately 32Sv and a standard deviation during these five years of 16Sv. Both the barotropic (full-depth vertical mean) and baroclinic flows have significant variability (changes exceeding 10Sv) on time scales ranging from a few days to months, with the barotropic variations being larger and more energetic at all time scales. The annual cycle of the deep transport is highly dependent on the horizontal integration distance; integrating ∼100km offshore yields an annual cycle of roughly similar magnitude but shifted in phase relative to that found from current meter arrays in the 1980–1990s, while the annual cycle becomes quite weak when integrating ∼450km offshore. Variations in the DWBC transport far exceed those of the total basin-wide Meridional Overturning Circulation (standard deviations of 16Sv vs. 5Sv). Transport integrated in the deep layer out to the west side of the Mid-Atlantic Ridge still demonstrates a surprisingly high variance, indicating that some compensation of the western basin deep variability must occur in the eastern basin.

The Key Role of the Western Boundary in Linking the AMOC Strength to the North–South Pressure Gradient

Abstract A key idea in the study of the Atlantic meridional overturning circulation (AMOC) is that its strength is proportional to the meridional density gradient or, more precisely, to the strength of the meridional pressure gradient. A physical basis that would indicate how to estimate the relevant meridional pressure gradient locally from the density distribution in numerical ocean models to test such an idea has been lacking however. Recently, studies of ocean energetics have suggested that the AMOC is driven by the release of available potential energy (APE) into kinetic energy (KE) and that such a conversion takes place primarily in the deep western boundary currents. In this paper, the authors develop an analytical description linking the western boundary current circulation below the interface separating the North Atlantic Deep Water (NADW) and Antarctic Intermediate Water (AAIW) to the shape of this interface. The simple analytical model also shows how available potential energy is converted into kinetic energy at each location and that the strength of the transport within the western boundary current is proportional to the local meridional pressure gradient at low latitudes. The present results suggest, therefore, that the conversion rate of potential energy may provide the necessary physical basis for linking the strength of the AMOC to the meridional pressure gradient and that this could be achieved by a detailed study of the APE to KE conversion in the western boundary current.

Sensitivity of Atlantic Meridional Overturning Circulation Variability to Parameterized Nordic Sea Overflows in CCSM4

Abstract The inclusion of parameterized Nordic Sea overflows in the ocean component of the Community Climate System Model version 4 (CCSM4) results in a much improved representation of the North Atlantic tracer and velocity distributions compared to a control CCSM4 simulation without this parameterization. As a consequence, the variability of the Atlantic meridional overturning circulation (AMOC) on decadal and longer time scales is generally lower, but the reduction is not uniform in latitude, depth, or frequency–space. While there is dramatically less variance in the overall AMOC maximum (at about 35°N), the reduction in AMOC variance at higher latitudes is more modest. Also, it is somewhat enhanced in the deep ocean and at low latitudes (south of about 30°N). The complexity of overturning response to overflows is related to the fact that, in both simulations, the AMOC spectrum varies substantially with latitude and depth, reflecting a variety of driving mechanisms that are impacted in different ways by the overflows. The usefulness of reducing AMOC to a single index is thus called into question. This study identifies two main improvements in the ocean mean state associated with the overflow parameterization that tend to damp AMOC variability: enhanced stratification in the Labrador Sea due to the injection of dense overflow waters and a deepening of the deep western boundary current. Direct driving of deep AMOC variance by overflow transport variations is found to be a second-order effect.

Mean Atlantic meridional overturning circulation across 26.5°N from eddy‐resolving simulations compared to observations

Observations along 26.5°N are used to examine the time mean structure of the Atlantic meridional overturning circulation (AMOC) in eddy‐resolving simulations with the Hybrid Coordinate Ocean Model (HYCOM). The model results yield a 5 year mean AMOC transport of 18.2 Sv, compared to 18.4 Sv based on data. The modeled northward limb of the AMOC has a vertical structure similar to observations. The southward limb is shallower than observed but deeper than other ocean general circulation models and includes a secondary transport maximum near 4000 m corresponding to Nordic Seas Overflow Water. The modeled flow through the Florida Strait and the deep western boundary current (DWBC) east of Abaco, Bahamas, are also approximately consistent with observations. The model results are used to clarify the sources of the northward AMOC transport and to explore the circulation pattern of the southward transport in the western subtropical North Atlantic in the range 18–33°N. About 14.1 Sv of the modeled northward AMOC transport is through the Florida Strait and the remainder through the mid‐ocean, primarily in the Ekman layer, but also below 600 m. The modeled AMOC transport is about 2/3 surface water and 1/3 Antarctic Intermediate Water with no contribution from the thermocline water in between. In the western subtropical North Atlantic the model results depict a complicated deep circulation pattern, associated with the complex bathymetry. The DWBC flows southward then eastward in both the upper and lower North Atlantic Deep Water (NADW) layers but with different offshore recirculation pathways, and there exists a second, more northern branch of eastward flow in the lower NADW layer.

The East Greenland boundary current system south of Denmark Strait

Four repeat sections across the East Greenland shelf and slope south of Denmark Strait are analysed to investigate the components of the boundary current system. The sections were occupied in summer 2001, 2003, 2004 and 2007, and included use of a vessel-mounted acoustic Doppler current profiler, enabling the computation of absolute geostrophic velocities. The components of the boundary current are the East Greenland/Irminger Current (EGIC) in the upper layer, the Deep Western Boundary Current (DWBC) at the base of the continental slope, and the East Greenland spill jet which resides inshore and beneath the EGIC. Special emphasis is placed on the spill jet, a recently discovered feature about which relatively little is known. The spill jet was observed in each occupation, transporting 5.0±2.2Sv equatorward in the mean, which is similar to the DWBC at this latitude (4.9±1.4Sv). The spill jet displayed considerable variability between sections, which appears to be linked to the geographical location of the upper-layer hydrographic front associated with the EGIC. When the front is located near the shelfbreak, the spill jet is confined to the outer shelf/upper slope and its transport is smaller. During these times there is less mixing and the water advected by the jet is generally lighter than that transported by the DWBC. In contrast, when the front is located seaward of the shelfbreak, the spill jet extends farther down the continental slope and its volume flux is larger. At these times, there is stronger mixing and the spill jet can transport water as dense as the Denmark Strait Overflow Water. A vorticity analysis indicates that the jet is susceptible to a variety of instability processes including baroclinic, barotropic and symmetric instability. In addition, it is subject to double diffusive mixing that may influence its downstream evolution. It appears that the spill jet is a permanent feature of the summertime circulation in this region and contributes significantly to the intermediate, and at times deep, limb of the Atlantic Meridional Overturning Circulation.

The Role of Bottom Pressure Torques on the Interior Pathways of North Atlantic Deep Water

Abstract Four versions of the same global climate model, one with horizontal resolution of 1.8° × 3.6° and three with 0.2° × 0.4°, are employed to evaluate the role of ocean bottom topography and viscosity on the spatial structure of the deep circulation. This study is motivated by several recent observational studies that find that subsurface floats injected near the western boundary of the Labrador Sea most often do not continuously follow the deep western boundary current (DWBC), in contrast to the traditional view that the deep water formed in the North Atlantic predominantly follows the DWBC. It is found that, with imposed large viscosity values, the model reproduces the traditional view. However, as viscosity is reduced and the model bathymetry resolution increased, much of the North Atlantic Deep Water (NADW) separates from the western boundary and enters the low-latitude Atlantic via interior pathways distinct from the DWBC. It is shown that bottom pressure torques play an important role in maintaining these interior NADW outflows.

Ocean circulations driven by meridional density gradients

State-of-the-art ocean models spinning up from realistic density fields rapidly develop deep western boundary currents and meridional overturning circulations (MOCs). Wajsowicz and Gill [Wajsowicz, R. and Gill, A.E., Adjustment of the ocean under buoyancy forces, Part I: the role of Kelvin waves. J. Phys. Oceanogr. 1986, 16, 2097–2114] found that the initial spin-up of a flat-bottomed ocean model from a meridionally varying density field is well described by the shallow water equations for a two-layer fluid and that the initial evolution on a β-plane could be understood in terms of f-plane dynamics: Kelvin waves propagate rapidly round the ocean boundaries establishing eastern and western boundary currents. The time-mean baroclinic motion of a two-layer fluid in a closed basin on an f-plane which spins up from an initial state of rest with a meridionally sloping interface is derived here and compared with Gill's [Gill, A.E., Adjustment under gravity in a rotating channel, J. Fluid Mech. 1976, 77, 603–621] steady-state solution for an open channel. These solutions are used to illustrate the constraints imposed by the no normal flow inviscid boundary conditions which also apply to solutions on a sphere or β-plane. Miles’ [Miles, J.W., Kelvin waves on oceanic boundaries. J. Fluid Mech. 1972, 55, 113–127] solution for Kelvin waves on a sphere is used to analyse the initial spin-up on a β-plane. Motivated by the slow speed of the geostrophic adjustment by the planetary waves at mid- to high-latitudes and the influence of the inviscid boundary conditions, simple, analytical steady-state solutions driven by relaxation of the internal interface towards a meridionally varying reference field and closed by dissipative boundary layers are derived using the planetary geostrophic equations for the baroclinic motion in a two-layer fluid. The solutions can be applied to basins which span the equator and derived using the full nonlinear continuity equation for any shape of basin. The depth of the internal interface is constant along the eastern boundaries and the equator but its east–west variations can be a large fraction of the pole to equator difference at high latitudes. The solutions support significant MOCs and, when periodic east–west boundary conditions are imposed at the southern boundary, can be shown to have a significant cross-equatorial baroclinic flow in the western boundary layer with greater southward flow in the lower layer than the surface layer.

Coupled deep-water flow and climate variability in the middle Pleistocene North Atlantic

Abrupt changes in the strength of the Atlantic Meridional Overturning Circulation (comprising northward flow of warm water and a cold southward return flow) are implicated in abrupt climate changes in the late Pleistocene. A sensitive place to assess this circulation is in the cold return flow of Deep Western Boundary Currents. Here, in records of flow speed and isotopic composition of surface and bottom waters from a Deep Western Boundary Current location near the northern source of North Atlantic Deep Water, we show both orbital and millennial-scale coupling between deep ocean flow and climate in the middle Pleistocene (0.75–0.87 Ma), when the boundary conditions in terms of the mean state and amplitude of climate change were different from more recent periods. The coupling appears as a phased series of events initiated by reduced vertical density gradients and initial ventilation of deep waters. The occurrence of these events in interglacials during the middle Pleistocene suggests that the millennial-scale climate variability in the North Atlantic was more pronounced at that time than previously thought. This demonstrates that, given the right boundary conditions, rapid climate shifts can also occur during relatively warm climate conditions.

Deciphering the pathways for the deep limb of the Meridional Overturning Circulation

For the past 50 years it has been assumed that the principal pathway for the deep limb of the Atlantic Meridional Overturning Circulation (AMOC) is the Deep Western Boundary Current (DWBC). However, recent observations of Lagrangian floats have shown that the DWBC is not necessarily a unique, dominant, or continuous pathway for these deep waters. A significant portion of the deep water export from the subpolar to the subtropical gyres follows a pathway through the interior of the Newfoundland and subtropical basins, which is constrained by the western boundary and the western flank of the Mid-Atlantic Ridge. The hypothesis that deep eddy-driven recirculation gyres are a mechanism for partitioning the deep limb of the AMOC into the DWBC and this interior pathway is investigated here. Eulerian and Lagrangian analyses of the output of ocean general circulation models at eddy-resolving, eddy-permitting, and non-eddy permitting resolutions are used to test this hypothesis. Eddy-driven recirculation gyres, simulated in the eddy-resolving and eddy-permitting models and similar to recirculations inferred from hydrographic data, are shown to shape the export pathways of deep water from the subpolar to the subtropical gyres.

Export of Labrador Sea Water from the subpolar North Atlantic: A Lagrangian perspective

Results from 59 acoustically tracked RAFOS floats obtained during 2003–2008 are analyzed here to study the spreading pathways of Labrador Sea Water (LSW) from the subpolar to subtropical North Atlantic. An earlier paper based on a subset of these float data presented evidence of a potentially important interior pathway for the southward spreading of LSW. Here those results are reinforced with the full data set and new information on LSW pathways around the Grand Banks is presented. About 70% of the RAFOS floats were expelled into the ocean interior east of the Grand Banks and meandered slowly eastward toward the mid-Atlantic ridge, a pathway observed previously with profiling floats. Less than 10% was advected continuously around the Grand Banks by the Deep Western Boundary Current (DWBC). A larger fraction (∼17%) drifted into the subtropical interior from the Tail of the Grand Banks, suggesting that this pathway is at least if not more important than the DWBC pathway for the export of LSW to the subtropics. RAFOS floats released closest to the continental slope at 700 m depth were more likely to rapidly reach subtropical latitudes, mainly because they drifted through Flemish Pass, a 1100 m deep channel between the Grand Banks and the Flemish Cap, which protects fluid parcels from being swept off the continental slope by meanders of the North Atlantic Current. A statistical comparison of the RAFOS trajectories with more than 5000 simulated floats obtained from the Family of Linked Atlantic Models Experiment (FLAME) high-resolution ocean general circulation model reveals a similar pattern of LSW spreading. The RAFOS and simulated floats are also used to show that contrary to a previous modeling study, the isobaric RAFOS floats do not underestimate the amount of LSW that continuously follows the DWBC around the Grand Banks compared to isopycnal floats. These results have implications for the connectivity of the deep limb of the Atlantic Meridional Overturning Circulation between the subpolar and the subtropical North Atlantic.