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Michael A Spall - One of the best experts on this subject based on the ideXlab platform.
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mid depth ventilation in the Western Boundary Current system of the sub polar gyre
Deep Sea Research Part I: Oceanographic Research Papers, 1997Co-Authors: Robert S. Pickart, Michael A Spall, John R N LazierAbstract:Two processes are investigated that result in the rapid (order of months) export of newly-ventilated water from the sub-polar north Atlantic. Both mechanisms involve mid-depth water mass formation within the Western Boundary Current system, which leads to such rapid spreading. The first mechanism, which apparently occurs every winter, forms upper Labrador Sea water (LSW), which is a source of the high CFC layer of the upper deep Western Boundary Current (DWBC). A mixed-layer model shows that this water mass can be formed by convection in the main branch of the Labrador Current. Strong heat loss near the Boundary together with the existing potential vorticity structure of the Current enables overturning to 1000 m. A regional numerical model of the circulation near Flemish Cap reveals how eddies of upper LSW are then shed by the baroclinically unstable Labrador Current. The eddies become detached from the Boundary at the entrance to Flemish Cap and are entrained in to the offshore (barotropic) branch of the Labrador Current, which brings them seaward of Flemish Cap (where they have been previously observed). The second mechanism presented occurs only under extreme winter forcing, such as that experienced in the Labrador Sea in recent years. The enhanced heat loss forms classical LSW south of the cyclonic gyre, where the DWBC and North Atlantic Current can then quickly transport the water away from the Labrador Sea. It is shown that newly-ventilated lenses of classical LSW observed in the DWBC likely originate from this southern region, consistent with their sudden appearance downstream in the early 1990s.
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dynamics of the gulf stream deep Western Boundary Current crossover part i entrainment and recirculation
Journal of Physical Oceanography, 1996Co-Authors: Michael A SpallAbstract:Abstract A regional primitive equation model is applied to the study of the interaction between the Gulf Stream and the deep Western Boundary Current (DWBC) where they cross at Cape Hatteras. It is found that for a wide range of forcing parameters the upper core of the DWBC is split into two mean paths at the crossover, one flowing toward the south along the Western Boundary and the other flowing toward the cast under the Gulf Stream. The eastward branch is entrained Into the southern recirculation gyre and, after being diverted into the interior for up to 1500 km, eventually returns to the Western Boundary Current and continues to flow southward. This recirculation and mixing is shown to have a significant impact on the separation point and mean path of the Gulf Stream, the basin-scale stratification, and the properties of the DWBC south of the crossover. For most configurations, the lower DWBC remains largely on the Western Boundary and interacts only weakly with the interior. The entrainment of the upp...
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dynamics of the gulf stream deep Western Boundary Current crossover part ii low frequency internal oscillations
Journal of Physical Oceanography, 1996Co-Authors: Michael A SpallAbstract:Abstract A low-frequency oscillation in the Gulf Stream/deep Western Boundary Current (DWBC) system is identified and its influences on several important aspects of the basin-scale circulation are investigated. An eddy-resolving regional primitive equation model is used to demonstrate that feedbacks between the Gulf Stream, with its associated northern and southern recirculation gyres, and the upper core of the DWBC can lead to self-sustaining large amplitude internal oscillations of roughly decadal frequency. The oscillator cycle is described as follows: The upper core of the DWBC is entrained under the Gulf Stream through interaction with the eddy-driven northern and southern recirculation gyres, as described in Part I of this study. Once entrained, the low potential vorticity DWBC water stabilizes the Gulf Stream and suppresses the eddy fluxes that maintained the interior recirculation gyres. This causes the upper DWBC to switch to a southward path along the Western Boundary, thus removing the source o...
Silvia L Garzoli - One of the best experts on this subject based on the ideXlab platform.
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characteristics and causes of deep Western Boundary Current transport variability at 34 5 s during 2009 2014
Ocean Science, 2016Co-Authors: Christopher S Meinen, Edmo J D Campos, Silvia L Garzoli, Renellys C Perez, Alberto R Piola, Maria Paz ChidichimoAbstract:Abstract. The Deep Western Boundary Current (DWBC) at 34.5° S in the South Atlantic carries a significant fraction of the cold deep limb of the Meridional Overturning Circulation (MOC), and therefore its variability affects the meridional heat transport and consequently the regional and global climate. Nearly 6 years of observations from a line of pressure-equipped inverted echo sounders (PIESs) have yielded an unprecedented data set for studying the characteristics of the time-varying DWBC volume transport at 34.5° S. Furthermore, the horizontal resolution of the observing array was greatly improved in December 2012 with the addition of two Current-and-pressure-equipped inverted echo sounders (CPIESs) at the midpoints of the two Westernmost pairs of PIES moorings. Regular hydrographic sections along the PIES/CPIES line confirm the presence of recently ventilated North Atlantic Deep Water carried by the DWBC. The time-mean absolute geostrophic transport integrated within the DWBC layer, defined between 800–4800 dbar and within longitude bounds of 51.5 to 44.5° W, is −15 Sv (1 Sv = 106 m3 s−1; negative indicates southward flow). The observed peak-to-peak range in volume transport using these integration limits is from −89 to +50 Sv, and the temporal standard deviation is 23 Sv. Testing different vertical integration limits based on time-mean water-mass property levels yields small changes to these values, but no significant alteration to the character of the transport time series. The time-mean southward DWBC flow at this latitude is confined west of 49.5° W, with recirculations dominating the flow further offshore. As with other latitudes where the DWBC has been observed for multiple years, the time variability greatly exceeds the time mean, suggesting the presence of strong coherent vortices and/or Rossby Wave-like signals propagating to the Boundary from the interior.
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the fate of the deep Western Boundary Current in the south atlantic
Deep Sea Research Part I: Oceanographic Research Papers, 2015Co-Authors: Silvia L Garzoli, Shenfu Dong, Rana A Fine, Christopher S Meinen, Renellys C Perez, Claudia Schmid, Erik Van SebilleAbstract:Abstract The pathways of recently ventilated North Atlantic Deep Water (NADW) are part of the lower limb of the Atlantic Meridional Overturning Circulation (AMOC). In the South Atlantic these pathways have been the subject of discussion for years, mostly due to the lack of observations. Knowledge of the pathways of the AMOC in the South Atlantic is a first order prerequisite for understanding the fluxes of climatically important properties. In this paper, historical and new observations, including hydrographic and oxygen sections, Argo data, and chlorofluorocarbons (CFCs), are examined together with two different analyzes of the Ocean general circulation model For the Earth Simulator (OFES) to trace the pathway of the recently ventilated NADW through the South Atlantic. CLIVAR-era CFCs, oxygen and salinity clearly show that the strongest NADW pathway in the South Atlantic is along the Western Boundary (similar to the North Atlantic). In addition to the Western Boundary pathway, tracers show an eastward spreading of NADW between ~17 and 25°S. Analyzed together with the results of earlier studies, the observations and model output presented here indicate that after crossing the equator, the Deep Western Boundary Current (DWBC) transports water with the characteristics of NADW and a total volume transport of approximately 14 Sv (1 Sv=10 6 m 3 s − 1 ). It crosses 5°S as a narrow Western Boundary Current and becomes dominated by eddies further south. When this very energetic eddying flow reaches the Vitoria-Trindade Ridge (~20°S), the flow follows two different pathways. The main portion of the NADW flow continues along the continental shelf of South America in the form of a strong reformed DWBC, while a smaller portion, about 22% of the initial transport, flows towards the interior of the basin.
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variability of the deep Western Boundary Current at 26 5 n during 2004 2009
Deep-sea Research Part Ii-topical Studies in Oceanography, 2013Co-Authors: Christopher S Meinen, William E. Johns, Silvia L Garzoli, Erik Van Sebille, Torsten Kanzow, Darren Rayner, Molly O BaringerAbstract: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 26.51N 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 þ BPR and those made by the PIES demonstrate that the two systems are collecting equivalent volume transport information (correlation coefficient r¼ 0.96, root-mean-square difference ¼6 Sv; 1 Sv ¼10 6 m 3 s � 1 ). Integrated to � 450 km off from the continental shelf and between 800 and 4800 dbar, the DWBC has a mean transport of approximately 32 Sv and a standard deviation during these five years of 16 Sv. Both the barotropic (full-depth vertical mean) and baroclinic flows have significant variability (changes exceeding 10 Sv) 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 � 100 km 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 � 450 km offshore. Variations in the DWBC transport far exceed those of the total basin-wide Meridional Overturning Circulation (standard deviations of 16 Sv vs. 5 Sv). 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. Published by Elsevier Ltd.
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deep Western Boundary Current transport variability in the south atlantic preliminary results from a pilot array at 34 5 s
Ocean Science, 2012Co-Authors: Christopher S Meinen, Renellys C Perez, Alberto R Piola, Silvia L GarzoliAbstract: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.
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variability in deep Western Boundary Current transports preliminary results from 26 5 n in the atlantic
Geophysical Research Letters, 2006Co-Authors: Christopher S Meinen, Molly O Baringer, Silvia L GarzoliAbstract:[1] Transport fluctuations of the deep limb of the Meridional Overturning Circulation (MOC) near the Western Boundary are presented from a line of inverted echo sounders, bottom pressure sensors, and a deep Current meter east of Abaco Island, Bahamas, at 26.5°N from September 2004 through September 2005. The mean southward flow between 800 dbar and 4800 dbar was 39 × 106 m3 s−1, with a northward recirculation of 28 × 106 m3 s−1, leaving a net southward flow of 11 × 106 m3 s−1 as the through-flow of the Deep Western Boundary Current (DWBC). The mean southward DWBC flow essentially equals previous values that were measured at the same location by arrays of Current meters deployed from 1986 to 1992. DWBC transport spectra indicate that barotropic and baroclinic changes have very similar energy levels at most periods less than 10 days and that barotropic changes dominate at periods of 10–80 days.
John M. Toole - One of the best experts on this subject based on the ideXlab platform.
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moored observations of the deep Western Boundary Current in the nw atlantic 2004 2014
Journal of Geophysical Research, 2017Co-Authors: John M. Toole, Terrence M. Joyce, Isabela Le Bras, Magdalena Andres, Michael S MccartneyAbstract:A moored array spanning the continental slope southeast of Cape Cod sampled the equatorward-flowing Deep Western Boundary Current (DWBC) for a 10-year period: May 2004 - May 2014. Daily profiles of subinertial velocity, temperature, salinity and neutral density are constructed for each mooring site and cross-line DWBC transport time series are derived for specified water mass layers. Time-averaged transports based on daily estimates of the flow and density fields in stream coordinates are contrasted with those derived from the Eulerian-mean flow field, modes of DWBC transport variability are investigated through compositing, and comparisons are made to transport estimates for other latitudes. Integrating the daily velocity estimates over the neutral density range of 27.8 - 28.125 kg/m3 (encompassing Labrador Sea and Overflow Water layers), a mean equatorward DWBC transport of 22.8 x 106 m3/s ± 1.9 x 106 m3/s is obtained. Notably, a statistically-significant trend of decreasing equatorward transport is observed in several of the DWBC components as well as the Current as a whole. The largest linear change (a 4% decrease per year) is seen in the layer of Labrador Sea Water that was renewed by deep convection in the early 1990s whose transport fell from 9.0 x 106 m3/s at the beginning of the field program to 5.8 x 106 m3/s at its end. The corresponding linear fit to the combined Labrador Sea and Overflow Water DWBC transport decreases from 26.4 x 106 m3/s to 19.1 x 106 m3/s. In contrast, no long-term trend is observed in upper-ocean Slope Water transport. These trends are discussed in the context of decadal observations of the North Atlantic circulation, and subpolar air-sea interaction/water mass transformation.
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tracking labrador sea water property signals along the deep Western Boundary Current
Journal of Geophysical Research, 2017Co-Authors: Isabela Le Bras, Igor Yashayaev, John M. TooleAbstract:Observations of the Deep Western Boundary Current (DWBC) at Line W on the Western North Atlantic continental slope southeast of Cape Cod from 1995 to 2014 reveal water mass changes that are consistent with changes in source water properties upstream in the Labrador Sea. This is most evident in the cold, dense, and deep class of Labrador Sea Water (dLSW) that was created and progressively replenished and deepened by recurring winter convection during the severe winters of 1987-1994. The arrival of this record cold, fresh and low potential vorticity anomaly at Line W lags its formation in the Labrador Sea by 3-7 years. Complementary observations along the path of the DWBC provide further evidence that this anomaly is advected along the Boundary and indicate that stirring between the Boundary and the interior intensifies south of the Flemish Cap. Finally, the consistency of the data with realistic advective and mixing time scales is assessed using the Waugh and Hall [2005] model framework. The data are found to be best represented by a mean transit time of 5 years from the Labrador Sea to Line W, with a leading order role for both advection by the DWBC and mixing between the Boundary flow and interior waters.
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variability in the deep Western Boundary Current local versus remote forcing
Journal of Geophysical Research, 2012Co-Authors: Beatriz Penamolino, Terrence M. Joyce, John M. TooleAbstract:[1] 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.
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recent changes in the labrador sea water within the deep Western Boundary Current southeast of cape cod
Deep Sea Research Part I: Oceanographic Research Papers, 2011Co-Authors: Beatriz Penamolino, Terrence M. Joyce, John M. TooleAbstract:Abstract Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the Western Boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.
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transport of the north atlantic deep Western Boundary Current about 39 n 70 w 2004 2008
Deep-sea Research Part Ii-topical Studies in Oceanography, 2011Co-Authors: John M. Toole, Terrence M. Joyce, Michael S Mccartney, Ruth G Curry, Beatriz PenamolinoAbstract:Abstract Begun in spring 2004, a sustained measurement program – Line W – is returning high-resolution observations of the North Atlantic's Deep Western Boundary Current (DWBC) southeast of New England. The study focuses on the cold limb of the Atlantic Meridional Overturning Circulation near the Boundary between the subpolar and subtropical gyres. The field study consists of a 6-element, continental-slope-spanning moored array on a line underlying an altimeter satellite ground track, and periodic reoccupations of a full-depth hydrographic section along the line extending from the continental shelf towards Bermuda. Here, data from the first 4 years of the array (May 10, 2004–April 9, 2008) are analyzed along with 9 realizations of the section. The array, a mix of Moored Profiler and discrete, fixed-depth instrument moorings, returned temperature, salinity and horizontal velocity data with various temporal and depth resolutions. After averaging to filter inertial, tidal and other high-frequency motions, the combined moored data set was binned to the lowest common temporal resolution of 5-days (the nominal burst sample interval of the Moored Profilers) and interpolated to 2-dbar vertical resolution. Temperature, salinity, dissolved oxygen, tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises. The present work focuses on the 4-year-mean and time-varying meridional transport in 4 layers bounded by neutral density surfaces: Upper and Classical Labrador Sea Waters, Iceland-Scotland Overflow Waters and Denmark Strait Overflow Waters. The 5-d, 4-layer-summed meridional transport estimates range between −3.5 and −79.9 Sv with a record mean average transport of −25.1 Sv and standard deviation of 12.5 Sv. Bias adjustment to account for the finite width of the moored array increases the 4-layer mean transport estimate to −28.7 Sv. At time scales longer than about 1 month, the variations in equatorward DWBC transport appear correlated with meridional position of the Gulf Stream North Wall with stronger transport observed when the Stream is displaced south.
Robert S. Pickart - One of the best experts on this subject based on the ideXlab platform.
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On the Deep Western Boundary Current south of Cape Cod
Deep-sea Research Part Ii-topical Studies in Oceanography, 2005Co-Authors: Terrence M. Joyce, Robert S. Pickart, Jane Dunworth-baker, Daniel J. Torres, Stephanie WatermanAbstract:Abstract Using CTD/oxygen data from eight cruises in the decade from 1994–2003, we have constructed a mean ‘section’ of properties across the Deep Western Boundary Current (DWBC) south of Cape Cod near 70°W. Since all sections included direct velocity measurements, our composite section enables us to portray the flow field as well as the mean water mass structure. Inshore of the Gulf Stream between the 2500 and 4000 m isobaths, the flow is to the southwest along the bathymetry and is remarkably barotropic. The equatorward flowing Labrador Sea Water is shown to have high dissolved oxygen, low salinity, and low potential vorticity, while the underlying Overflow Water is high in oxygen. Transport estimates for the cold limb of the thermohaline circulation give a range of −14 to −19 Sv for the N. Atlantic Deep Water found on the section. The greatest uncertainty is due to the presence of a Warm-Core Ring on one of the sections, which apparently completely reversed the flow in the DWBC. Offshore of the DWBC, some of the deep source waters are returned to the north in the deep Gulf Stream. The section is compared to two other, widely separated locations (Abaco and 55°W) that have markedly different DWBC characteristics.
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temporal evolution of the deep Western Boundary Current where it enters the sub tropical domain
Deep Sea Research Part I: Oceanographic Research Papers, 1998Co-Authors: Robert S. Pickart, William M SmethieAbstract:Abstract Four repeat hydrographic sections across the Deep Western Boundary Current (DWBC) at 55°W, occupied between 1983–1995, are used to investigate the inter-annual variability of the deep flow. The sections include measurement of tracers (oxygen, CFCs) and absolute geostrophic velocity. All properties are interpolated onto a regular grid, both in depth space and density space. The analysis focuses on two of the water masses of the DWBC: the Denmark Strait overflow water (DSOW) and classical Labrador Sea water (CLSW), both of which are clearly revealed in the property sections. The mean volume flux of water denser than σ θ =27.8 kg/m 3 is 13.3(±4.2) Sv, comparable to that measured south of Greenland in the DWBC, suggesting that this is an accurate measure of the deep throughput into the sub-tropical North Atlantic. The largest property variability over the 12 yr period occurs in the CLSW, which in the 1990s became markedly colder, fresher, more weakly stratified, and higher in oxygen and CFCs – all indicative of new ventilation. By contrast, over this same period the deeper DSOW became less well-ventilated. Such opposite behavior of the two water masses is consistent with the large-scale atmospheric forcing in the North Atlantic. The deep absolute velocities were decomposed into a barotropic (CLSW) and baroclinic (DSOW) contribution. The DSOW flow intensified with the appearance of the new CLSW in 1991. An attempt is made to explain this, but it remains unclear to what extent the change could be due to local versus remote forcing.
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mid depth ventilation in the Western Boundary Current system of the sub polar gyre
Deep Sea Research Part I: Oceanographic Research Papers, 1997Co-Authors: Robert S. Pickart, Michael A Spall, John R N LazierAbstract:Two processes are investigated that result in the rapid (order of months) export of newly-ventilated water from the sub-polar north Atlantic. Both mechanisms involve mid-depth water mass formation within the Western Boundary Current system, which leads to such rapid spreading. The first mechanism, which apparently occurs every winter, forms upper Labrador Sea water (LSW), which is a source of the high CFC layer of the upper deep Western Boundary Current (DWBC). A mixed-layer model shows that this water mass can be formed by convection in the main branch of the Labrador Current. Strong heat loss near the Boundary together with the existing potential vorticity structure of the Current enables overturning to 1000 m. A regional numerical model of the circulation near Flemish Cap reveals how eddies of upper LSW are then shed by the baroclinically unstable Labrador Current. The eddies become detached from the Boundary at the entrance to Flemish Cap and are entrained in to the offshore (barotropic) branch of the Labrador Current, which brings them seaward of Flemish Cap (where they have been previously observed). The second mechanism presented occurs only under extreme winter forcing, such as that experienced in the Labrador Sea in recent years. The enhanced heat loss forms classical LSW south of the cyclonic gyre, where the DWBC and North Atlantic Current can then quickly transport the water away from the Labrador Sea. It is shown that newly-ventilated lenses of classical LSW observed in the DWBC likely originate from this southern region, consistent with their sudden appearance downstream in the early 1990s.
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how does the deep Western Boundary Current cross the gulf stream
Journal of Physical Oceanography, 1993Co-Authors: Robert S. Pickart, William M SmethieAbstract:Abstract The manner in which the deep Western Boundary Current (DWBC) crosses the Gulf Stream is investigated using data from a hydrographic survey conducted in 1990. Absolute geostrophic velocity vectors are computed using in situ float data to obtain the reference level. Three density layers are considered in detail: two mid-depth layers, which together make up the shallowest water mass component of the DWBC (500–1200 m), and a deep layer consisting of the Norwegian–Greenland overflow water (2500–3500 m). The shallowest layer does not make it through the crossover and is completely entrained by the Gulf Stream; however, the resulting drop in equatorward transport is almost completely replenished by offshore entrainment just south of the crossover. In the intermediate layer, which is denser than the Gulf Stream coming off the shelf, part of the DWBC recirculates to the northeast while the onshoremost portion continues equatorward. In the deep layer only a small amount of recirculation occurs. The lateral...
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Water mass components of the North Atlantic deep Western Boundary Current
Deep Sea Research Part A. Oceanographic Research Papers, 1992Co-Authors: Robert S. PickartAbstract:Abstract Four hydrographic sections across the North Atlantic deep Western Boundary Current from 55°W to 70°W are analysed to distinguish the Current's different water mass components. The deepest component is the Norwegian-Greenland overflow water (2–3°C) which is characterized most readily by a core of high oxygen, tritium, chlorofluorocarbons (CFCs) and low silicate anomaly. The above lying Labrador Sea Water (3–4°C) is distinguishable at this latitude only by its core of low potential vorticity. The shallowest component of the Boundary Current (4–5°C) is revealed by a core of high tritium, CFCs and low anomaly nut has no corresponding oxygen signal because of its proximity to the pronounced oxygen minimum layer. A careful analysis of the shallow water mass reveals that it is not dense enough to be formed in the central Labrador Sea even during warm winters. Rather, based on historical hydrography its area of formation is the southern Labrador Sea inshore of the North Atlantic Current where surface layer salinities are particularly low. A simple scale analysis shows that lateral mixing with the adjacent North Atlantic Current can increase the salinity of this component to the values observed in the mid-latitude data set.
Beatriz Penamolino - One of the best experts on this subject based on the ideXlab platform.
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variability in the deep Western Boundary Current local versus remote forcing
Journal of Geophysical Research, 2012Co-Authors: Beatriz Penamolino, Terrence M. Joyce, John M. TooleAbstract:[1] 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.
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recent changes in the labrador sea water within the deep Western Boundary Current southeast of cape cod
Deep Sea Research Part I: Oceanographic Research Papers, 2011Co-Authors: Beatriz Penamolino, Terrence M. Joyce, John M. TooleAbstract:Abstract Water properties measured by the central mooring in the Line W mooring array southeast of Cape Cod document a large character shift during the period of November 2001 to April 2008. The observed temperature, salinity and planetary potential vorticity (PPV) anomalies manifest changes in the formation region of the water masses present at Station W, specifically upper Labrador Sea Water (uLSW), deep Labrador Sea Water (dLSW) and Overflow Water (OW). During the observation period, the minimum in the PPV anomaly field relative to the record mean PPV profile migrated from 1500 m, where it was originally found, to 700 m. Temporal changes in the vertical distribution of temperature and salinity were correlated with the PPV changes. This suggests a dLSW-dominated first half of the record versus an uLSW-dominated second half. The structure of these anomalies is consistent with observations within the Labrador Sea, and their transit time to Line W agrees well with tracer-derived times for signals spreading along the Western Boundary. In that context, the observed water properties at Line W in the early 2000s reflected the intense deep convection in the Labrador Sea in the mid-1990s, with less intense convection subsequently affecting lighter isopycnals. The observed velocity field is dominated by high-frequency (periods of days to months) fluctuations, however, a fraction of the velocity variability is correlated with changes in water mass properties, and indicate a gradual acceleration of the southwestward flow, with a corresponding increase in Deep Western Boundary Current transport.
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transport of the north atlantic deep Western Boundary Current about 39 n 70 w 2004 2008
Deep-sea Research Part Ii-topical Studies in Oceanography, 2011Co-Authors: John M. Toole, Terrence M. Joyce, Michael S Mccartney, Ruth G Curry, Beatriz PenamolinoAbstract:Abstract Begun in spring 2004, a sustained measurement program – Line W – is returning high-resolution observations of the North Atlantic's Deep Western Boundary Current (DWBC) southeast of New England. The study focuses on the cold limb of the Atlantic Meridional Overturning Circulation near the Boundary between the subpolar and subtropical gyres. The field study consists of a 6-element, continental-slope-spanning moored array on a line underlying an altimeter satellite ground track, and periodic reoccupations of a full-depth hydrographic section along the line extending from the continental shelf towards Bermuda. Here, data from the first 4 years of the array (May 10, 2004–April 9, 2008) are analyzed along with 9 realizations of the section. The array, a mix of Moored Profiler and discrete, fixed-depth instrument moorings, returned temperature, salinity and horizontal velocity data with various temporal and depth resolutions. After averaging to filter inertial, tidal and other high-frequency motions, the combined moored data set was binned to the lowest common temporal resolution of 5-days (the nominal burst sample interval of the Moored Profilers) and interpolated to 2-dbar vertical resolution. Temperature, salinity, dissolved oxygen, tracer chemical concentrations and direct velocity data were acquired on the hydrographic cruises. The present work focuses on the 4-year-mean and time-varying meridional transport in 4 layers bounded by neutral density surfaces: Upper and Classical Labrador Sea Waters, Iceland-Scotland Overflow Waters and Denmark Strait Overflow Waters. The 5-d, 4-layer-summed meridional transport estimates range between −3.5 and −79.9 Sv with a record mean average transport of −25.1 Sv and standard deviation of 12.5 Sv. Bias adjustment to account for the finite width of the moored array increases the 4-layer mean transport estimate to −28.7 Sv. At time scales longer than about 1 month, the variations in equatorward DWBC transport appear correlated with meridional position of the Gulf Stream North Wall with stronger transport observed when the Stream is displaced south.
