Deep Western Boundary Current Variability Research Articles

Attribution of Deep Western Boundary Current variability at 26.5°N

Observed variations in the Deep Western Boundary Current (DWBC) at 26.5°N, which carries the deep limb of the Atlantic Meridional Overturning Circulation (MOC), have been shown to greatly exceed in magnitude the variations of the overall basin-wide MOC, with strong variability at a range of time scales from weeks to multiple-months. Attribution of these strong DWBC variations will be crucial for understanding variations in the MOC itself. Nevertheless, despite many years of moored observations of the DWBC at 26.5°N, understanding of these variations has been elusive. Two years of observations from a high horizontal resolution array of pressure-equipped inverted echo sounders are used together with output from a modern high-resolution numerical model to investigate the mechanisms behind these ±20×106m3s−1 volume transport variations. The model and observational results together suggest that the strongest variations cannot be explained solely via either of the two most commonly proposed mechanisms – meandering or pulsation of the DWBC. The dominant mechanism appears to be propagation of Rossby Wave-like structures into the region from the east, and it is the impact of these features in the region that yield the largest transport anomalies. These waves have been observed and discussed in the past – however their key role as the dominant source of DWBC variability has not previously been recognized. The implications of these results are also discussed in the context of future observing systems for the DWBC.

Intermediate water links to Deep Western Boundary Current variability in the subtropical NW Atlantic during marine isotope stages 5 and 4

Records from Ocean Drilling Program Sites 1057 and 1059 (2584 m and 2985 m water depth, respectively) have been used to reconstruct the behavior of the Deep Western Boundary Current (DWBC) on the Blake Outer Ridge (BOR) from 130 to 60 kyr B.P. (marine isotope stage (MIS) 5 and the 5/4 transition). Site 1057 lies within Labrador Sea Water (LSW) but close to the present‐day boundary with Lower North Atlantic Deep Water (LNADW), while Site 1059 lies within LNADW. High‐resolution sortable silt mean () grain size and benthic δ13C records were obtained, and changes in the DWBC intensity and spatial variability were inferred. Comparisons are made with similar proxy records generated for the Holocene from equivalent depth cores on the BOR. During MIS 5e, evidence at Site 1057 suggests slower relative flow speeds consistent with a weakening and a possible shoaling of the LSW‐sourced shallower limb of the DWBC that occupies these depths today. In contrast, the paleocurrent record from the deeper site suggests that the fast flowing deep core of the DWBC was located close to its modern depth below 3500 m. During this interval the benthic δ13C suggests little chemical stratification of the water column and the presence of a near‐uniform LNADW‐dominated water mass. After ∼111 kyr B.P. the record at Site 1057 increases to reach values similar to Site 1059 for the rest of MIS 5. The strengthening of flow speeds at the shallow site may correspond to the initiation of Glacial North Atlantic Intermediate Water formation also suggested by a divergence in the benthic δ13C records with Site 1057 values increasing to ∼1.2‰. Coupled suborbital oscillations in DWBC flow variability and paleohydrography persisted throughout MIS 5. Comparison of these data with planktonic δ18O records from the sites and alkenone‐derived sea surface temperature (SST) estimates from the nearby Bermuda Rise suggest a hitherto unrecognized degree of linkage between oscillations in subtropical North Atlantic SST and DWBC flow.

Deep Western Boundary Current variability in the subtropical northwest Atlantic Ocean during marine isotope stages 12–10

High‐resolution (125–500 year temporal resolution) sortable silt mean grain size data of subtropical northwest Atlantic Ocean Drilling Program (ODP) Leg 172, Site 1061, monitor fluctuations of the Deep Western Boundary Current (DWBC) throughout marine oxygen isotope stages (MIS) 12–10 (330–460 kyr) in great detail. Comparison with foraminiferal stable isotope data from the same site and ODP Site 1063 (Bermuda Rise, data of Poli et al. (2000)) shows that the DWBC shoals during glacial times in relation to a reduced North Atlantic Deep Water production and increased advection of southern ocean waters. Superimposed fluctuations in the flow speed/position of the DWBC at a semiprecession scale are persistent throughout MIS 10–12 and independent of the glacial state. Additional millennial‐scale variations in DWBC flow speed parallel surface water cooling events and ice‐rafted debris episodes especially at the transition of MIS 11/10, indicating the sensitivity of deep current flow variations to changes in the surface water conditions. These millennial‐scale variations are amplified when benthic foraminiferal δ18O values exceed ∼3.8‰ and may be related to the transition of an ice volume threshold where climate variability is possibly amplified by feedback mechanisms associated with ice sheet growth.

Magnetic proxy for the deep (Pacific) western boundary current variability across the mid-Pleistocene climate transition

The Deep Western Boundary Current (DWBC) inflow to the SW Pacific is one of the largest, transporting ∼ 40% of the total input of deep water to the world's oceans. Here we use a sedimentary record from the giant piston core MD97-2114 collected on the northern flank of the Chatham Rise located at 1935 m water depth, east of New Zealand, to investigate DWBC variability during the Pleistocene epoch when the period of glacial cycles changed progressively from a 41 kyr to 100 kyr rhythm. Magnetic grain-size may be directly related to orbitally forced fluctuations in the strength of the upper circumpolar deep water (UCDW) through its interaction with terrigenous sediments supplied from the south and west. The long-term trends in magnetic properties are characterized by two main perturbations centered at 870 ka (Marine Isotope Stage, MIS 22) 450 ka (MIS 12), which is broadly consistent with the inferred perturbation during the mid-Pleistocene climate transition based on sedimentological paleocurrent reconstruction from Ocean Drilling Program Site 1123 located at 3290 m water depth in the main core of the DWBC flow on the North Chatham Drift. This similarity suggests that both the upper and middle CDW are modulated by similar processes and fluctuations of Antarctic Bottom Water production could be directly responsible for this deep Pacific Ocean inflow variability over the past 1.2 Ma.

Seasonal variability of deep currents in the equatorial Atlantic: a model study

A suite of high-resolution models of the Atlantic Ocean circulation is used to study the deep seasonal current variability in the equatorial regime, with a particular emphasis on its manifestation in the variability of the interhemispheric transports near the western boundary. The basic experiment has a resolution of 1/3∘ horizontally and 45 vertical levels, and is subject to a monthly mean atmospheric forcing based on ECMWF flux fields. Sensitivity experiments explore the effects of higher horizontal resolution (1/12∘), and alternative mixing parameterizations. The model behavior near the equator confirms previous suggestions based on solutions of the WOCE Community Modelling Effort (“CME”) and the “DYNAMO” model intercomparison project, of the presence of a system of vigorous seasonal current oscillations, spanning the whole water column and nearly the whole zonal extent of the basin. The patterns of the primarily zonal current anomalies are fairly robust across the range of model cases investigated, i.e., show relatively little sensitivity to horizontal resolution/mixing, or to the different choices of vertical discretization and vertical mixing as in the DYNAMO cases. The amplitude of the seasonal variation exceeds 10cm/s in the surface layer, and decreases to about 5cm/s near 1000m and 2–3cm/s in the deep ocean in both the basic 1/3∘- and the 1/12∘-cases, thereby leading to seasonally reversing current signatures at all depths below the EUC.A particular aspect of the seasonal current variability concerns its manifestation in the southward transport of North Atlantic Deep Water (NADW) by the Deep Western Boundary Current (DWBC). The temporal characteristics of the DWBC variability are in agreement with moored current meter observations at 44∘W, with simulated DWBC transports varying between a maximum of more than 30Sv in January/February, and almost vanishing transport in September. However, in contrast to the annual-mean deep water transport which is confined to the DWBC and tight, O(100)km-recirculation cells, the seasonal cycle of transport is not trapped near the boundary: the simulations show that the zonal current variations of the equatorial wave guide, near the western boundary give rise to a broad system of seasonal recirculation cells of the DWBC. Calculations of the amplitude of the seasonal variability in the deep water transport near the equator are therefore strongly dependent of the spatial extent of the cross-section considered; in particular, for being approximately representative of low-frequency variations in the net, zonally-integrated meridional transport of deep water in the equatorial regime, transport sections would need to extend over nearly the whole western basin.

Circulation and Deep-Water Export at the Western Exit of the Subpolar North Atlantic

The current system east of the Grand Banks was intensely observed by World Ocean Circulation Experiment (WOCE) array ACM-6 during 1993–95 with eight moorings, reaching about 500 km out from the shelf edge and covering the water column from about 400-m depth to the bottom. More recently, a reduced array by the Institut für Meerskunde (IfM) at Kiel, Germany, of four moorings was deployed during 1999–2001, focusing on the deep-water flow near the western continental slope. Both sets of moored time series, each about 22 months long, are combined here for a mean current boundary section, and both arrays are analyzed for the variability of currents and transports. A mean hydrographic section is derived from seven ship surveys and is used for geostrophic upper-layer extrapolation and isopycnal subdivision of the mean transports into deep-water classes. The offshore part of the combined section is dominated by the deep-reaching North Atlantic Current (NAC) with currents still at 10 cm s−1 near the bottom and a total northward transport of about 140 Sv (Sv ≡ 106 m3 s−1), with the details depending on the method of surface extrapolation used. The mean flow along the western boundary was southward with the section-mean North Atlantic Deep Water outflow determined to be 12 Sv below the σθ = 27.74 kg m−3 isopycnal. However, east of the deep western boundary current (DWBC), the deep NAC carries a transport of 51 Sv northward below σθ = 27.74 kg m−3, resulting in a large net northward flow in the western part of the basin. From watermass signatures it is concluded that the deep NAC is not a direct recirculation of DWBC water masses. Transport time series for the DWBC variability are derived for both arrays. The variance is concentrated in the period range from ∼2 weeks to 2 months, but there are also variations at interannual and longer periods, with much of the DWBC variability being related to fluctuations and meandering of the NAC. A significant annual cycle is not recognizable in the combined current and transport time series of both arrays. The moored array results are compared with other evidence on deep outflow and recirculation, including recent models of different types and complexity.

Seasonal transport variability of the Deep Western Boundary Current in the equatorial Atlantic

A total of 21 about year‐long current meter records in the depth range of the upper and middle North Atlantic Deep Water (NADW) were analyzed to determine the mean and the fluctuations of the upper Deep Western Boundary Current (DWBC) in the equatorial Atlantic. The investigation was based on moored arrays at 44°W from three different deployment periods, 1989/1990, 1990/1991 and 1992/1994, and was supplemented by current profiling along 44°W and 35°W. The approximately 100‐km‐wide DWBC at 44°W, just north of the equator, was attached to the topography with the current maximum exceeding 70 cm s−1. Currents within the DWBC core followed the topography, and the close agreement between the mean current direction and the direction of maximum variance indicated that the major contribution to the DWBC variability near the equator was due to pulsing rather than meandering. For mean transports of upper and middle NADW, the current meter records were averaged over their deployment duration yielding a best estimate of 13 Sv in the depth range 1000 to 3100 m. The mean transport appeared robust, as subsets of the data from two different years yielded about the same mean transport, namely, 12.4 and 13.6 Sv. The DWBC transport time series showed a definite seasonal cycle, ranging from less than 7 Sv during September/October to about 25 Sv during January/February. Annual and semiannual transport harmonics had similar amplitudes, at about 6 Sv each, and together they explained about two thirds of the total transport variability. After crossing the equator, the DWBC splits into two cores with the major flow along a chain of seamounts near 3.5°S, near 35°W. Magnitudes and phases of the transport variability at 35°W, south of approximately 1.5°S, were similar to that at 44°W. Further, for the flow of lower NADW which was detached from the upper DWBC core, similar periodicity and phases were observed in the deep records at 44°W.

Interaction of the Gulf Stream and Deep Western Boundary Current where they cross

The long‐timescale variability of the Deep Western Boundary Current (DWBC) where it crosses under the Gulf Stream is analyzed using a 3‐year array of bottom current meters and inverted echo sounders. The relationship of this variability to fluctuations in the upper layer Gulf Stream is specifically addressed. At periods shorter than a year the DWBC variability shows no relationship to the local Gulf Stream, 100‐day fluctuations show a pulsing of the DWBC transport, whereas longer‐period fluctuations are indicative of a meandering‐type variability. By contrast, on timescales greater than a year, variations in the orientation and transport of the DWBC are directly related to such fluctuations in the local Gulf Stream. The coupling reveals that the DWBC is forced by changes in the Gulf Stream rather than causing such variations, which is in contrast to earlier modeling results.

Deep western boundary current variability off northeastern Brazil

Observations from a year-long deep current meter mooring at 8°N, 52°W are used to describe the structure and variability of the Deep Western Boundary Current (DWBC) in the tropical Atlantic. The DWBC has a deep core near 4300 m depth, is extremely swift and narrow (35 cm s −1 core speed, 60 km width), and shows a total transport of 22 Sv between 2500 m and the bottom. Approximately 3 Sv of the DWBC transport are carried in near-bottom layers below θ=1.8° C, which appears to be recirculating in the western tropical Atlantic north of about 4°N. Variations in the DWBC are dominated by large-amplitude vertical displacements of the deep temperature field at 60–70 day periodicities that cause significant changes in the deep stratification and vertical transport structure of the DWBC. These fluctuaions appear to be related to periodic surges of cold bottom waters up onto the base of the continental rise that become entrained in the DWBC. While the forcing mechanism for these deep fluctuations remains unexplained, the picture emerging from these data is that of a highly active abyssal layer in the tropical Atlantic.

Deep Western Boundary Current variability at Cape Hatteras

Deep Western Boundary Current variability at Cape Hatteras