The Deep Western Boundary Current (DWBC) along the western margin of the subpolar North Atlantic is an important component of the deep limb of the Meridional Overturning near its northern origins. A network of moored arrays from Denmark Strait to the tail of the Grand Banks has been installed for almost two decades to observe the boundary currents and transports of North Atlantic Deep Water as part of an internationally coordinated observatory for the Atlantic Meridional Overturning Circulation.The dominant variability in all of the moored velocity time series is in the week-to-month period range. While the temporal characteristics of this variability change only gradually between Denmark Strait and Flemish Cap, a broad band of longer term variability is present farther along the path of the DWBC at the Grand Banks and in the interior basins (Labrador and Irminger Seas). The vigorous intra-seasonal variability may well mask possible interannual to decadal variability that is typically an order of magnitude smaller than the high-frequency fluctuations. Here, the intra-seasonal variability is quantified at key positions along the DWBC path using both, observations and high resolution model data. The results are used to evaluate the model circulation, and in turn the model is used to relate the discrete measurements to the overall pattern of the subpolar circulation. Topographic waves are found to be trapped by the steep topography all around the western basins, the Labrador and Irminger Seas. In the Labrador Sea, the high intra-seasonal variability of the boundary current regime is separated by a region of extremely low variability in narrow recirculation cells from the basin interior. There, the variability is also on intra-seasonal time scales, but at much longer periods around 50 days. (C) 2014 Elsevier Ltd. All rights reserved.

Repeated shipboard observation sections across the boundary flow off northeastern Brazil as well as acoustic Doppler current profiler (ADCP) and current-meter records from a moored boundary array deployed during 2000 - 04 near 11 S are analyzed here for both the northward warm water flow by the North Brazil Undercurrent (NBUC) above approximately 1100 m and the southward flow of North Atlantic Deep Water (NADW) underneath. At 5 S, the mean from nine sections yields an NBUC transport of 26.5 +/- 3.7 Sv (Sv = 10(6) m(3) s(-1)) along the boundary; at 11 degrees S the mean NBUC transport from five sections is 25.4 +/- 7.4 Sv, confirming that the NBUC is already well developed at 11 S. At both latitudes a persistent offshore southward recirculation between 200- and 1100-m depth reduces the net northward warm water flow through the 5 degrees S section ( west of 31.5 W) to 22.1 +/- 5.3 Sv and through the 11 degrees S section to 21.7 +/- 4.1 Sv ( west of 32.0 degrees W). The 4-yr-long NBUC transport time series from 11 degrees S yields a seasonal cycle of 2.5 Sv amplitude with its northward maximum in July. Interannual NBUC transport variations are small, varying only by +/- 1.2 Sv during the four years, with no detectable trend. The southward flow of NADW within the deep western boundary current at 5 degrees S is 25.5 +/- 8.3 Sv with an offshore northward recirculation, yielding a nine-section mean of 20.3 +/- 10.1 Sv west of 31.5 degrees W. For Antarctic Bottom Water, a net northward flow of 4.4 +/- 3.0 Sv is determined at 5 degrees S. For the 11 degrees S section, the moored array data show a pronounced energy maximum at 60 - 70-day period in the NADW depth range, which was identified in related work as deep eddies translating southward along the boundary. Based on a kinematic eddy model fit to the first half of the moored time series, the mean NADW transfer by the deep eddies at 11 degrees S was estimated to be about 17 Sv. Given the large interannual variability of the deep near-boundary transport time series, which ranged from 14 to 24 Sv, the 11 degrees S mean was considered to be not distinguishable from the mean at 5 degrees S.

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 fur 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 21 near the bottom and a total northward transport of about 140 Sv (Sv ( 106 m3 s 21), 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 su 5 27.74 kg m23 isopycnal. However, east of the deep western boundary current (DWBC), the deep NAC carries a transport of 51 Sv northward below su 5 27.74 kg m23, 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.

Observations in the central tropical Atlantic are used to investigate the circulation, the variability, and the near-equatorial meridional flow in this oceanic region. Meridional sections confirm that the southern band of the South Equatorial Current is a broad sluggish flow transporting subtropical water northwestward toward the western boundary. Variability in the South Equatorial Current is weak with an annual signal of about 2 cm/s. Recent equatorial flow observations agree with the previously proposed mean flow field, indicating that a permanent tropical circulation exists that is composed of several zonal current and countercurrent bands of small vertical and meridional extent compared to the subtropical gyres. However, wave phenomena superimpose on the mean flow field. On seasonal time scales the variability in the zonal flow field near the equator is dominated by the semiannual cycle in the central and eastern part while the annual cycle dominates in the western part. This seasonal variability is caused by the propagation of equatorial Rossby and Kelvin waves generated mainly by the zonal wind anomaly at the equator. Despite the observations of instantaneous cross-equatorial velocities and of floats crossing the equator it remains unclear whether there is a net cross-equatorial flow in the central tropical Atlantic in addition to cross-equatorial exchanges via thermocline convergence, upwelling and Ekman divergence. Three floats deployed at 200 m and 400 m depth either leave their deployment region at the equator to join the North Equatorial Undercurrent and progress further northward or in two cases have been deployed in the southern hemisphere and drift towards the equator.

A new shipboard current profiler, a 75-kHz ocean surveyor, was operationally used during two research cruises in the tropical Atlantic and the subpolar North Atlantic, respectively. Here, a report is presented on the first experience with this instrument in two very different current regimes, in the Tropics with large vertical shears, and in the subpolar regime with mainly barotropic flow. The ocean surveyor continuously measured currents in the upper ocean from near the surface to about 500-700-m depth. The measurement range showed a dependence on the regional and temporal variations of scattering particles and on the intensity of swell and wind waves. Statistical comparisons are performed with on-station lowered acoustic Doppler current profiler (LADCP) profiles and underway measurements by classic shipboard acoustic Doppler current profiler (ADCP) measurements. Accuracy estimates for hourly averaged ocean surveyor currents result in errors of about 1 cm s(-1) for on-station data and of 2-4 cm s(-1) for underway measurements, depending on the regional abundance of scatterers and on the weather conditions encountered.

The bottom and deep circulation in the Somali Basin are investigated on the basis of hydrographic and direct velocity profiles from three shipboard surveys carried out during the southwest monsoon in 1995 and of velocity time series from the WOCE mooring array ICM7. The inflow of bottom water into the Somali Basin through the Amirante Passage drives a thermohaline circulation, which may be modulated by the monsoon wind forcing. Details of the abyssal circulation have been discussed controversially. Deep velocity records from the mooring array in the northern Somali Basin are dominated by fluctuations with periods of 30–50 days and amplitudes above 5cms−1. Despite this strong variability annual record averages indicate the existence of a deep western boundary current (DWBC) below 3000m at the base of the continental slope south of Socotra Island as part of a cyclonic bottom circulation. The southwestward DWBC transport off Socotra Island is estimated to 2±1.3Sv. The bottom and deep water exchange between the Somali and Arabian Basin north of 7°N is estimated from two cross-basin geostrophic velocity sections referenced by vertically averaged LADCP currents. For the bottom water, an eastward transport into the Arabian Basin of 1.4±0.5Sv and 2.1±0.6Sv was determined in June and August, respectively, while for the deep-water layer above 3500m eastward transports of 3.6±2.3Sv in June and 4.0±2.4Sv in August were obtained.

In 1997, a unique hydrographic and chlorofluorocarbon (CFC: component CFC-11) dataset was obtained in the subpolar North Atlantic. To estimate the synopticity of the 1997 data, the recent temporal evolution of the CFC and Labrador Sea Water (LSW) thickness fields are examined. In the western Atlantic north of 50degreesN, the LSW thickness decreased considerably from 1994-97, while the mean CFC concentrations did not change much. South of 50degreesN and in the eastern Atlantic, the CFC concentration increased with little or no change in the LSW thickness. On shorter timescales, local anomalies due to the presence of eddies are observed, but for space scales larger than the eddies the dataset can be treated as being synoptic over the 1997 observation period.The spreading of LSW in the subpolar North Atlantic is described in detail using gridded CFC and LSW thickness fields combined with Profiling Autonomous Lagrangian Circulation Explorer (PALACE) float trajectories. The gridded fields are also used to calculate the CFC-11 inventory in the LSW from 40degrees to 65degreesN, and from 10degrees to 60degreesW. In total, 2300 +/- 250 tons of CFC-11 (equivalent to 16.6 million moles) were brought into the LSW by deep convection. In 1997, 28% of the inventory was still found in the Labrador Sea west of 45degreesW and 31% of the inventory was located in the eastern Atlantic.The CFC inventory in the LSW was used to estimate the lower limits of LSW formation rates. At a constant formation rate, a value of 4.4-5.6 Sv (Sv = 10(6) m(3) s(-1)) is obtained. If the denser modes of LSW are ventilated only in periods with intense convection, the minimum formation rate of LSW in 1988-94 is 8.1-10.8 Sv, and 1.8-2.4 Sv in 1995-97.

Sea-surface height data acquired by the TOPEX/POSEIDON satellite over the Arabian Sea from October 1992 to October 1998 are analyzed. Strong seasonal fluctuations are found between 6° and 10°N, which are mainly associated with westward propagating annual Rossby waves radiated from the western side of the Indian subcontinent and that are continuously forced by the action of the wind-stress curl over the central Arabian Sea. An analysis of hydrographic data acquired during August 1993 and during January 1998 at 8°N in the Arabian Sea reveals the existence of first- and second-mode annual Rossby waves. These waves, which can be traced as perturbations in the density fields, have wavelengths of 12×103 and 4.4×103km as well as phase velocities of 0.38 and 0.14m/s, respectively. The waves are associated with a time-dependent meridional overturning cell that sloshes water northward and southward. Between 58° and 68°E in the central Arabian Sea, we found a Rossby-wave induced transport in the upper 500m of about 10Sv southward in August 1993 and northward in January 1998. Below 2000m, there was still a northward transport of 3.2Sv in August 1993 and a southward transport of 4.8Sv in January 1998. A comparison of steric height differences between August 1993 and January 1998 calculated from the observed density fields as well as calculated from the reconstructed density fields using first- and second-mode annual Rossby waves agree quite well with the corresponding sea-surface height differences. Implications resulting from the reflection of annual Rossby waves, like fluctuations of the western boundary currents, are discussed.