Description: Current measurements from two consecutive yearlong deployments of three moored stations at the western end of the equator in the Atlantic, along 44°W, are used to determine the northwestward flow of warm water in the upper several 100 m and of the southeastward counterflow of North Atlantic Deep Water (NADW). Measurements from three acoustic Doppler current profilers (ADCPs) looking upward from 300 m toward the surface allowed calculation of a time series of upper layer transports over 1 year. Mean transport through the array for the upper 300 m is 23.8 Sv with an annual cycle of only ±3 Sv that has its maximum in June-August and minimum in northern spring. Estimated additional mean northwestward transport in the range 300–600 m is 6.7 Sv, based on moored data and shipboard Pegasus and lowered ADCP profiling. In the depth range 1400–3100 m a current core with maximum annual mean southeastward speed of 30 cm s−1 is found along the continental slope that carries an estimated upper NADW transport of 14.2–17.3 Sv, depending on the extrapolation used between the mooring in the core and the continental slope. This transport is higher than off-equatorial estimates and suggests near-equatorial recirculation at the upper NADW level, in agreement with northwestward mean flow found about 140 km offshore. Below 3100 m and above the 1.8°C isotherm, only a small core of lower NADW flow with speeds of 10–15 cm s−1 is found over the flat part of the basin near 1.5°N, clearly separated from the continental slope by a zone of near-zero mean speeds. Estimated transport of that small current core is about 4.5 Sv, which is significantly below other estimates of near-equatorial transport of lower NADW and suggests that a major fraction of lower NADW may cross the 44°W meridian north of the Ceara Rise. Intraseasonal variability is large, although smaller than observed at 8°N near the western boundary. It occurs at a period of about 1 month when it is dominant in the near-surface records and corresponds to earlier observations in the equatorial zones of all oceans and at a period of about 2 months when it is dominant at the NADW level and could be imported either from the north along the boundary or from the east along the equator. The existence of an annual cycle in the deep currents of a few centimeters per second amplitude, as suggested by high-resolution numerical model results, could neither be proven nor disproven because of the high amount of shorter-period variability.

Description: The zonal monsoon circulation south of India/Sri Lanka is a crucial link for the exchange between the northeastern and the northwestern Indian Ocean. The first direct measurements from moored stations and shipboard profiling on the seasonal and shorter‐period variability of this flow are presented here. Of the three moorings deployed from January 1991 to February 1992 along 80°30′E between 4°11′N and 5°39′N, the outer two were equipped with upward looking acoustic Doppler current profilers (ADCPs) at 260‐m depth. The moored and shipboard ADCP measurements revealed a very shallow structure of the near‐surface flow, which was mostly confined to the top 100 m and required extrapolation of moored current shears toward the surface for transport calculations. During the winter monsoon, the westward flowing Northeast Monsoon Current (NMC) carried a mean transport of about 12 Sv in early 1991 and 10 Sv in early 1992. During the summer monsoon, transports in the eastward Southwest Monsoon Current (SMC) were about 8 Sv for the region north of 3°45′N, but the current might have extended further south, to 2°N, which would increase the total SMC transport to about 15 Sv. The circulation during the summer was sometimes found to be more complicated, with the SMC occasionally being separated from the Sri Lankan coast by a band of westward flowing low‐salinity water originating in the Bay of Bengal. The annual‐mean flow past Sri Lanka was weakly westward with a transport of only 2–3 Sv. Using seasonal‐mean ship drift currents for surface values in the transport calculations yielded rather similar results to upward extrapolation of the moored profiles. The observations are compared with output of recent numerical models of the Indian Ocean circulation, which generally show the origin of the zonal flow past India/Sri Lanka to be at low latitudes and driven by the large‐scale tropical wind field. Superimposed on this zonal circulation is local communication along the coast between the Bay of Bengal and the Arabian Sea

Description: Results from an interannually forced, 0.08 degrees eddy-resolving simulation based on the Hybrid Coordinate Ocean Model, in conjunction with a small but well-determined transport database, are used to investigate the currents and transports associated with the Atlantic meridional overturning circulation (AMOC) in the subpolar North Atlantic (SPNA). The model results yield a consistent warming in the western SPNA since the early 1990s, along with mean transports similar to those observed for the trans-basin AMOC across the World Ocean Circulation Experiment hydrographic section AR19 (16.4 Sv) and boundary currents at the exit of the Labrador Sea near 53 degrees N (39.0 Sv) and east of the Grand Banks near 43 degrees N (15.9 Sv). Over a 34 year integration, the model-determined AMOC across the AR19 section and the western boundary current near 53 degrees N both exhibit no systematic trend but some long-term (interannual and longer) variabilities, including a decadal transport variation of 3-4 Sv from relatively high in the 1990s to low in the 2000s. The decadal variability of the model boundary current transport near 53 degrees N lags the observed winter time North Atlantic Oscillation index by about 2 years and leads the model AMOC across the AR19 section by about 1 year. The model results also show that the long-term variabilities are low compared to those on shorter time scales. Thus, rapid sampling of the current over long time intervals is required to filter out high-frequency variabilities in order to determine the lower frequency variabilities of interest

Description: Highlights: • A joint analysis of deep current meter records in the western North Atlantic. • Intra-seasonal variability dominates the deep boundary current. • Topographic waves near 10d periods trapped over steep topography. • Basin centers are showing longer periods (50d) caused by the eddy field. • Observed variability characteristics compared to high resolution model simulation. Abstract 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 timescales, but at much longer periods around 50 days.

Description: A decade of weak convection in the Labrador Sea associated with decreasing water mass transformation, in combination with advective and eddy fluxes into the convection area, caused significant warming of the deep waters in both the central Labrador Sea and boundary current system along the Labrador shelf break. The connection to the export of Deep Water was studied based on moored current meter stations between 1997 and 2009 at the exit of the Labrador Sea, near the shelf break at 5˚3N. More than 100 year -long current meter records spanning the full water column have been analyzed with respect to high frequency variability, decaying from the surface to the bottom layer, and for the annual mean flow, showing intra- to interannual variability but no detectable decadal trend in the strength of the deep and near-bottom flow out of the Labrador Sea.

Description: Equatorial deep jets (EDJs) are a prominent flow feature of the equatorial Atlantic below the Equatorial Undercurrent down to about 3000 m. Here we analyze long-term moored velocity and oxygen observations, as well as shipboard hydrographic and current sections acquired along 23{degree sign}W and covering the depth range of the oxygen minimum zones of the eastern tropical North and South Atlantic. The moored zonal velocity data show high-baroclinic mode EDJ oscillations at a period of about 4.5 years. Equatorial oxygen observations which do not resolve or cover a full 4.5-yr EDJ cycle nevertheless reveal large variability, with oxygen concentrations locally spanning a range of more than 60 μmol kg−1. We study the effect of EDJs on the equatorial oxygen concentration by forcing an advection-diffusion model with the velocity field of the gravest equatorial basin mode corresponding to the observed EDJ cycle. The advection-diffusion model includes an oxygen source at the western boundary and oxygen consumption elsewhere. The model produces a 4.5-yr cycle of the oxygen concentration and a temporal phase difference between oxygen concentration and eastward velocity that is less than quadrature, implying a net eastward oxygen flux. The comparison of available observations and basin-mode simulations indicates that a substantial part of the observed oxygen variability at the equator can be explained by EDJ oscillations. The respective role of mean advection, EDJs, and other possible processes in shaping the mean oxygen distribution of the equatorial Atlantic at intermediate depth is discussed. Key Points: - Equatorial Deep Jets strongly affect oxygen distribution/variability - Mean oxygen ditribution in the equatorial Atlantic at intermediate depth - Gravest equatorial basin mode forces an advection-diffusion model

Description: The flow field in the area of what was thought to be the source region of the North Brazil Current (NBC) off the northeast coast of Brazil between 5 degrees 30'S and 10 degrees S was investigated in austral spring during November 1992 and compared with observations in October 1990. The data were taken with several different instruments, including vessel-mounted ADCP, lowered-ADCP, Pegasus, CTD and XBTs. The flow was found off the coast at 5 degrees 30'S as well as at 10 degrees S as an undercurrent, the North Brazil Undercurrent (NBUC). The NBUC shows a subsurface core at about 200 m depth with velocities of up to 90.0 cm s(-1), resulting in large northward transports of more than 22 Sv in the upper 1000 m. The transport is about the same at 5 degrees 30'S and 10 degrees S, hence no net inflow from the east is required to feed the NBUC. The climatological Ekman transport is to the south between 5 degrees 30'S and 10 degrees S, and in consequence the northward flow near the surface was reduced and might be one reason for the existence of the undercurrent. The flow near the coast was to the north at 10 degrees S, therefore the Brazil Current had to start as a coastal current south of 10 degrees S. For the zonal sections at 5 degrees 30'S and 10 degrees S the geostrophic computations relative to the density surface sigma(1) = 32.15 kg m(-3) (about 1150 m depth) resulted in transports comparable to those obtained from direct measurements. The results further show that the choice of a correct level of no motion can be supported by the direct observations. A shallower reference based on water mass boundaries alone would reduce the NBUC transport to almost zero. Computations with data from the historical data base for austral fall resulted in a weaker NBUC of less than 20 Sv near 10 degrees S, indicating a possible seasonal signal in the NBUC with a stronger NBUC in austral spring.

Description: During March 1994 a survey of the western boundary of the tropical Atlantic, between 10 degrees N and 10 degrees S, was carried out by conductivity-temperature-depth and current profiling using shipboard and lowered acoustic Doppler current profilers. In the near-surface layer, above sigma. = 24.5, the inflow into the boundary regime came dominantly from low latitudes; out of the 14 Sv that crossed the equator in the upper part of the North Brazil Current (NBC), only 2 Sv originated from south of 5 degrees S, while 12 Sv came in from the east at 1 degrees-5 degrees S with the South Equatorial Current (SEC). After crossing the equator near 44 degrees W, only a minor fraction of the near-surface NBC retroflected eastward, while a net through flow of about 12 Sv above sigma. = 24.5 continued northwestward along the boundary, By contrast, in the isopycnal range sigma. = 24.5-26.8 encompassing the Equatorial Undercurrent (EUC), the source waters of the equatorial circulation were dominantly of higher-latitude South Atlantic origin. While only 3 Sv of eastern equatorial water entered the region through the SEC at 3 degrees-5 degrees S, there was an inflow of 10 Sv of South Atlantic water in the North Brazil Undercurrent (NBUC) along the South American coast that originated south of 10 degrees S, The transport of 14 Sv arriving at the equator along the boundary in the undercurrent layer was almost entirely retroflected into the EUC with only marginal northern water additions along its path to 35 degrees W. The off-equatorial undercurrents in the upper thermocline, the South and North Equatorial Undercurrents carried only small transports across 35 degrees W, of 5 Sv and 3 Sv, respectively, dominantly supplied out of SEC recirculation rather than out of the boundary current. Still deeper, three zonal undercurrents were observed: the westward-flowing Equatorial Intermediate Current (EIC) in the depth range 200-900 m below the EUC, and two off-equatorial eastward undercurrents, the Northern and Southern Intermediate Countercurrents (NICC, SICC) at 400-1000 m and 1 degrees-3 degrees latitude. In the lower part of the NBUC there was an Antarctic Intermediate Water (AAIW) inflow along the coast of 6 Sv, and there was a clear connection at the AAIW level to the SICC by low salinities and high oxygens and a weaker suggestion also that some supply of the NICC might be through AAIW out of the deep NBUC.

Description: During the winter of 1988–1989 five acoustic Doppler current profilers (ADCPs) were moored in the central Greenland Sea to measure vertical currents that might occur in conjunction with deep mixing and convection. Two ADCPs were looking up from about 300 m and combined with thermistor strings in the depth range 60–260 m, two were looking downward from 200 m, and one was looking upward from 1400 m. First maxima of vertical velocity variance occurred at two events of strong cold winds in October and November when cooling and turbulence in the shallow mixed layer generated internal waves in the thermocline. Beginning in late November the marginal ice zone expanded eastward over the central Greenland Sea, reaching its maximum extent in late December. In mid-January a bay of ice-free water opened over the central Greenland Sea, leaving a wedge of ice, the “is odden,” curled around it along the axis of the Jan Mayen Current and then northeastward and existing well into April 1989. Below the ice a mixed layer at freezing temperatures developed that increased in thickness from 60 to 120 m during the period of ice cover, corresponding to an average heat loss of about 40 W m−2. Through brine rejection, mixed-layer salinity increased steadily, reducing stability to underlying weakly stratified layers (Roach et al., 1993). During the ice cover period, vertical currents were at a minimum. After the opening of the ice-free bay, successive mixed-layer deepening to 〉350 m occurred in conjunction with cooling events around February 1 and 15, accompanied by strong small-scale vertical velocity variations. Upward mixing of more saline waters of Atlantic origin during this phase reduced the stability further, generating a pool of homogeneous water of 〉50 km horizontal extent in the central Greenland Sea, preconditioned for subsequent convection to greater depths. Individual convection events were observed during March 6–16, associated with downward velocities at the 1400-m level of about 3 cm s−l. One event was identified as a plume of about 300-m horizontal scale, in agreement with recently advanced scaling arguments and model results, and with earlier similar observations in the Gulf of Lions, western Mediterranean. The deep convection occurred in the center of the ice-free bay; hence brine rejection did not seem necessary for its generation. Plume temperatures at 1400 m were generally higher than that of the homogeneous surface pool, suggesting entrainment of surrounding warmer waters on the way down. Mean vertical velocity over a period of convection events was indistinguishable from zero, suggesting that plumes served as a mixing agent rather than causing mean downward transport of water masses. However, different from the surface pool that was governed by mixed-layer physics, the water between 400 and 1400 m was not horizontally homogenized in a large patch by the sporadic plumes. Overall, and compared to results from the Gulf of Lions, convection activity in the central Greenland Sea was weak and limited to intermediate depths in winter 1988–1989.

Description: 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.