Flux comparison of Eulerian and Lagrangian estimates of Agulhas leakage: A case study using a numerical model

Estimating the magnitude of Agulhas leakage, the volume flux of water from the Indian to the Atlantic Ocean, is difficult because of the presence of other circulation systems in the Agulhas region. Indian Ocean water in the Atlantic Ocean is vigorously mixed and diluted in the Cape Basin. Eulerian integration methods, where the velocity field perpendicular to a section is integrated to yield a flux, have to be calibrated so that only the flux by Agulhas leakage is sampled. Two Eulerian methods for estimating the magnitude of Agulhas leakage are tested within a high-resolution two-way nested model with the goal to devise a mooring-based measurement strategy. At the GoodHope line, a section halfway through the Cape Basin, the integrated velocity perpendicular to that line is compared to the magnitude of Agulhas leakage as determined from the transport carried by numerical Lagrangian floats. In the first method, integration is limited to the flux of water warmer and more saline than specific threshold values. These threshold values are determined by maximizing the correlation with the float-determined time series. By using the threshold values, approximately half of the leakage can directly be measured. The total amount of Agulhas leakage can be estimated using a linear regression, within a 90% confidence band of 12 Sv. In the second method, a subregion of the GoodHope line is sought so that integration over that subregion yields an Eulerian flux as close to the float-determined leakage as possible. It appears that when integration is limited within the model to the upper 300 m of the water column within 900 km of the African coast the time series have the smallest root-mean-square difference. This method yields a root-mean-square error of only 5.2 Sv but the 90% confidence band of the estimate is 20 Sv. It is concluded that the optimum thermohaline threshold method leads to more accurate estimates even though the directly measured transport is a factor of two lower than the actual magnitude of Agulhas leakage in this model.     

Isopycnal displacements within the Cape Basin thermocline as revealed by the Hydrographic Data Archive

The transfer of upper kilometer water from the Indian Ocean into the South Atlantic, the Agulhas leakage, is believed to be accomplished primarily through meso-scale eddy processes. There have been various studies investigating eddies of the “Cape Basin Cauldron” from specific data sets. The hydrographic data archive acquired during the last century within the Cape Basin region of the South Atlantic provides additional insight into the distribution and water mass properties of the Cape Basin eddies. Eddies are identified by mid-thermocline isopycnal depth anomalies relative to the long-term mean. Positive depth anomalies (the reference isopycnal is deeper than the long-term mean isopycnal depth) mark the presence of anticyclonic eddies; negative anomalies mark cyclonic eddies. Numerous eddies are identified in the whole region; the larger isopycnal displacements are attributed to the energetic eddies characteristic of the Cape Basin and indicate that there is a 2:1 anticyclone/cyclone ratio. Smaller displacements of the less energetic features are almost equally split between anticyclones and cyclones (1.4:1 ratio). Potential temperature, salinity and oxygen relationships at thermocline and intermediate levels within each eddy reveal their likely origin. The eddy core water is not solely drawn from Indian Ocean: tropical and subtropical South Atlantic water are also present. Anticyclones and cyclones carrying Agulhas Water properties are identified throughout the Cape Basin. Anticyclones with Agulhas Water characteristics show a predominant northwest dispersal, whereas the cyclones are identified mainly along the western margin of the African continent, possibly related to their origin as shear eddies at the boundary between the Agulhas axis and Africa. Cyclones and anticyclones carrying pure South Atlantic origin water are identified south of 30°S and west of the Walvis Ridge. Tropical Atlantic water at depth is found for cyclones north of the Walvis Ridge, west of 10°E and for stations deeper than 4000 m, and a few anticyclones with the same characteristics are found south of the ridge.     

Role of the Agulhas Current in Indian Ocean circulation and associated heat and freshwater fluxes

A reduced estimate of Agulhas Current transport provides the motivation to examine the sensitivity of Indian Ocean circulation and meridional heat transport to the strength of the western boundary current. The new transport estimate is 70 Sv, much smaller than the previous value of 85 Sv. Consideration of three case studies for a large, medium and small Agulhas Current transport demonstrate that the divergence of heat transport over the Indian Ocean north of 32°S has a sensitivity of 0.08 PW per 10 Sv of Agulhas transport, and freshwater convergence has a sensitivity of 0.03×10⁹ kg s⁻¹ per 10 Sv of transport. Moreover, a smaller Agulhas Current leads to a better silica balance and a smaller meridional overturning circulation for the Indian Ocean. The mean Agulhas Current transport estimated from time-series current meter measurements is used to constrain the geostrophic transport in the western boundary region in order to re-evaluate the circulation, heat and freshwater transports across 32°S. The Indonesian Throughflow is taken to be 12 Sv at an average temperature of 18°C. The constrained circulation exhibits a vertical–meridional circulation with a net northward flow below 2000 dbar of 10.1 Sv. The heat transport divergence is estimated to be 0.66 PW, the freshwater convergence to be 0.54×10⁹ kg s⁻¹, and the silica convergence to be 335 kmol s⁻¹. Meridional transports are separated into barotropic, baroclinic and horizontal components, with each component conserving mass. The barotropic component is strongly dependent on the estimated size of the Indonesian Throughflow. Surprisingly, the baroclinic component depends principally on the large-scale density distribution and is nearly invariant to the size of the overturning circulation. The horizontal heat and freshwater flux components are strongly influenced by the size of the Agulhas Current because it is warmer and saltier than the mid-ocean. The horizontal fluxes of heat and salt penetrate down to 1500 m depth, suggesting that warm and salty Red Sea Water may be involved in converting the intermediate and upper deep waters which enter the Indian Ocean from the Southern Ocean into warmer and saltier waters before they exit in the Agulhas Current.     

Kinematic elements of Antarctic Intermediate Water in the western South Atlantic

The northward flowing Antarctic Intermediate Water (AAIW) is a major contributor to the large-scale meridional circulation of water masses in the Atlantic. Together with bottom and thermocline water, AAIW replaces North Atlantic Deep Water that penetrates into the South Atlantic from the North. On the northbound propagation of AAIW from its formation area in the south-western region of the Argentine Basin, the AAIW progresses through a complex spreading pattern at the base of the main thermocline. This paper presents trajectories of 75 subsurface floats, seeded at AAIW depth. The floats were acoustically tracked, covering a period from December 1992 to October 1996. Discussions of selected trajectories focus on mesoscale kinematic elements that contribute to the spreading of AAIW. In the equatorial region, intermittent westward and eastward currents were observed, suggesting a seasonal cycle of the AAIW flow direction. At tropical latitudes, just offshore the intermediate western boundary current, the southward advection of an anticyclonic eddy was observed between 5^°S and 11°S. Farther offshore, the flow lacks an advective pattern and is governed by eddy diffusion. The westward subtropical gyre return current at about 28°S shows considerable stability, with the mean kinetic energy to eddy kinetic energy ratio being around one. Farther south, the eastward deeper South Atlantic Current is dominated by large-scale meanders with particle velocities in excess of 60 cm s^-1. At the Brazil–Falkland Current Confluence Zone, a cyclonic eddy near 40°S 50°W seems to act as injector of freshly mixed AAIW into the subtropical gyre. In general, much of the mixing of the various blends of AAIW is due to the activity of mesoscale eddies, which frequently reoccupy similar positions.     

Upper ocean circulation in the western tropical Atlantic in boreal fall 2000

The upper ocean large-scale circulation of the western tropical Atlantic from 11.5°S to the Caribbean in November and December 2000 is investigated from a new type of shipboard ADCP able to measure accurate velocities to 600 m depth, combined with lowered ADCP measurements. Satellite data and numerical model output complement the shipboard measurements to better describe the large-scale circulation. In November 2000 the North Brazil Undercurrent (NBUC) was strongly intensified between 11 and 5°S by inflow from the east, hence the NBUC was formed further to the north than in the mean. The NBUC was transporting 23.1 Sv northward at 5°S, slightly less than the mean of six cruises (Geophysical Research Letters (2002) 29 (7) 1840). At 35°W the North Brazil Current (NBC) transported 29.4 Sv westward, less than the mean of 13 cruises (Geophysical Research Letters (2003) 30 (7) 1349). A strong retroflection ring had just pinched off the NBC retroflection according to the satellite information. The inflow into the Caribbean south of 16.5°N originated in part of a leakage from the NBC retroflection zone and in part from the North Equatorial Current. A thermocline intensified ring with a transport of about 30 Sv was located off Guadeloupe carrying South Atlantic Central Water towards the north. Observed deviations of the November/December 2000 flow field from the November long-term mean flow field were related to an enhanced Intertropical Convergence Zone (ITCZ) associated with an increased North Equatorial Countercurrent (NECC), as well as to boundary current rings and Rossby waves with zonal wavelength of the order of 1000 km. At 44°W the presence of a Rossby wave associated with an anticyclonic circulation led to a strongly enhanced NBC of 65.0 Sv as well as to a combined NECC and Equatorial Undercurrent transport of 52.4 Sv, much stronger than during earlier cruises. While the 1/3°-FLAME model is unable to reproduce details of the vertical distribution of the observed horizontal flow at 44 °W for November 2000 as well as the horizontal distribution of some of the observed permanent current bands, a climatological simulation with the 1/12°-FLAME agrees much better with the observations and provides information on the spreading path between the sections. E.g., the interpretation that the widening in the Antarctic Intermediate Water layer of the westward flowing NBC at 44°W in November was caused by water from the Equatorial Intermediate Current was further supported by the model results.     

A North Atlantic deep-water eddy in the Agulhas Current system

One hundred and twelve stations of CTDO2 and LADCP were collected in the Agulhas Current system as part of the Agulhas Undercurrent experiment (AUCE) in March 2003. Along an offshore section, at approximately 35.6°S and 27.3°E to the northwest of the tip of the Agulhas Plateau, an unusual feature was revealed between 2200 and 3500 m depth, imbedded in the northward moving NADW layer. An anomalously high salinity of 34.83, 0.03 saltier than the surrounding water, was observed. Maximums in the potential temperature and oxygen were also found, with isotherms dropping by about 250 m over 50 km and a doming of the oxygen layers. From the convex lens structure of the neutral surfaces, we conclude that we sampled an anticyclonic eddy of NADW. Since the LADCP data reveal deep velocities up to 20 cm s⁻¹, yet no anticyclonic circulation, whereas the geostrophic velocity referenced to the bottom shows a weak anticyclonic circulation, we inferred that we sampled the outer edge of the eddy and not its core. From an analysis of the water properties within the eddy and a comparison with known properties in the SE Atlantic Ocean and SW Indian Ocean, we conclude that the eddy was formed in the Agulhas Retroflection region. We speculate that the eddy was the result of an instability in the NADW slope current, which flows from the SE Atlantic around the Agulhas Bank. A deeply penetrating Agulhas Ring spun up the deep waters, pinching off an eddy, which later detached from the slope current and was carried southward. Once offshore, it coupled with the surface Agulhas Return Current, whose meandering path advected the eddy northeastward and ejected it over the Agulhas Plateau.     

Red Sea Intermediate Water in the source regions of the Agulhas Current

Red Sea Intermediate Water (RSIW) has been shown to move down the Agulhas Current as distinct lenses. It has been assumed that this intermittency is the result of variable input. To clarify and quantify the nature of RSIW contributions from the source regions of the Agulhas Current observations at 15 hydrographic sections were examined using a multi-parameter analysis. In the northern Mozambique Channel RSIW is found to be layer-like, but with patches of distinctly different contributions. In the southern part of the channel the layer-like distribution disappears with RSIW mostly confined within anticyclonic and cyclonic eddies exhibiting varying maximum contributions ranging from 15–20% to 25–30% purity. Net transports across the channel ranged from ?0.45 to ?0.7 Sv. At the southern tip of Madagascar RSIW contributions exhibited similar purity variability ranging from 10–15% to 15–20%. The net southward transport of RSIW in the East Madagascar Current displayed an even greater variability due to changes in the flux of the undercurrent ranging from negligible to ?0.3 Sv. Indications therefore were that the transport of RSIW to the Agulhas Current occurs in both cyclones and anti-cyclones through the Mozambique Channel whilst from the East Madagascar Current it is mostly confined to anti-cyclones. This variability in the inflow was also reflected in the northern part of the Agulhas Current proper. The maximum contributions of RSIW range here from 10–15% to 20–25% purity and net transports from ?0.75 to ?1.39 Sv off Durban. As it was east of Madagascar RSIW was mostly confined to the slope.     

Float trajectories in the deep western boundary current and deep equatorial jets of the tropical Atlantic

Fourteen neutrally buoyant SOFAR floats at a nominal depth of 1800 m were tracked acoustically for 3.7 yr in the vicinity of the western boundary and the equator of the Atlantic Ocean. The trajectories revealed a swift, narrow, southward-flowing deep western boundary current (DWBC) extending from 7N across the equator. Two floats crossed the equator in the DWBC and went to 10S. Two other floats left the DWBC and drifted eastward in the equatorial band (3S–3N). Three floats entered the DWBC from the equatorial current system and drifted southward. These results suggest that at times the DWBC flows directly southward across the equator with a mean velocity of 8–9 cm/s averaged over long distances (∼2800 km). At other times DWBC water is diverted eastward near the equator for long periods (2–3 yr), which can reduce the mean along-boundary velocity to 1–2 cm/s. This is much less than the instantaneous along-boundary velocities in the DWBC, which are often above 25 cm/s and occasionally exceed 50 cm/s. Mean eastward-flowing jets were observed near 2N and 2S bounding a mean westward jet centered on the equator (1S–1N). The southern jet at 2S coincides with a CFC-rich plume centered south of the equator. The CFC plume is inferred to have been advected by the southern jet across the Atlantic and into the Gulf of Guinea.     

Dianeutral mixing,transformation and transport of Antarctic Intermediate Water in the South Atlantic Ocean

Recently obtained World Ocean Circulation Experiment (WOCE) sections combined with a specially prepared pre-WOCE South Atlantic data set are used to study the dianeutral (across neutral surface) mixing and transport achieving Antarctic Intermediate Water (AAIW) being transformed to be part of the North Atlantic Deep Water (NADW) return cell. Five neutral surfaces are mapped, encompassing the AAIW from 700 to 1100 db at the subtropical latitudes.Coherent and significant dianeutral upwelling is found in the western boundary near the Brazil coast north of the separation point (about 25°S) between the anticyclonic subtropical and cyclonic south equatorial gyres. The magnitude of dianeutral upwelling transport is 10^-3 Sv (1 Sv=10⁶ m³ s^-1) for 1°×1° square area. It is found that the AAIW sources from the southwestern South Atlantic and southwestern Indian Ocean do not rise significantly into the Benguela Current. Instead, they contribute to the NADW return formation by dianeutral upwelling into the South Equatorial Current. In other words, the AAIW sources cannot obtain enough heat/buoyancy to rise until they return to the western boundary region but north of the separation point. The basin-wide integration of dianeutral transport shows net upward transports, ranging from 0.25 to 0.6 Sv, across the lower and upper boundary of AAIW north of 40°S. This suggests that the equatorward AAIW is a slow rising water on a basin average. Given one order of uncertainty in evaluating the along-neutral-surface and dianeutral diffusivities from the assumed values, K=10³ m² s^-1 and D=10^-5 m² s^-1, the integrated dianeutral transport has an error band of about 10–20%. The relatively weak integrated dianeutral upwelling transport compared with AAIW in other oceans implies much stronger lateral advection of AAIW in the South Atlantic.Mapped Turner Angle in diagnosing the double-diffusion processes shows that the salty Central Water can flux salt down to the upper half of AAIW layer through salt-fingering. Therefore, the northward transition of AAIW can gain salt either through along-neutral-surface advection and diffusion or through salt fingering from the Central Water and heat through either along-neutral-surface advection and diffusion or dianeutral upwelling. Cabbeling and thermobaricity are found significant in the Antarctic frontal zone and contribute to dianeutral downwelling with velocity as high as −1.5×10^-7 m s^-1. A schematic AAIW circulation in the South Atlantic suggests that dianeutral mixing plays an essential role in transforming AAIW into NADW return formation.     

Spatio-temporal characteristics of the Agulhas Current retroflection

A 12.7-year series of weekly absolute sea surface height (SSH) data in the region south of Africa is used for a statistical characterization of the location of the Agulhas Current retroflection and its variations at periods up to 2 years. The highest probability of presence of the retroflection point is at ～39.5°S/18–20°E. The longitudinal probability density is negatively skewed. A sharp eastward decrease at 22°E is related to detachments of the Agulhas Current from the continental slope at this longitude. The asymmetry in the central part of the distribution might reflect a westward increase of the zonal velocity of the retroflection point during its east–west pulsations. The western tail of the distribution reveals larger residence times of the retroflection at 14°E–15°E, possibly related to a slowing down of its westward motion by seamounts. While the averaged zonal velocity component of the retroflection point increases westward, its modulus exhibits an opposite trend, the result of southward velocity components more intense in the northeastern Agulhas Basin than farther west. These meridional motions likely reflect influences by cyclones adjacent to the Agulhas Current south of the Agulhas Bank, and farther west in the Cape Basin. In the latter area, variations of the meridional motions result in different positions of the westernmost retroflection patterns relative to the neighbouring seamounts, likely influencing the future behaviour of Agulhas rings shed at these locations. Agulhas ring formation at an average yearly rate of 5.8, similar to previous findings, was observed to occur west of ～19°E, in the western half of the retroflection probability domain. A well-defined seasonal signal of the retroflection longitude was found throughout the first 5 years of the time series, characterized by amplitudes of 1–1.3° of longitude, and western (eastern) extremes during austral summer (winter). This annual cycle was strongly phase shifted during and after the upstream retroflection event of 2000–2001.     

Caribbean current variability and the influence of the Amazon and Orinoco freshwater plumes

The variability of the Caribbean Current is studied in terms of the influence on its dynamics of the freshwater inflow from the Orinoco and Amazon rivers. Sea-surface salinity maps of the eastern Caribbean and SeaWiFS color images show that a freshwater plume from the Orinoco and Amazon Rivers extends seasonally northwestward across the Caribbean basin, from August to November, 3–4 months after the peak of the seasonal rains in northeastern South America. The plume is sustained by two main inflows from the North Brazil Current and its current rings. The southern inflow enters the Caribbean Sea south of Grenada Island and becomes the main branch of the Caribbean Current in the southern Caribbean. The northern inflow (14°N) passes northward around the Grenadine Islands and St. Vincent. As North Brazil Current rings stall and decay east of the Lesser Antilles, between 14°N and 18°N, they release freshwater into the northern part of the eastern Caribbean Sea merging with inflow from the North Equatorial Current. Velocity vectors derived from surface drifters in the eastern Caribbean indicate three westward flowing jets: (1) the southern and fastest at 11°N; (2) the center and second fastest at 14°N; (3) the northern and slowest at 17°N. The center jet (14°N) flows faster between the months of August and December and is located near the southern part of the freshwater plume. Using the MICOM North Atlantic simulation, it is shown that the Caribbean Current is seasonally intensified near 14°N, partly by the inflow of river plumes. Three to four times more anticyclonic eddies are formed during August–December, which agrees with a pronounced rise in the number of anticyclonic looper days in the drifter data then. A climatology-forced regional simulation embedding only the northern (14°N) Caribbean Current (without the influence of the vorticity of the NBC rings), using the ROMS model, shows that the low salinity plume coincides with a negative potential vorticity anomaly that intensifies the center jet located at the salinity front. The jet forms cyclones south of the plume, which are moved northwestward as the anticyclonic circulation intensifies in the eastern Caribbean Sea, north of 14°N. Friction on the shelves of the Greater Antilles also generates cyclones, which propagate westward and eastward from 67°W.     

The seasonal intrusion of relatively saline water on the shelf off northern and central Namibia

The surface and subsurface waters of the Angola and Agulhas Current systems significantly influence the Benguela region and its living resources, and it is probable that the movement of Central Water, which plays a key role in the coastal upwelling process, is controlled by circulation of underlying Antarctic Intermediate Water (AIW) as well as by the dynamics of the overlying subtropical water. The movement of AIW can be inferred from a study of the t-s characteristics, and the data holdings and data base of the South African Data Centre for Oceanography facilitated this investigation. Key findings of the investigation, some confirming earlier theories and hypotheses, are as follows. The mean depth of the AIW core in the South-East Atlantic is 750 m, and in the South-West Indian Ocean, 1 100 m. Agulhas Current AIW, which is modified by Red Sea Water, becomes fresher en route because of entrainment and mixing of water from the south and west. Most of the Agulhas Current AIW per se retroflects east of 18°E. A poleward movement of AIW along the West Coast to around 32°S may be inferred from the salinity and oxygen data, with a freshening en route analogous to the Agulhas Current. Relatively fresh AIW (s < 34,35 × 10^?3) is present off the South-Western Cape, the only part of the Benguela where the overlying virgin Central Water upwells. No statistically significant seasonal differences could be resolved.     

Near-surface circulation of the northeast Pacific Ocean derived from WOCE-SVP satellite-tracked drifters

Statistics of the near-surface circulation in the northeast Pacific Ocean were derived from the trajectories of nearly 100 surface drifters tracked between August 1990 and December 1995 as part of the World Ocean Circulation Experiment's (WOCE) Surface Velocity Program (SVP). Drifters were drogued within the mixed layer (15 m drogue depth) or near the top of the permanent halocline (120 m). All branches of the Alaskan Gyre were well-sampled at both depths, revealing a weak Subarctic Current, a bifurcation of the Subarctic Current near 48°N, 130°W at 15 m depth, and strong, variable flow in the Alaska Current and Alaskan Stream. At 120 m depth, northward flow in the Alaska Current occurred much farther offshore than within the mixed layer. The drifter trajectories revealed interannual variability, with evidence of an intensified Alaskan Gyre during the winters of 1991–92 and 1992–93 and more southerly transport during winter 1994–95. A minimum in eddy kinetic energy was found at both depths within the northern branch of the Subtropical Gyre. Eddy kinetic energies were nearly twice as high in the mixed layer compared to below, and were 2–3 times larger in winter than in summer throughout most of the near-surface Alaskan Gyre. High eddy energies observed near the eastern perimeter of the Alaskan Gyre may be due to the offshore intrusion of eddies formed by coastal current instabilities.Taylor's theory of single-particle dispersion was applied to the drifter ensembles to estimate Lagrangian decorrelation scales and eddy diffusivities. Both the initial dispersion and random walk regimes were identified in the dispersion time series computed for several regions of both ensembles. The integral time scales and eddy diffusivities computed from the dispersion scale linearly with r.m.s. velocity, which is consistent with drifter studies from the Atlantic. An exception is the meridional integral time scales, which were nearly constant throughout the study area and at both drogue depths. The magnitudes of the derived eddy statistics are comparable to those derived from surface drifters in other parts of the world ocean. These are the first Lagrangian estimates of particle dispersion over a broad region of the near-surface North Pacific, and the consistency of the results with previous studies from the Atlantic lends credence to the idea that the simplifying assumptions of Taylor (1921) (Proceedings of the London Mathematical Society Series A 20, 196–221) are reasonably valid throughout the upper ocean. This bodes well for the effective parameterization of near-surface diffusivities in general circulation models. Finally, the drifter-derived velocity statistics were used to speculate on the source regions of waters of possible coastal origin observed at offshore stations during the field studies of the Canadian Joint Global Ocean Flux Study.     

Year-long float trajectories in the Labrador Sea Water of the eastern North Atlantic Ocean

Year-long Lagrangian trajectories within the Labrador Sea Water of the eastern North Atlantic Ocean are analysed for basic flow statistics. Root-mean-square velocities at 1750 m depth are about 2 cm/s, except within the North Atlantic Current, where they are twice as large. These values are consistent with previous Eulerian measurements and extend those results to a much larger domain of the eastern basin. Mean flow estimates in boxes large enough to contain about 1 float-year of data indicate that Labrador Sea Water, having crossed the Mid- Atlantic Ridge (not resolved) near 50–55^°N, presumably with the North Atlantic Current, partially recirculates to the north in the subpolar gyre, as well as entering the subtropical gyre and continuing south and west. The circulation of this water mass, as defined by the 1 yr average velocities, is stronger than traditional models of deep circulation would suggest, with an interior flow of roughly 1 cm/s. Mean speeds up to 3 cm/s were observed, with the highest values near the Azores Plateau. North of 45°N–55°N, mean eastward speeds closer to 0.2 cm/s were observed. Wind-generated barotropic fluctuations may be responsible for some part of the transport at this depth.     

Tracing the North Atlantic Deep Water through the Romanche and Chain fracture zones with chlorofluoromethanes

Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatorial Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deeper maximum around 3800 m at level of the Lower North Atlantic Deep Water (LNADW). These observations suggest a bifurcation at the western boundary, some of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core (σ_1.5=34.70 kg m^-3) was observed eastward as far as 5°W. The deep CFM maximum (σ₄=45.87 kg m^-3), associated with an oxygen maximum, decreased dramatically at the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water “ages” computed from the F-11/F-12 ratio are estimated. Near the bottom, no enrichment in CFMs is detected at the entrance of the fracture zones in the cold water mass originating from the Antarctic Bottom Water flow.     

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 ±20×10⁶ m³ s⁻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.     

Dianeutral mixing,transformation and transport of the deep water of the Indian Ocean

The realization of North Atlantic Deep Water (NADW) replacement in the deep northern Indian Ocean is crucial to the “conveyor belt” scheme. This was investigated with the updated 1994 Levitus climatological atlas. The study was performed on four selected neutral surfaces, encompassing the Indian deep water from 2000 to 3500 m. The Indian deep water comprises three major water masses: NADW, Circumpolar Deep Water (CDW) and North Indian Deep Water (NIDW). Since NADW flowing into the southwest Indian Ocean is largely blocked by the ridges (the Madagascar Ridge in the east and Davie Ridge in the north in the Mozambique Channel) and NIDW is the only source in the northern Indian Ocean that cannot provide a large amount of volume transport, CDW has to be a major source for the Indian deep circulation and ventilation in the north. Thus the question of NADW replacement becomes that of how the advective flows of CDW from the south are changed to be upwelled flows in the north—a water-mass transformation scenario. This study considered various processes causing motion across neutral surfaces. It is found that dianeutral mixing is vital to achieve CDW transformation. Basin-wide uniform dianeutral upwelling is detected in the entire Indian deep water north of 32°S, somewhat concentrated in the eastern Indian Ocean on the lowest surface. However, the integrated dianeutral transport is quite low, about a net of 0.2 Sv (1 Sv=10⁶ m³ s^-1) across the lowermost neutral surface upward and 0.4 Sv across the uppermost surface upward north of 32°S with an error band of about 10–20% when an uncertainty of half-order change in diffusivities is assumed. Given about 10–15% of rough ridge area where dianeutral diffusivity could be about one order of magnitude higher (10^-4 m² s^-1) due to internal-wave breaking, the additional amount of increased net dianeutral transport across the lowest neutral surface is still within that error band. The averaged net upward transport in the north is matched with a net downward transport of 0.3 Sv integrated in the Southern Ocean south of 45°S across the lowermost surface. With the previous works of You (1996. Deep Sea Research 43, 291–320) in the thermocline and You (Journal of Geophysical Research) in the intermediate water combined, a schematic dianeutral circulation of the Indian Ocean emerges. The integrated net dianeutral upwelling transport shows a steady increase from the deep water to the upper thermocline (from 0.2 to 4.6) north of 32°S. The dianeutral upwelling transport is accumulated upward as the northward advective transport provided from the Southern Ocean increases. As a result, the dianeutral upwelling transport north of 32°S can provide at least 4.6 Sv to south of 32°S from the upper main thermocline, most likely to the Agulhas Current system. This amount of dianeutral upwelling transport does not include the top 150–200 m, which may contribute much more volume transport to the south.     

Interhemispheric exchanges of mass and heat in the Atlantic Ocean in January–March 1993

Hydrographic, geochemical, and direct velocity measurements along two zonal (7.5°N and 4.5°S) and two meridional (35°W and 4°W) lines occupied in January–March, 1993 in the Atlantic are combined in an inverse model to estimate the circulation. At 4.5°S, the Warm Water (potential temperature θ>4.5°C) originating from the South Atlantic enters the equatorial Atlantic, principally at the western boundary, in the thermocline-intensified North Brazil Undercurrent (33±2.7×10⁶ m³ s⁻¹ northward) and in the surface-intensified South Equatorial Current (8×10⁶ m³ s⁻¹ northward) located to the east of the North Brazil Undercurrent. The Ekman transport at 4.5°S is southward (10.7±1.5×10⁶ m³ s⁻¹). At 7.5°N, the Western Boundary Current (WBC) (17.9±2×10⁶ m³ s⁻¹) is weaker than at 4.5°S, and the northward flow of Warm Water in the WBC is complemented by the basin-wide Ekman flow (12.3±1.0×10⁶ m³ s⁻¹), the net contribution of the geostrophic interior flow of Warm Water being southward. The equatorial Ekman divergence drives a conversion of Thermocline Water (24.58⩽σ₀<26.75) into Surface Water (σ₀<24.58) of 7.5±0.5×10⁶ m³ s⁻¹, mostly occurring west of 35°W. The Deep Water of northern origin flows southward at 7.5°N in an energetic (48±3×10⁶ m³ s⁻¹) Deep Western Boundary Current (DWBC), whose transport is in part compensated by a northward recirculation (21±4.5×10⁶ m³ s⁻¹) in the Guiana Basin. At 4.5°S, the DWBC is much less energetic (27±7×10⁶ m³ s⁻¹ southward) than at 7.5°N. It is in part balanced by a deep northward recirculation east of which alternate circulation patterns suggest the existence of an anticyclonic gyre in the central Brazil Basin and a cyclonic gyre further east. The deep equatorial Atlantic is characterized by a convergence of Lower Deep Water (45.90⩽σ₄<45.83), which creates an upward diapycnal transport of 11.0×10⁶ m³ s⁻¹ across σ₄=45.83. The amplitude of this diapycnal transport is quite sensitive to the a priori hypotheses made in the inverse model. The amplitude of the meridional overturning cell is estimated to be 22×10⁶ m³ s⁻¹ at 7.5°N and 24×10⁶ m³ s⁻¹ at 4.5°S. Northward heat transports are in the range 1.26–1.50 PW at 7.5°N and 0.97–1.29 PW at 4.5°S with best estimates of 1.35 and 1.09 PW.     

An inverse modeling study in Fram Strait. Part I: dynamics and circulation

In order to reconstruct the circulation in the northern Greenland Sea, between 77°N and 81°N, and the exchanges with the Arctic Ocean through Fram Strait, a variational inverse model is applied to the density field observed in summer 1984 during the MIZEX 84 experiment. An estimate of the three-dimensional large-scale pressure field is obtained in which the solution is decomposed into a limited number of vertical modes and the mode amplitudes are described by piece-wise polynomials on a finite-element grid. The solution should be consistent with a frictional depth-integrated vorticity balance and with the density data. The global model parameters are tuned to ensure agreement between the retrieved geostrophic velocity and independent currentmeter data. In a companion paper (Schlichtholz and Houssais, 1999b), the same method, but without dynamical constraint, is applied to the same hydrographic dataset to perform a detailed water mass analysis and to estimate individual water mass transports.A comprehensive picture of the summer geostrophic circulation in Fram Strait is obtained in which northward recirculations in the East Greenland Current (EGC) and various recirculations from the West Spitsbergen Current (WSC) to the EGC are identified. It is suggested that the branch of the WSC following the upper western slope of the Yermak Plateau turns westward beyond 81°N and recirculates southward along the lower slope, then merging with a westward recirculating branch south of 79°N. At 79°N, a southward net transport of 6.5 Sv is found in the EGC which, combined with a northward net transport of only 1.5 Sv in the WSC, results in a fairly large outflow of 5 Sv from the Arctic Ocean to the Greenland Sea.The inverse solutions show that, in summer, the local induction of vorticity by the wind stress curl or by meridional advection of planetary vorticity should be small, so that, in the EGC and in the WSC, the vorticity balance is mainly achieved between the bottom pressure torque and dissipation of vorticity through bottom friction. A substantial barotropic flow associated with along-slope potential energy gradients is indeed identified on both sides of the strait.