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.     

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.     

On the circulation of bottom water in the region of the Vema Channel

The circulation and transport of Antarctic Bottom Water (σ₄<45.87) in the region of the Vema Channel are studied along three WOCE hydrographic lines, the geostrophic velocities referenced to previously published direct current measurements. The primary supply of water to the deep Vema Channel is from the Argentine Basin's deep western boundary current, with no indication of an inflow from the southeast. In the northern Argentine Basin, detachment of lower North Atlantic Deep Water from the continental slope is associated with a deep thermohaline front near 34°S. To the north of this front, the upper part of the AABW bound for the Vema Channel (σ₄<46.01) exhibits a significant NADW influence. Further modification of the throughflow water occurs near 30°30′S, where the channel orientation changes by ∼50°. Southward flow of bottom water on the eastern flank of the Vema Channel, amounting to ∼1.5 Sv, represents a significant countercurrent to the deep channel transport. Inclusion of this countercurrent reduces the net flow of AABW through the Vema Channel from 3.2±0.7 to 1.7±1.1 Sv. Water properties imply that the near-zero net flow over the Santos Plateau results from a near-closed cyclonic circulation fed by the deep Vema Channel throughflow. A disruption of the northward boundary current in the upper AABW (lower circumpolar water) is required by this flow pattern. The extension of the cyclonic circulation on the Santos Plateau enters the Brazil Basin as a ∼1 Sv flow distinct from the outflow in the Vema Channel Extension (6.2 Sv). The high magnitude of the latter suggests a southward recirculation of bottom water near the western boundary to the north of the region of study.     

Volume transport and distribution of deep circulation at 165°W in the North Pacific

We conducted full-depth hydrographic observations between 8°50′ and 44°30′N at 165°W in 2003 and analyzed the data together with those from the World Ocean Circulation Experiment and the World Ocean Database, clarifying the water characteristics and deep circulation in the Central and Northeast Pacific Basins. The deep-water characteristics at depths greater than approximately 2000 dbar at 165°W differ among three regions demarcated by the Hawaiian Ridge at around 24°N and the Mendocino Fracture Zone at 37°N: the southern region (10–24°N), central region (24–37°N), and northern region (north of 37°N). Deep water at temperatures below 1.15 °C and depths greater than 4000 dbar is highly stratified in the southern region, weakly stratified in the central region, and largely uniform in the northern region. Among the three regions, near-bottom water immediately east of Clarion Passage in the southern region is coldest (θ<0.90 °C), most saline (S>34.70), highest in dissolved oxygen (O₂>4.2 ml l^?1), and lowest in silica (Si<135 μmol kg^?1). These characteristics of the deep water reflect transport of Lower Circumpolar Deep Water (LCDW) due to a branch current south of the Wake–Necker Ridge that is separated from the eastern branch current of the deep circulation immediately north of 10°N in the Central Pacific Basin. The branch current south of the Wake–Necker Ridge carries LCDW of θ<1.05 °C with a volume transport of 3.7 Sv (1 Sv=10⁶ m³ s^?1) into the Northeast Pacific Basin through Horizon and Clarion Passages, mainly through the latter (～3.1 Sv). A small amount of the LCDW flows northward at the western boundary of the Northeast Pacific Basin, joins the branch of deep circulation from the Main Gap of the Emperor Seamounts Chain, and forms an eastward current along the Mendocino Fracture Zone with volume transport of nearly 1 Sv. If this volume transport is typical, a major portion of the LCDW (～3 Sv) carried by the branch current south of the Wake–Necker and Hawaiian Ridges may spread in the southern part of the Northeast Pacific Basin. In the northern region at 165°W, silica maxima are found near the bottom and at 2200 dbar; the minimum between the double maxima occurs at a depth of approximately 4000 dbar (θ～1.15 °C). The geostrophic current north of 39°N in the upper deep layer between 1.15 and 2.2 °C, with reference to the 1.15 °C isotherm, has a westward volume transport of 1.6 Sv at 39–44°30′N, carrying silica-rich North Pacific Deep Water from the northeastern region of the Northeast Pacific Basin to the Northwest Pacific Basin.     

An inverse modeling study in Fram Strait. Part II: Water mass distribution and transports

A water-mass analysis is carried out in Fram Strait, between 77.15 and 81.15°N, based on three-dimensional large-scale potential temperature and salinity distributions reconstructed from the MIZEX 84 hydrographic data collected in summer 1984. Combining these distributions with the geostrophic flow field derived from the same data in a companion paper (Schlichtholz and Houssais, 1999), the heat, fresh water and volume transports are estimated for each of the water masses identified in the strait. Twelve water masses are selected based on their different origins. Among them, the Polar Water (PW) enters Fram Strait from the Arctic Ocean both over the Greenland Slope and over the western slope of the Yermak Plateau. In the Atlantic Water (AW) range, four modes with distinct geographical distributions are indentified. In the Deep Water range, the Eurasian Basin Deep Water (EBDW) is confined to the Lena Trough and to the Molloy Deep area where it is involved in a cyclonic circulation. The warm and shallower mode of the Norwegian Sea Deep Water (NSDW), concentrated to the west, is mainly seen as an outflow from the Arctic Ocean while the cold and deeper mode, essentially observed to the east, enters the strait from the Greenland Sea. Apart from the EBDW, there is a tendency for all water masses of polar origin to flow along the Greenland Slope. The two most abundant water masses, the AW and the NSDW, occupy as much as 67% of the total water volume. The southward net transport of PW through Fram Strait is about 1 Sv at 78.9°N. At the same latitude, the net transport of AW is southward and equal to about 1.7 Sv. Only the transport of the warm mode (AWw) is northward, amounting to 0.2 Sv. The overall net outflow of the Deep Waters to the Greenland Sea is about 2.6 Sv. Two upper water masses, the fresh (AWf) and the cold (AWc) mode of the AW, and one deep-water mass, the NSDW, appear to be produced in the strait, with production rates, between 77.6 and 79.9°N, of about 0.2, 1.0 and 1.7 Sv, respectively. A southward net fresh-water transport through the strait of about 2000 km³ yr⁻¹ (relative to a salinity of 34.93) is mainly due to the PW. The net heat transport relative to −0.1°C is northward, but undergoes a rapid northward decrease, suggesting an area-averaged surface heat loss of 50–100 W m⁻² in the strait.     

Rates of North Atlantic Deep Water formation calculated from chlorofluorocarbon inventories

Chlorofluorocarbon (CFC) inventories provide an independent method for calculating the rate of North Atlantic Deep Water (NADW) formation. From data collected between 1986 and 1992, the CFC-11 inventories for the major components of NADW are: 4.2 million moles for Upper Labrador Sea Water (ULSW), 14.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million moles for Iceland–Scotland Overflow Water (ISOW), and 5.9 million moles for Denmark Strait Overflow Water (DSOW). The inventories directly reflect the input of newly formed water into the deep Atlantic Ocean from the Greenland, Iceland and Norwegian Seas and from the surface of the subpolar North Atlantic during the time of the CFC-11 transient. Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970 and 1990, the formation rates estimated by this method represent an average over this time period. Formation rates based on best estimates of source water CFC-11 saturations are: 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for DSOW. To our knowledge, this is the first calculation for the rate of ULSW formation. The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of CLSW in the central Labrador Sea with a 10 : 1 ratio of high to low rates. The best estimate of these rates are 12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970–1990 time period. The average formation rate for the sum of CLSW, ISOW and DSOW is 15.0 Sv, which is similar to (within our error) previous estimates (which do not include ULSW) using other techniques. Including ULSW, the total NADW formation rate is about 17.2 Sv. Although ULSW has not been considered as part of the North Atlantic thermohaline circulation in the past, it is clearly an important component that is exported out of the North Atlantic with other NADW components.     

Deep-water circulation at low latitudes in the western North Pacific

Full-depth conductivity-temperature-depth-oxygen profiler (CTDO₂) data at low latitudes in the western North Pacific in winter 1999 were analyzed with water-mass analysis and geostrophic calculations. The result shows that the deep circulation carrying the Lower Circumpolar Water (LCPW) bifurcates into eastern and western branch currents after entering the Central Pacific Basin. LCPW colder than 0.98°C is carried by the eastern branch current, while warmer LCPW is carried mainly by the western branch current. The eastern branch current flows northward in the Central Pacific Basin, supplying water above 0.94°C through narrow gaps into an isolated deep valley in the Melanesian Basin, and then passes the Mid-Pacific Seamounts between 162°10′E and 170°10′E at 18°20′N, not only through the Wake Island Passage but also through the western passages. Except near bottom, dissolved oxygen of LCPW decreases greatly in the northern Central Pacific Basin, probably by mixing with the North Pacific Deep Water (NPDW). The western branch current flows northwestward over the lower Solomon Rise in the Melanesian Basin and proceeds westward between 10°40′N and 12°20′N at 150°E in the East Mariana Basin with volume transport of 4.1 Sv (1 Sv=10⁶ m³ s⁻¹). The current turns north, west of 150°E, and bifurcates around 14°N, south of the Magellan Seamounts, where dissolved oxygen decreases sharply by mixing with NPDW. Half of the current turns east, crosses 150°E at 14–15°N, and proceeds northward primarily between 152°E and 156°E at 18°20′N toward the Northwest Pacific Basin (2.1 Sv). The other half flows northward west of 150°E and passes 18°20′N just east of the Mariana Trench (2.2 Sv). It is reversed by a block of topography, proceeds southward along the Mariana Trench, then detours around the south end of the trench, and proceeds eastward along the Caroline Seamounts to the Solomon Rise, partly flowing into the West Mariana and East Caroline Basins. A deep western boundary current at 2000–3000 m depth above LCPW (10.0 Sv) closes to the coast than the deep circulation. The major part of it (8.5 Sv) turns cyclonic around the upper Solomon Rise from the Melanesian Basin and proceeds along the southern boundary of the East Caroline Basin. Nearly half of it proceeds northward in the western East Caroline Basin, joins the current from the east, then passes the northern channel, and mostly enters the West Caroline Basin (4.6 Sv), while another half enters this basin from the southern side (>3.8 Sv). The remaining western boundary current (1.5 Sv) flows over the middle and lower Solomon Rise, proceeds westward, then is divided by the Caroline Seamounts into southern (0.9 Sv) and northern (0.5 Sv) branches. The southern branch current joins that from the south in the East Caroline Basin, as noted above. The northern branch current proceeds along the Caroline Seamounts and enters the West Mariana Basin.     

Deep flow and transport through the Ulleung Interplain Gap in the Southwestern East/Japan Sea

Deep circulation in the southwestern East/Japan Sea through the Ulleung Interplain Gap (UIG), a possible pathway for deep-water exchange, was directly measured for the first time. Five concurrent current meter moorings were positioned to effectively span the UIG between the islands of Ulleungdo to the west and Dokdo to the east. They provided a 495-day time series of deep currents below 1800 m depth spanning the full breadth of the East Sea Deep and Bottom Water flowing from the Japan Basin into the Ulleung Basin. The UIG circulation is found to be mainly a two-way flow with relatively weak southward flows directed into the Ulleung Basin over about two-thirds of the western UIG. A strong, persistent, and narrow compensating northward outflow occurs in the eastern UIG near Dokdo and is first referred to here as the Dokdo Abyssal Current. The width of the abyssal current is about 20 km below 1800 m depth. The low-frequency variability of the transports is dominated by fluctuations with a period of about 40 days for inflow and outflow transports. The 40-day fluctuations of both transports are statistically coherent, and occur almost concurrently. The overall mean transport of the deep water below 1800 m into the Ulleung Basin over the 16.5 months is about 0.005 Sv (1 Sv=10⁶ m³ s^?1), with an uncertainty of 0.025 Sv indicating net transport is negligible below 1800 m through the UIG.     

A late winter hydrographic section from Tasmania to Antarctica

A hydrographic section between Tasmania and Antarctica was occupied in late winter 1991 as part of the World Ocean Circulation Experiment (WOCE). The primary purpose of the WOCE repeat section SR3 is to measure the exchange between the Indian and Pacific Oceans south of Australia. This paper describes the fronts, water masses and transport observed on the first occupation of the repeat section. The Subantarctic Front (SAF) is located between 50°S and 51°S and is the most striking feature of the vertical sections. Two additional fronts at 53°S and 59°S are associated with the Polar Front (PF), part of which turns northward to flow along the section before turning back to the east near 53°S. Very deep (>500 m) mixed layers are found north of the SAF, confirming that Subantarctic Mode Water (SAMW) is formed in this region by deep convection in winter. Chlorofluorocarbons (CFCs) are significantly undersaturated (≈90–92% of equilibrium values) in these deep mixed layers, indicating that gas exchange rates are not rapid enough to bring these deep mixed layers to equilibrium by the end of the winter period of deep convective mixing. Northward Ekman drift of cold, fresh water across the SAF is likely to be responsible for the cooler, fresher mixed layers observed immediately north of the SAF. The Antarctic Intermediate Water (AAIW) on the SR3 section is relatively low in oxygen and CFCs (≈60–70% and 10–20% of saturation values, respectively), high in potential vorticity, and high in nutrients. These characteristics suggest that the AAIW on this section is not renewed by direct and rapid ventilation near this location. Water mass properties suggest that water from the Tasman Sea spreads south and west across the northern portion of the SR3 section between 800 and 3000 m depth. A cold, fresh, CFC-rich variety of Antarctic Bottom Water is formed along the Wilkes-Adelie coast of Antarctica. The net transport across the section relative to the deepest common depth is 160 Sv. The band of eastward flow between 50°S and 53°S including the SAF carries 137 Sv to the east and dominates the net transport. Weaker flow south of 58°S contributes an additional 70 Sv. The eastward flow is compensated in part by 37 Sv of westward flow between Tasmania and 48.5°S and 8 Sv of flow to the west over the southern flank of the mid-ocean ridge. The trajectories of six ALACE floats deployed at about 950 m confirm the sense of flow inferred from the choice of a deep reference level.     

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.     

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.     

Dissolved organic carbon export and subsequent remineralization in the mesopelagic and bathypelagic realms of the North Atlantic basin

Dissolved organic carbon (DOC) data are presented from three meridional transects conducted in the North Atlantic as part of the US Climate Variability (CLIVAR) Repeat Hydrography program in 2003. The hydrographic sections covered a latitudinal range of 6°S to 63°N along longitudes 20°W (CLIVAR line A16), 52°W (A20) and 66°W (A22). Over 3700 individual measurements reveal unprecedented detail in the DOC distribution and systematic variations in the mesopelagic and bathypelagic zones of the North Atlantic basin. Latitudinal gradients in DOC concentrations combined with published estimates of ventilation rates for the main thermocline and North Atlantic Deep Water (NADW) indicate a net DOC export rate of 0.081 Pg C yr⁻¹ from the epipelagic zone into the mesopelagic and bathypelagic zones. Model II regression and multiple linear regression models applied to pairwise measures of DOC and chlorofluorocarbon (CFC-12) ventilation age, retrieved from major water masses within the main thermocline and NADW, indicate decay rates for exported DOC ranging from 0.13 to 0.94 μmol kg⁻¹ yr⁻¹, with higher DOC concentrations driving higher rates. The contribution of DOC oxidation to oxygen consumption ranged from 5 to 29% while mineralization of sinking biogenic particles drove the balance of the apparent oxygen utilization.     

The deep-water motion through the Lifamatola Passage and its contribution to the Indonesian throughflow

In order to estimate the contribution of cold Pacific deep water to the Indonesian throughflow (ITF) and the flushing of the deep Banda Sea, a current meter mooring has been deployed for nearly 3 years on the sill in the Lifamatola Passage as part of the International Nusantara Stratification and Transport (INSTANT) programme. The velocity, temperature, and salinity data, obtained from the mooring, reflect vigorous horizontal and vertical motion in the lowest 500 m over the ～2000 m deep sill, with speeds regularly surpassing 100 cm/s. The strong residual flow over the sill in the passage and internal, mainly diurnal, tides contribute to this bottom intensified motion. The average volume transport of the deep throughflow from the Maluku Sea to the Seram Sea below 1250 m is 2.5 Sv (1 Sv=10⁶ m³/s), with a transport-weighted mean temperature of 3.2 °C. This result considerably increases existing estimates of the inflow of the ITF into the Indonesian seas by about 25% and lowers the total mean inflow temperature of the ITF to below 13 °C. At shallower levels, between 1250 m and the sea surface, the flow is directed towards the Maluku Sea, north of the passage. The typical residual velocities in this layer are low (～3 cm/s), contributing to an estimated northward flow of 0.9–1.3 Sv. When more results from the INSTANT programme for the other Indonesian passages become available, a strongly improved estimate of the mass and heat budget of the ITF becomes feasible.     

The influence of the south-west monsoon upon the nutrient biogeochemistry of the Arabian Sea

Variations in the nutrient concentrations were studied during two cruises to the Arabian Sea. The situation towards the end of the southwest monsoon season (September/October 1994) was compared with the inter-monsoonal season during November and December 1994. Underway surface transects showed the influence of an upwelling system during the first cruise with deep, colder, nutrient-rich water being advected into the surface mixed layer. During the southwesterly monsoon there was an area of coastal Ekman upwelling, bringing colder water (24.2°C) into the surface waters of the coastal margin. Further offshore at about 350 km there was an area of Ekman upwelling, as a result of wind-stress curl, north of the Findlater Jet axis; this area also had cooler surface water (24.6°C). Further offshore (>1000 km) the average surface temperatures increased to >27°C. These waters were oligotrophic with no evidence of the upwelling effects observed further inshore. In the upwelling regions nutrient concentrations in the close inshore coastal zone were elevated (NO₃=18 μmol l^-1, PO₄=1.48 μmol l^-1); higher concentrations also were measured at the region of offshore upwelling off the shelf, with a maximum nitrate concentration of 12.5 μmol l^-1 and a maximum phosphate concentration of 1.2 μmol l^-1. Nitrate and phosphate concentrations decreased with increasing distance offshore to the oligotrophic waters beyond 1400 km, where typical nitrate concentrations were 35.0 nmol l^-1 (0.035 μmol l^-1) in the surface mixed layer. A CTD section from the coastal shelf, to 1650 km offshore to the oligotrophic waters, clearly showed that during the monsoon season, upwelling is one of the major influences upon the nutrient concentrations in the surface waters of the Arabian Sea off the coast of Oman. Productivity of the water column was enhanced to a distance of over 800 km offshore. During the intermonsoon period a stable surface mixed layer was established, with a well-defined thermocline and nitracline. Surface temperature was between 26.8 and 27.4°C for the entire transect from the coast to 1650 km offshore. Nitrate concentrations were typically between 2.0 and 0.4 μmol l^-1 for the transect, to about 1200 km where the waters became oligotrophic, and nitrate concentrations were then typically 8–12 nmol l^-1. Ammonia concentrations for the oligotrophic waters were typically 130 nmol l^-1, and are reported for the first time in the Indian Ocean. The nitrogen/phosphorus (N/P) ratios suggest that phytoplankton production was potentially nitrogen-limited in all the surface waters of the Arabian Sea, with the greatest nitrogen limitation during the intermonsoon period.     

Observation of Luzon Strait transport in summer 2007

A field experiment was conducted across the Luzon Strait in July 2007, and a total of 51 profiles covering variables of horizontal velocity, temperature, salinity, and pressure were collected at 11 stations. Using this observation, the volume transport through the Luzon Strait, its differences between July 2007 and October 2005, and the distribution of subtidal flow and geostrophic flow have been investigated. The net transport has a two-layer vertical structure, which is eastward both in the upper layer (<26 kg m^?3 σ₀), and in the intermediate layer (26–27.3 kg m^?3 σ₀), while it is westward in the deeper layer (>27.3 kg m^?3 σ₀), with respective values of 3.0, 4.0, and ?1.5 Sv. The net transport is eastward, and estimated to be 5.5 Sv. The distribution of the subtidal flow in the intermediate layer shows that a westward flow exists in the northern part, countered by an eastward flow existing in the southern part of the strait. This distribution is in direct contrast to the previous results obtained in October 2005, in which a westward flow occurs in the south countered by an eastward flow in the north in the intermediate layer. This suggests that the flow pattern varies greatly from October 2005 to July 2007 not only in the upper layer but also in the intermediate layer. The deep layer, on the other hand, shows few changes between the two observation periods.     

Agulhas leakage into the Atlantic estimated with subsurface floats and surface drifters

Surface drifters and subsurface floats drifting at depths near 800 m were used to study the pathways of warm, salty Indian Ocean water leaking into the South Atlantic that is a component of the upper limb of the Atlantic meridional overturning circulation (MOC). Four drifters and 5 floats drifted from the Agulhas Current directly into the Benguela Current. Others looped for various amounts of time in Agulhas rings and cyclones, which translated westward into the Atlantic, contributing a large part of Indian Ocean leakage. Agulhas rings translated into the Benguela Current, where they slowly decayed. Some large, blob-like Agulhas rings with irregular shapes were found in the southeastern Cape Basin. Drifter trajectories suggest these rings become more circular with time, eventually evolving into the circular rings observed west of the Walvis Ridge. Agulhas cyclones, which form on the north side of the Agulhas Current south of Africa, translated southwestward (to 6°E) and contributed water to the southern Cape Basin. A new discovery is a westward extension from the mean Agulhas retroflection measured by westward drifting floats near 41°S out to at least 5°W, with some floats as far west as 25°W. The Agulhas extension appears to split the South Atlantic Current (SAC) into two branches and to transport Agulhas water westward, where it is mixed and blended with eastward-flowing water from the western Atlantic. The blended mixture flows northeastward in the northern branch of the SAC and into the Benguela Current. Agulhas leakage transport was estimated from drifters and floats to be at least 15 Sv in the upper 1000 m, which is equivalent to the transport of the upper layer MOC. It is suggested that the major component of the upper layer overturning circulation in the Atlantic is Agulhas leakage in the form of Agulhas rings.     

Latitudinal distribution of microbial plankton abundance,production, and respiration in the Equatorial Atlantic in autumn 2000

Phytoplankton and bacterial abundance, size-fractionated phytoplankton chlorophyll-a (Chl-a) and production together with bacterial production, microbial oxygen production and respiration rates were measured along a transect that crossed the Equatorial Atlantic Ocean (10°N–10°S) in September 2000, as part of the Atlantic Meridional Transect 11 (AMT 11) cruise. From 2°N to 5°S, the equatorial divergence resulted in a shallowing of the pycnocline and the presence of relatively high nitrate (>1 μM) concentrations in surface waters. In contrast, a typical tropical structure (TTS) was found near the ends of the transect. Photic zone integrated ¹⁴C primary production ranged from ∼200 mg C m⁻² d⁻¹ in the TTS region to ∼1300 mg C m⁻² d⁻¹ in the equatorial divergence area. In spite of the relatively high primary production rates measured in the equatorial upwelling region, only a moderate rise in phytoplankton biomass was observed as compared to nearby nutrient-depleted areas (22 vs. 18 mg Chl-a m⁻², respectively). Picophytoplankton were the main contributors (>60%) to both Chl-a biomass and primary production throughout the region. The equatorial upwelling did not alter the phytoplankton size structure typically found in the tropical open ocean, which suggests a strong top-down control of primary producers by zooplankton. However, the impact of nutrient supply on net microbial community metabolism, integrated over the euphotic layer, was evidenced by an average net microbial community production within the equatorial divergence (1130 mg C m⁻² d⁻¹) three-fold larger than net production measured in the TTS region (370 mg C m⁻² d⁻¹). The entire region under study showed net autotrophic community metabolism, since respiration accounted on average for 51% of gross primary production integrated over the euphotic layer.     

Atlantic inflow to the Nordic Seas: current structure and volume fluxes from moored current meters,VM-ADCP and SeaSoar-CTD observations, 1995–1999

This study deals with the inflow of warm and saline Atlantic water to the Nordic Seas, an important factor for climate, ecology and biological production in Northern Europe. The investigations are carried out along the Svinøy standard hydrographic section, which cuts through the Atlantic inflow to the Norwegian Sea just to the north of the Faroe–Shetland Channel. In the Svinøy section, we consider the Atlantic inflow as water with salinity above 35.0, corresponding to temperatures above 5°C. Current measurements for the period April 1995 to February 1999, positioned on the continental slope in water depths between 490 and 990 m, are combined with VM-ADCP, SeaSoar-CTD and CTD transects to estimate long-term transports and spatial features of the Atlantic inflow. A well-defined two-branched Norwegian Atlantic Current was revealed with an eastern and a western branch. The eastern branch appears as a narrow, topographically trapped, near barotropic, 30–50 km wide current, with a maximum speed of 117 cm/s. The western branch is also about 30–50 km wide, and appears as an unstable frontal jet about 400 m deep with a maximum speed of 87 cm/s. Between these two prominent branches, the observations show an average eddy field with a recirculation to the southwest. Transport estimates from the current records in the eastern branch show an annual mean inflow of 4.2 Sv (1 Sv=10⁶ m³/s) with variation on a 25 h time scale ranging from −2.2 to 11.8 Sv, and between 2.0 and 8.0 Sv on a monthly time scale. The current record in the core of the eastern branch mirrors the estimated transport on a monthly time scale with a correlation coefficient of 0.86. Except for the year 1995–1996, this nearly four-year current record shows evidence of a systematic annual cycle with summer to winter variations in the proportion of 1 to 2. Comparison between the North Atlantic Oscillation (NAO) index and the current record on a three-month time scale shows a strong connection for most of the period. This reflects the strong coupling between the westerly winds and the inflow. The baroclinic transport west of the eastern branch, including the frontal jet, is inferred from hydrography in combination with VM-ADCP transects, and has a total mean of 3.4 Sv. Thus, investigations to date indicate a yearly mean Atlantic inflow of 7.6 Sv in the Svinøy section.     

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.