Antarctic Intermediate Water Layer Research Articles

The Origin and Fate of Antarctic Intermediate Water in the Southern Ocean

Abstract Using observationally based hydrographic and eddy diffusivity datasets, a volume budget analysis is performed to identify the main mechanisms governing the spatial and seasonal variability of Antarctic Intermediate Water (AAIW) within the density range γn = (27.25–27.7) kg m−3 in the Southern Ocean. The subduction rates and water mass transformation rates by mesoscale and small-scale turbulent mixing are estimated. First, Ekman pumping upwells the dense variety of AAIW into the mixed layer south of the Polar Front, which can be advected northward by Ekman transport into the subduction regions of lighter-variety AAIW and Subantarctic Mode Water (SAMW). The subduction of light AAIW occurs mainly by lateral advection in the southeast Pacific and Drake Passage as well as eddy-induced flow between the Subantarctic and Polar Fronts. The circumpolar-integrated total subduction yields from −5 to 19 Sv (1 Sv ≡ 106 m3 s−1) of AAIW volume loss. Second, the diapycnal transport from subducted SAMW into the AAIW layer is predominantly by mesoscale mixing (2–13 Sv) near the Subantarctic Front and vertical mixing in the South Pacific, while AAIW is further replenished by transformation from Upper Circumpolar Deep Water by vertical mixing (1–10 Sv). Last, 3–14 Sv of AAIW are exported out of the Southern Ocean. Our results suggest that the distribution of AAIW is set by its formation due to subduction and mixing, and its circulation eastward along the ACC and northward into the subtropical gyres. The volume budget analysis reveals strong seasonal variability in the rate of subduction, vertical mixing, and volume transport driving volume change within the AAIW layer. The nonzero volume budget residual suggests that more observations are needed to better constrain the estimate of geostrophic flow and mesoscale and small-scale mixing diffusivities.

Observational estimates of turbulent mixing in the southeast Indian Ocean

AbstractThis study investigates the spatio-temporal variability of turbulent mixing in the eastern South Indian Ocean using a collection of data from EM-APEX profiling floats, shipboard CTD and microstructure profilers. The floats collected 1566 profiles of temperature, salinity and horizontal velocity data down to 1200 m over a period of about four months. A fine-scale parameterization is applied to the float and CTD data to estimate turbulent mixing. Elevated mixing is observed in the upper ocean, over bottom topography and in mesoscale eddies. Mixing is enhanced in the anticyclonic eddies due to trapped near-inertial waves within the eddy. We found that cyclonic eddies contribute to turbulent mixing in the depth range of 500 – 1000 m, which is associated with downward propagating internal waves. The mean diapycnal diffusivity over 250 – 500 m depth is O(10−6) m2 s−1 and it increases to O(10−5) m2 s−1 in 500 – 1000 m in cyclonic eddies. The turbulent mixing in this region has implications for watermass transformation and large-scale circulation. Higher diffusivity (O(10−5) m2 s−1) is observed in the Antarctic Intermediate Water (AAIW) layer in cyclonic eddies whereas weak diffusivity is observed in the Subantarctic Mode Water (SAMW) layer (O(10−6) m2 s−1). Counter-intuitively, then, the SAMW watermass properties are strongly affected in cyclonic eddies whereas the AAIW layer is less affected. Comparatively high diffusivity at the location of the South Indian Countercurrent (SICC) jets suggests there are wave-mean flow interactions in addition to the wave-eddy interactions that warrant further investigation.

Decoupled Freshwater Transport and Meridional Overturning in the South Atlantic

AbstractFreshwater transports (Fov) by the Atlantic meridional overturning circulation (AMOC) are sensitive to salinity distributions and may determine AMOC stability. However, climate models show large salinity biases, distorting the relation between Fov and the AMOC. Using free‐running models and ocean reanalyses with realistic salinities but quite different AMOCs, we show that the fresh Antarctic Intermediate Water layer eliminates salinity differences across the AMOC branches at ~1,200 m, ∆S1200m, which decouples Fov from the AMOC south of ~10°N. As the Antarctic Intermediate Water disappears north of ~10°N, a large ∆S1200m allows the AMOC to drive substantial southward Fov in the North Atlantic. In the South Atlantic the 0–300 m zonal salinity contrasts control the gyre freshwater transports Fgyre, which also determine the total freshwater transports. This decoupling makes the southern Fov unlikely to play any role in AMOC stability, leaving indirect Fgyre feedbacks or Fov in the north, as more relevant factors.

Freshening of Antarctic Intermediate Water in the South Atlantic Ocean in 2005–2014

Abstract. Basin-scale freshening of Antarctic Intermediate Water (AAIW) is reported to have occurred in the South Atlantic Ocean during the period from 2005 to 2014, as shown by the gridded monthly means of the Array for Real-time Geostrophic Oceanography (Argo) data. This phenomenon was also revealed by two repeated transects along a section at 30° S, performed during the World Ocean Circulation Experiment Hydrographic Program. Freshening of the AAIW was compensated for by a salinity increase of thermocline water, indicating a hydrological cycle intensification. This was supported by the precipitation-minus-evaporation change in the Southern Hemisphere from 2000 to 2014. Freshwater input from atmosphere to ocean surface increased in the subpolar high-precipitation region and vice versa in the subtropical high-evaporation region. Against the background of hydrological cycle changes, a decrease in the transport of Agulhas Leakage (AL), which was revealed by the simulated velocity field, was proposed to be a contributor to the associated freshening of AAIW. Further calculation showed that such a decrease could account for approximately 53 % of the observed freshening (mean salinity reduction of about 0.012 over the AAIW layer). The estimated variability of AL was inferred from a weakening of wind stress over the South Indian Ocean since the beginning of the 2000s, which would facilitate freshwater input from the source region. The mechanical analysis of wind data here was qualitative, but it is contended that this study would be helpful to validate and test predictably coupled sea–air model simulations.

Influence of Antarctic Intermediate Water on the deoxygenation of the Atlantic Ocean

Hydrographic trends in the Antarctic Intermediate Water (AAIW) layer that may be associated with changes in the thickness and oxygen content of oxygen minimum zones (OMZs) in the eastern tropical South Atlantic (ETSA) and eastern tropical North Atlantic (ETNA) are investigated by using historical data (1960 to 2015). Our results reveal that the thickness of these OMZs has continually increased (2.58±0.67myr−1 for the ETSA and 3.37±0.73myr−1 for the ETNA), the mean oxygen concentration has decreased (– 0.12±0.03μmolkg−1yr−1 for the ETSA and − 0.17±0.05μmolkg−1yr−1 for the ETNA), and the mean temperature has increased. The optimum multiparameter analysis method is used to track modifications in the AAIW along its path through the South Atlantic Subtropical Gyre. We observe an AAIW layer vertical expansion rate of 1.67±0.71myr−1, a decrease in the mean oxygen concentration of − 0.18±0.04μmolkg−1yr−1 and an increase in the mean temperature of 0.010±0.005°Cyr−1. Moreover, a similar decrease in oxygen concentrations is observed in the AAIW layer of the studied OMZ regions compared to those in the non-AAIW portions of these OMZs, which indicates strong deoxygenation in this water mass over time. Our results suggest that warming in the AAIW source region and in its extensive temporal displacement through the SASG to the eastern tropical Atlantic Ocean appreciably shifted this water mass toward lower densities with depleted oxygen (increases in ventilation age and oxygen consumption). The warming trend that is reported here suggests that global warming is one of the factors that influence oxygen solubility changes during the deoxygenation and expansion of OMZs.

Diapycnal oxygen supply to the tropical North Atlantic oxygen minimum zone

Abstract. The replenishment of consumed oxygen in the open ocean oxygen minimum zone (OMZ) off northwest Africa is accomplished by oxygen transport across and along density surfaces, i.e. diapycnal and isopycnal oxygen supply. Here the diapycnal oxygen supply is investigated using a large observational set of oxygen profiles and diapycnal mixing data from years 2008 to 2010. Diapycnal mixing is inferred from different sources: (i) a large-scale tracer release experiment, (ii) microstructure profiles, and (iii) shipboard acoustic current measurements plus density profiles. From these measurements, the average diapycnal diffusivity in the studied depth interval from 150 to 500 m is estimated to be 1 × 10−5 m2 s−1, with lower and upper 95% confidence limits of 0.8 × 10−5 m2 s−1 and 1.4 × 10−5 m2 s−1. Diapycnal diffusivity in this depth range is predominantly caused by turbulence, and shows no significant vertical gradient. Diapycnal mixing is found to contribute substantially to the oxygen supply of the OMZ. Within the OMZ core, 1.5 μmol kg−1 yr−1 of oxygen is supplied via diapycnal mixing, contributing about one-third of the total demand. This oxygen which is supplied via diapycnal mixing originates from oxygen that has been laterally supplied within the upper Central Water layer above the OMZ, and within the Antarctic Intermediate Water layer below the OMZ. Due to the existence of a separate shallow oxygen minimum at about 100 m depth throughout most of the study area, there is no net vertical oxygen flux from the surface layer into the Central Water layer. Thus all oxygen supply of the OMZ is associated with remote pathways.

Estimating the storage of anthropogenic carbon in the subtropical Indian Ocean: a comparison of five different approaches

Abstract. The subtropical Indian Ocean along 32° S was for the first time simultaneously sampled in 2002 for inorganic carbon and transient tracers. The vertical distribution and inventory of anthropogenic carbon (CANT) from five different methods: four data-base methods (ΔC*, TrOCA, TTD and IPSL) and a simulation from the OCCAM model are compared and discussed along with the observed CFC-12 and CCl4 distributions. In the surface layer, where carbon-based methods are uncertain, TTD and OCCAM yield the same result (7±0.2 molC m−2), helping to specify the surface CANT inventory. Below the mixed-layer, the comparison suggests that CANT penetrates deeper and more uniformly into the Antarctic Intermediate Water layer limit than estimated from the much utilized ΔC* method. Significant CFC-12 and CCl4 values are detected in bottom waters, associated with Antarctic Bottom Water. In this layer, except for ΔC* and OCCAM, the other methods detect significant CANT values. Consequently, the lowest inventory is calculated using the ΔC* method (24±2 molC m−2) or OCCAM (24.4±2.8 molC m−2) while TrOCA, TTD, and IPSL lead to higher inventories (28.1±2.2, 28.9±2.3 and 30.8±2.5 molC m−2 respectively). Overall and despite the uncertainties each method is evaluated using its relationship with tracers and the knowledge about water masses in the subtropical Indian Ocean. Along 32° S our best estimate for the mean CANT specific inventory is 28±2 molC m−2. Comparison exercises for data-based CANT methods along with time-series or repeat sections analysis should help to identify strengths and caveats in the CANT methods and to better constrain model simulations.

On the role of mesoscale eddies in the ventilation of Antarctic intermediate water

The spatial distribution of Antarctic intermediate water (AAIW) formation and ventilation remains a matter of debate. Some studies suggest that AAIW forms nearly homogeneously in a circumpolar pattern, whereas others favor more localized formation particularly in the southeast Pacific Ocean. We show here that the patterns and magnitude of AAIW formation and ventilation are substantially affected by mesoscale eddies. To diagnose the role of eddies, we made global CFC-11 simulations in two versions of the ocean general circulation model OPA9, a “non-eddying”, coarse-resolution version ( 2 ∘ cos ϕ × 2 ∘ , ORCA2) and an “eddying” or eddy-permitting version ( 1 2 ∘ cos ϕ × 1 2 ∘ , ORCA05). In the non-eddying simulation, AAIW subducts in a near homogeneous, circumpolar pattern; in the eddying simulation, the distribution of AAIW ventilation is patchier. Increasing resolution causes the AAIW layer to thin by 32% on average in the Indian sector, but only by 11% in the Pacific sector. This patchiness appears due to the zonal wind stress, which is weak over much of the Pacific and southwest Atlantic sectors but is strong over the Indian sector. Consequently, the effect of eddies is largest in the Indian Ocean, moderate in the Atlantic, and smallest in the Pacific basin. Although the Gent and McWilliams (GM) eddy parameterization improves the overall vertical structure of density in the Southern Ocean, applying it in our non-eddying model still results in the nearly uniform circumpolar distribution of AAIW ventilation, in contrast to the observations.

Formation and pathways of intermediate water in the Parallel Ocean Circulation Model's Southern Ocean

The formation mechanisms and pathways of intermediate water in the Southern Ocean are analyzed from output of a high‐resolution ocean general circulation model. Deep winter mixed layer formation in the Southern Ocean is diagnosed from the model results and is found to be mostly consistent with observations. Diapycnal water mass transformations by air‐sea fluxes and internal mixing are quantified and split into mean and eddy components. The diapycnal formation of the water masses that constitute the Antarctic intermediate water layer in the southeast Pacific is found to occur mainly in the western Pacific Ocean in this model. In winter, convection up to 900 m is found to set the potential vorticity characteristics of this layer. Eddy fluxes of heat and buoyancy play an important role in the formation of the intermediate waters by transferring water from the southern parts of the subtropical gyres into the Antarctic Circumpolar Current (ACC) and vice versa. The effects of eddy fluxes are found to vary significantly along the path of the ACC. They are strongly concentrated in the regions near the Agulhas Return Current in the Indian Ocean and the Brazil‐Malvinas Confluence in the Atlantic.

Zonal intermediate currents in the equatorial Atlantic Ocean

Acoustic float data collected near 800 m depth, are used to map zonal mean currents within the Antarctic Intermediate Water (AAIW) tongue in the equatorial Atlantic. Alternating zonal jets of 2° latitudinal width are revealed between 6°S and 6°N. Displacements from profiling floats drifting near 1000 m depth, also reveal similar zonal jets at the base of the AAIW layer. The strongest jets (15 cm s−1 peak) are found at 4°S, 2°S, 0°, 2°N and 4°N. They are coherent longitudinally over order of 3000 km and, poleward of 1°S and 1°N, generally coherent vertically between 800 m and 1000 m. Large seasonal fluctuations exist at both levels: within 1° of equator, AAIW at 800 m flows westward (8 cm s−1 mean) in boreal summer and fall but eastward (3 cm s−1 mean) in winter, whereas the flow at 1000 m is eastward in late fall and winter.

Impact of the equatorial deep jets on estimates of zonal transports in the Atlantic

The structure and variability of the zonal equatorial flow in the Atlantic is studied on the basis of velocity profiles obtained with lowered Acoustic Doppler Current Profilers during multiple surveys. The vertical extent of the zonal currents is found to vary considerably. It can be as small as 100 m or as large as 1000 m. In the Atlantic, vertical scales of 400-600 m have been associated with the equatorial deep jets (they are also frequently called deep jets or stacked jets). Typical amplitudes of the zonal velocity are about 20 cm s(-1). An analysis of quasi-synoptic surveys indicates that the zonal extent of most jets is likely to be at least 27D. They can rise or deepen from west to east, although the deepening was observed more often and is often more pronounced. The west to east deepening can be as large as 320 m/10D. Basin-wide mean depth changes of the jets are mostly on the order of 50 m/10D, and the largest depth changes are typically observed between 35D and 23D W. The existence of these changes indicates that vertically propagating, equatorially trapped, waves might be one cause for the jet structure. However, the dependence of the slope on the longitude indicates that other processes must be involved as well. The typical vertical extent of the jets is small enough to result in several direction changes of the zonal flow in the Antarctic Intermediate Water (AAIW) and the North Atlantic Deep Water (NADW) layer. From transport estimates for 14 meridional sections it is found that the transport for the westward component of the flow within the AAIW layer (500-1000 m) can be as large as -24 Sv (1 Sv = 10(6) M, s(-1)) within 1D of the equator. For the eastward component of the flow in the AAIW layer the transport can be as large as 8 Sv. Adding the transport components for each section results in a range of total AAIW transports from -24 to 7 Sv. This suggest that the annual mean transport of AAIW is westward. The only months with eastward total transports are June and July. This is consistent with earlier Lagrangian and some other observations that indicated that the AAIW flow along the equator is governed by an annual cycle. In the NADW layer (1200-3900 m) the transport for the westward (eastward) flow can be as large as -25 Sv (23 Sv) within 1D of the equator. This results in a range of total NADW transports from -10 to 18 Sv. The variations of the total transports of AAIW and NADW are anti-correlated, with a correlation coefficient of -0.86. Since only eight sections reached deep enough to allow transport estimates in the NADW layer it is more difficult to come to a conclusion about the mean transport in this layer than for the transport in the AAIW layer (for the latter layer 14 sections were available). Nevertheless, the obtained estimates suggest that the total NADW transport may be eastward.

Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic

Abstract This study combines float data from different projects collected between 1991 and 2003 in the South Atlantic to describe the flow of Antarctic Intermediate Water (AAIW). Velocity space–time averages are calculated for various grid resolutions and with cells deformed to match the bathymetry, f/H or f/h (with H being the water depth and h being the thickness of the AAIW layer). When judged by the degree of alignment between respective isolines and the resulting average velocity fields, the best grid is based on a nominal cell size of 3° (latitude) by 4° (longitude) with cell shapes deformed according to f/h. Using this grid, objectively estimated mean currents (and their associated errors), as well as meridional and zonal volume transports are estimated. Results show an anticyclonic Subtropical Gyre centred near 36°S and spanning from 23±1°S to 46±1°S. The South Atlantic Current meanders from 33°S to 46°S and shows a mean speed of 9.6±7.8 cm s−1 (8.5±3.5 Sv; 1 Sv=1×106 m3 s−1). The northern branch of the Subtropical Gyre is located between 22°S and 32°S and flows westward with a mean speed of 4.7±3.3 cm s−1 (9.3±3.4 Sv). Evidence of a cyclonic Tropical Gyre divided in two sub-cells is visible on the stream function.

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.

Quantification of the interocean exchange of intermediate water masses around southern Africa

Abstract A large collection of historic and recent hydrographic data has been used in a water-mass mixing model to quantify the remote sources of intermediate water that pass through the region around southern Africa. Special emphasis has been given to deriving the fractional transports of water with Indian Ocean origins that enter the South Atlantic and that thus contribute to the overturning circulation of the Atlantic. The model includes five remote sources: Antarctic Intermediate Water (AAIW) from the Drake Passage (dAAIW), a component of AAIW from the South Indian Ocean (siAAIW), intermediate water from the Indonesian Seas (IIW) and from the Red Sea (RSIW), and a transformed end member of the mixture of these sources (aAAIW). The study is performed on five neutral density surfaces chosen to follow the core of AAIW and to span the AAIW layer, encompassing a depth range between about 700 and 1200 m . To resolve the mixing of water masses from the five remote sources, the six physical–chemical tracers, potential temperature, salinity, dissolved oxygen, silicate, initial phosphate, nitrate (NO) and potential vorticity, have been used as input information. The model-derived mixing fraction gives a quantitative description of the remote sources when they are mapped onto neutral density surfaces. Results show that dAAIW is a dominant source, with a large proportion of its water extending to the southwestern Indian Ocean in a continuous distribution, while the Indian Ocean sources spread to the southeastern South Atlantic mostly in a patchy distribution. Given the mixing fractions, the geostrophic velocity fields and the RAFOS velocity fields, the volume transport can be separated into individual contributions from each source. It is found that dAAIW provides most of the transport (63% or 1.9 Sv , 1 Sv =10 6 m 3 s −1 ) crossing the 25°S section (10°W–14°E). The rest of the equatorward transport is contributed by siAAIW (16% or 0.5 Sv ), by IIW (10% or 0.3 Sv ) and by RSIW (12% or 0.4 Sv ), respectively. The net eastward transport of dAAIW passing south of Africa into the Indian Ocean is estimated to be 13.5 Sv at 20°E (50°S–36°S). Its westward return transport from the Indian Ocean is 2.1 Sv , amounting to about one-seventh of its total transport into the Indian Ocean. The Benguela Current transport from RAFOS observations for the AAIW layer at 6°W (34°S–22°S) is 12.9 Sv westward. Of this 80% is contributed by dAAIW, 11% by siAAIW, 3% by IIW, and 6% by RSIW, respectively. This means that of the 12.9 Sv Benguela Current transport, 10.3 Sv is contributed by dAAIW alone, which is composed of 8.8 Sv directly fed from the South Atlantic and 1.5 Sv returned after a loop in the Indian Ocean. Due to a weak baroclinic shear the corresponding geostrophic transport is 6.5 Sv westward, only half of the RAFOS transport. Of this 80% is contributed by dAAIW, 15% by siAAIW, 2% by IIW and 3% by RSIW.

Quantitative estimate of Antarctic Intermediate Water contributions from the Drake Passage and the southwest Indian Ocean to the South Atlantic

Recently obtained World Ocean Circulation Experiment (WOCE) bottle sections and a pre‐WOCE bottle data set are used in a water mass mixing model. The mixing scheme comprises three intermediate water sources: Antarctic Intermediate Water (AAIW) from the northern Drake Passage, a combination source of the Indian Ocean intermediate waters entering from south of Africa, and a transformed end‐member of the former two sources. I call them dAAIW, iAAIW, and aAAIW, respectively. The dAAIW originates from the southeast South Pacific, enters the South Atlantic in the northern Drake Passage, and is modified in the Falkland Current loop. The iAAIW is a combination of the Indian Ocean sources including Red Sea Intermediate Water, Indonesian Intermediate Water, and AAIW formed locally in the south central Indian Ocean and transformed dAAIW that has returned following a loop through the Indian Ocean. The aAAIW is a transformed end‐member of a mixture of dAAIW and iAAIW located in the eastern tropical South Atlantic, characterized by an oxygen minimum and nutrient maxima. Although aAAIW is not an import source like dAAIW and iAAIW, it spans property fields to extrema as a result of water mass mixing and transformation processes and therefore must be included in the basin‐wide water mass mixing scheme. The study is performed on five neutral surfaces that encompass the AAIW layer from 700 to 1200 dbar in the subtropical latitudes with a distance of about 100 dbar between a pair of surfaces. Four conservative variables of potential temperature, salinity, initial phosphate (PO4o), and NO and one conservative dynamical tracer fN2 (where f is the Coriolis frequency and N2 is the squared buoyancy frequency) are used as input information to the mixing model. The model‐derived mixing fraction gives a quantitative description of AAIW sources when they are mapped onto neutral surfaces. The contoured pattern of mixing fraction shows water mass spreading paths, thus implying circulation and ventilation of AAIW in the South Atlantic. Results show that dAAIW is a dominant water mass and iAAIW is about 30–60% of dAAIW in the subtropical latitudes. With the mixing proportion of AAIW sources derived from the mixing model the geostrophic volume transport and dianeutral upwelling transport can be separated into the individual contributions from each AAIW source. It is found that the percentage of the transport contributed by dAAIW and iAAIW to the South Atlantic is almost constant at 64 ± 2% and 36 ± 2%, respectively. Meridional transport in the subtropical latitudes between 30° and 10°S by dAAIW and iAAIW (referenced to 2000 dbar) has a mean of 4.26 Sv northward (1 Sv = 106 m3 s−1) shared between dAAIW at 2.70 Sv (63%) and iAAIW at 1.56 Sv (37%). The mean zonal transport in the western and eastern South Atlantic between 40° and 3°S is −5.13 Sv westward, shared between dAAIW at −3.38 Sv (66%) and iAAIW at −1.75 Sv (34%). The dianeutral upwelling transport across the uppermost neutral surface σN = 27.25 in the northwest South Atlantic (north of 30°S and west of 10°W) is 2.26 Sv shared between dAAIW at 1.40 Sv (62%) and iAAIW at 0.86 Sv (38%).

On the water masses and mean circulation of the South Atlantic Ocean

We examine recent observations of water mass distribution and circulation schemes at different depths of the South Atlantic Ocean to propose a layered, qualitative representation of the mean distribution of flow in this region. This furthers the simple upper layer geostrophic flow estimates of Peterson and Stramma [1991]. In addition, we assess how well ocean general circulation models (GCMs) capture the overall structure of flow in the South Atlantic in this regard. The South Atlantic Central Water (SACW) is of South Atlantic origin in the subtropical gyre, while the SACW in the tropical region in part originates from the South Indian Ocean. The Antarctic Intermediate Water in the South Atlantic originates from a surface region of the circumpolar layer, especially in the northern Drake Passage and the Falkland Current loop, but also receives some water from the Indian Ocean. The subtropical South Atlantic above the North Atlantic Deep Water and north of the Antarctic Circumpolar Current (ACC) is dominated by the anticyclonic subtropical gyre. In the eastern tropical South Atlantic the cyclonic Angola Gyre exists, embedded in a large tropical cyclonic gyre. The equatorial part of the South Atlantic shows several depth‐dependent zonal current bands besides the Angola Gyre. Ocean GCMs have difficulty capturing this detailed zonal circulation structure, even at eddy‐permitting resolution. The northward extent of the subtropical gyre reduces with increasing depth, located near Brazil at 16°S in the near‐surface layer and at 26°S in the Antarctic Intermediate Water layer, while the tropical cyclonic gyre progresses southward. The southward shift of the northern part of the subtropical gyre is well resolved in global ocean GCMs. However, high horizontal resolution is required to capture the South Atlantic Current north of the ACC. The North Atlantic Deep Water in the South Atlantic progresses mainly southward in the Deep Western Boundary Current, but some water also moves southward at the eastern boundary.

An investigation of the circulation of the Gulf of Mexico

Empirical search and inverse techniques were used to reanalyze historical hydrographic data obtained in the Gulf of Mexico, February 12 to March 31, 1962. From these data, eight transects, which provide basin‐wide coverage of the gulf, were constructed. On the basis of water properties the circulation of the Gulf of Mexico was characterized as a three‐layer, mass‐conservative system. Along most sections an empirical search in density coordinates was sufficient to obtain an optimum reference level that balanced the transport in the three layers. An inverse correction was necessary to obtain a mass balance for those sections that crossed the Loop Current and mesoscale eddies. The density of the optimum reference level (σ1 = 32.17) obtained from this analysis corresponds approximately to the bottom of the Antarctic Intermediate Water layer that exists in the Gulf of Mexico. By using this optimum reference level, total velocity profiles at specific locations in the Gulf of Mexico were obtained. Taken together, these yielded a description of the winter circulation of this region. In general, the large‐scale circulation consists of a clockwise (anticyclonic) gyre which is most prominent in the upper 500 m. Eastward and westward transports associated with the gyre are approximately equal at about 5 × 106 m3 s−1. Variations to this flow are provided by the Loop Current in the eastern gulf and a cyclonic eddy in the northwestern gulf. Transports obtained for these circulation features are approximately 30 × 106 and 8 × 106 m3 s−1, respectively, which are in agreement with values obtained from other hydrographic surveys in the Gulf of Mexico.