Antarctic Intermediate Water Flow Research Articles

Astronomically‐Paced Changes in Paleoproductivity, Winnowing, and Mineral Flux Over Broken Ridge (Indian Ocean) Since the Early Miocene

AbstractA significant shift in Earth's climate characterizes the Neogene, transitioning from a single‐ice‐sheet planet to the current bipolar configuration. This climate evolution is closely linked to changing ocean currents, but globally‐distributed continuous high‐resolution sedimentary records are needed to fully capture this interaction. The Ocean Drilling Program (ODP) Site 752, located on Broken Ridge in the Indian Ocean, provides such a Miocene‐to‐recent archive. We use X‐ray fluorescence (XRF) core scanning to build an eccentricity‐tuned age‐depth model and reconstruct paleoceanographic changes since 23 Ma. We find two intervals of enhanced productivity, during the early and middle Miocene (18.5–13.7 Ma) and late Pliocene/early Pleistocene (3–1 Ma). We also report a mixed eccentricity‐obliquity imprint in the XRF‐derived paleoproductivity proxy. In terms of grain size, three coarsening steps occur between 19.2–16 Ma, 10.8–8 Ma, and since 2.6 Ma. The steps respectively indicate stronger current winnowing in response to vigorous Antarctic Intermediate Water flow over Broken Ridge in the early Miocene, the first transient onset of Tasman Leakage in the Late Miocene, and the intensification of global oceanic circulation at the Plio‐Pleistocene transition. High‐resolution iron and manganese series provide a detailed Neogene dust record. This study utilized a single hole from an ODP legacy‐site. Nevertheless, we managed to provide novel perspectives on past Indian Ocean responses to astronomical forcing. We conclude that Neogene sediments from Broken Ridge harbor the potential for even more comprehensive reconstructions. Realizing this potential necessitates re‐drilling of these sedimentary archives utilizing modern drilling strategies.

Diatomaceous ooze in a sedimentary core from Mariana Trench: implications for paleoceanography

Diatomaceous ooze sampled from near the Mariana Trench sediment surface by gravity corer (Core JL7KGC05) revealed a high sedimentary abundance of Ethmodiscus rex (Rattray, 1890) Wiseman and Hendey, 1953 fragments and tropical open ocean planktonic diatom taxa including Azpeitia nodulifera (Schmidt, 1878) Fryxell and Watkins in Fryxell, Sims and Watkins, 1986 and Alveus marinus (Grunow, 1880) Kaczmarska and Fryxell, 1996. Subsurficial sediments from the ooze are assigned a Marine Isotope Stage 2 age, approximately at the Last Glacial Maximum. The occurrence of Ethmodiscus ooze suggests massive late Pleistocene blooms in the Northwestern Pacific Ocean and provides a plausible link to paleoceanographic and paleoclimatic changes related to Antarctic Intermediate Water mass, which carried a high dissolved silica content as silicon leakage that reduced dissolution rate of diatom frustules. Northward flow of Antarctic Intermediate Water was probably related to surface current migration and southward shift of the Northwest Pacific Gyre to form oligotrophic conditions that triggered Ethmodiscus rex blooms under unusual nutrient recycling conditions within the ocean system. This bloom hypothesis may help explain differential silica dissolution during the last glacial stage.

Holocene changes in Antarctic Intermediate Water flow strength in the Southwest Atlantic

Antarctic Intermediate Water (AAIW) is an essential component of the Atlantic meridional overturning circulation (AMOC) contributing to balance the southward flow of North Atlantic Deep Water (NADW). However, the role of AAIW in Holocene abrupt climate changes remains poorly understood. Here we reconstruct changes in the flow strength of AAIW based on a high temporal resolution paleocurrent record from the Southwest Atlantic. Superimposed on a slight increase in AAIW strength at ~7ka BP, a succession of millennial-scale AAIW variations is recognized in our paleocurrent records indicating a highly variable intermediate water circulation throughout the Holocene. Although variations in the strength and position of the Southern Westerlies Winds (SWW) are proposed to greatly influence the formation and circulation of AAIW, we cannot confirm such a potential SWW-AAIW linkage since our records of AAIW flow strength do not correlate to Holocene shifts of the SWW across the Atlantic. However, our data shows a good correspondence with abrupt variations in the AMOC with enhanced (reduced) northward advection of AAIW during periods of reduced (enhanced) NADW circulation. These results provide evidence for a Holocene AAIW-NADW see-saw. Thus, although the exact forcing mechanism remains unresolved, we suggest that Holocene perturbations in AAIW exerted a significant impact on the AMOC.

Centennial to millennial-scale changes in oxygenation and productivity in the Eastern Tropical South Pacific during the last 25,000 years

Oxygen minimum zones (OMZ) have expanded in all tropical oceans during the last 50 years resulting in habitat contraction and considerable changes in marine biogeochemistry. However, for a better understanding of the OMZ dynamics under the current climate change, two questions are relevant: 1) how do the magnitude and temporal changes in oceanic dissolved oxygen of the last few decades compare to the natural variability on longer timescales, and 2) what were the local and remote factors driving OMZ changes in the past. In the present study we use a stacked record covering the last 25 kyr from the Eastern Tropical South Pacific (ETSP) OMZ to reconstruct changes in oxygenation and productivity. We use a suite of proxies including the presence of laminations, redox sensitive metals (U, Mo, Re, Ni and Cu), total organic carbon and δ15N measurements. Water column denitrification and sediment redox conditions show pronounced centennial to millennial-scale variability during the last 25 kyr, with oxygenation levels as low as at present. Global cold periods at different timescales such as the Last Glacial Maximum (23–19 kyr BP) and the Little Ice Age (1500–1850 AD) were associated with a weak OMZ and low export production, while warm intervals such as the deglaciation, part of the Medieval Climate Anomaly and the last 100 years are associated with a stronger OMZ and high export production. Water column denitrification and sediment redox conditions were strongly coupled during the last 25 kyr BP apart from one remarkable exception: during the Antarctic Cold Reversal, sediments were less reducing but the water column denitrification was high resulting in a strong but shallow OMZ. This may have been produced by an enhanced Antarctic Intermediate Water flow. Contrary to our expectations and modeling predictions for the next few decades, we observe a weak ETSP-OMZ during the warm mid-Holocene, which may have been the result of a stronger Walker Circulation that brought oxygen-rich waters to intermediate depths off Peru via Equatorial undercurrents. In combination with other paleoceanographic reconstructions, our results show that oxygenation variability in the ETSP-OMZ was influenced by ocean circulation changes in the Tropical Pacific, high latitude oceanographic and climatic changes, and local productivity.

Erosional and depositional contourite features at the transition between the western Scotia Sea and southern South Atlantic Ocean: links with regional water-mass circulation since the Middle Miocene

The aim of the present study was to characterise the morpho-sedimentary features and main stratigraphic stacking pattern off the Tierra del Fuego continental margin, the north-western sector of the Scotia Sea abyssal plain (Yaghan Basin) and the Malvinas/Falkland depression, based on single- and multi-channel seismic profiles. Distinct contourite features were identified within the sedimentary record from the Middle Miocene onwards. Each major drift developed in a water depth range coincident with a particular water mass, contourite terraces on top of some of these drifts being associated with interfaces between water masses. Two major palaeoceanographic changes were identified. One took place in the Middle Miocene with the onset of Antarctic Intermediate Water flow and the enhancement of Circumpolar Deep Water (CDW) flow, coevally with the onset of Weddell Sea Deep Water flow in the Scotia Sea. Another palaeoceanographic change occurred on the abyssal plain of the Yaghan Basin in the Late Miocene as a consequence of the onset of Southeast Pacific Deep Water flow and its complex interaction with the lower branch of the CDW. Interestingly, these two periods of change in bottom currents are coincident with regional tectonic episodes, as well as climate and Antarctic ice sheet oscillations. The results convincingly demonstrate that the identification of contourite features on the present-day seafloor and within the sedimentary record is the key for decoding the circulation of water masses in the past. Nevertheless, further detailed studies, especially the recovery of drill cores, are necessary to establish a more robust chronology of the evolutionary stages at the transition between the western Scotia Sea and the southern South Atlantic Ocean.

Meridional overturning transports at 7.5N and 24.5N in the Atlantic Ocean during 1992–93 and 2010–11

Transatlantic hydrographic sections along latitudes 7.5N and 24.5N have been repeated with about 20years difference, at the beginning of the 1990s and 2010s. For each period, an inverse model is applied to the closed box bound by both sections. The model imposes mass conservation for individual layers, defined by isoneutral surfaces, and the whole water column, using surface Ekman transport and several transport constraints for specific ranges of longitudes and depths. As a result, the velocities at the reference layer for each station pair and the dianeutral velocities between layers are estimated, and the horizontal velocity fields and the water, heat and freshwater transports are calculated; in particular, we find that mass transport per stratum at 24.5N in 2011 is in good agreement with the transport estimates from the RAPID-Watch array. During both realizations the dianeutral velocities downwell from the Upper North Atlantic Deep Water (UNADW) to the Lower North Atlantic Deep Water (LNADW) strata, resulting in the merging of the two southward flowing strata at 24.5N into one deep southward-moving stratum at 7.5N. At 24.5N, there is an increase in southward UNADW transport between 1992 and 2011, compensated by a decrease of southward LNADW transport; a descent in the upper limit of the Antarctic Bottom Water (AABW) from 1992 to 2011 is also inferred. The Atlantic Meridional Overturning Circulation (AMOC) is larger in 1992–93 than in 2010–11, decreasing from 24.7±1.7 to 20.1±1.4Sv at 24.5N and from 29.2±1.7 to 16.9±1.5Sv at 7.5N. Much of this decrease arises because of the northward flow of Antarctic Intermediate Water (AAIW), which was much more intense in 1992–93 than in 2010–11. As a consequence, heat transport at 24.5N is not significantly different in 1992 (1.4±0.1 PW) and 2011 (1.2±0.1 PW). The estimation of heat transport at 7.5N strongly depends on the magnitude of the North Brazil Current over the American continental platform. The freshwater flux into the box bounded by the two transoceanic sections decreases significantly from 1992–93 (−0.45±0.05Sv) to 2010–11 (−0.36±0.3Sv), meaning a decrease in net evaporation over this same area.

Decreased influence of Antarctic intermediate water in the tropical Atlantic during North Atlantic cold events

Antarctic Intermediate Water (AAIW) is a key player in the global ocean circulation, contributing to the upper limb of the Atlantic Meridional Overturning Circulation (AMOC), and influencing interhemispheric heat exchange and the distribution of salinity, nutrients and carbon. However, the deglacial history of AAIW flow into the North Atlantic is controversial. Here we present a multicore-top neodymium isotope calibration, which confirms the ability of unclean foraminifera to faithfully record bottom water neodymium isotopic composition (εNd) values in their authigenic coatings. We then present the first foraminifera-based reconstruction of εNd from three sediment cores retrieved from within modern AAIW, in the western tropical North Atlantic. Our records reveal similar glacial and interglacial contributions of AAIW, and a pronounced decrease in the AAIW fraction during North Atlantic deglacial cold episodes, Heinrich Stadial 1 (HS1) and Younger Dryas (YD). Our results suggest two separate phases of reduced fraction of AAIW in the tropical Atlantic during HS1, with a greater reduction during early HS1. If a reduction in AAIW fraction also reflects reduced AMOC strength, this finding may explain why, in many regions, there are two phases of hydrologic change within HS1, and why atmospheric CO2 rose more rapidly during early than late HS1. Our result suggesting less flow of AAIW into the Atlantic during North Atlantic cold events contrasts with evidence from the Pacific, where intermediate-depth εNd records may indicate increased flow of AAIW into the Pacific during the these same events. Antiphased εNd behavior between intermediate depths of the North Atlantic and Pacific implies that the flow of AAIW into Atlantic and Pacific seesawed during the last deglaciation.

Vertical structure, energetics, and dynamics of the Brazil Current System at 22°S–28°S

We use four current meter moorings and quasi‐synoptic hydrographic observations in conjunction with a one‐dimensional quasi‐geostrophic linear stability model to investigate downstream changes in the Brazil Current (BC) System between 22°S and 28°S. The data set depict the downstream thickening of the BC. Its vertical extension increases from 350 m at 22.7°S to 850 m at 27.9°S. Most of this deepening occurs between 25.5°S and 27.9°S and is linked to the bifurcation of the South Equatorial Current at intermediate depths (Santos bifurcation), which adds the Antarctic Intermediate Water flow to the BC. Geostrophic estimates suggest that the BC transport is increased by at least 4.3 Sv (∼70%) to the south of that bifurcation. Moreover, the Santos bifurcation is associated with a substantial increase in the barotropic component of the BC System. On average, the water column average kinetic energy (IKE) is 70% baroclinic to the north and 54% barotropic to the south of the bifurcation. Additionally, the BC shows conspicuous mesoscale activity off southeast Brazil. The water column average eddy kinetic energy accounts for 30–60% of the IKE. Instabilities of the mean flow may give rise to these mesoscale fluctuations. Indeed, the linear stability analysis suggests that the BC System is baroclinically unstable between 22°S and 28°S. In particular, the model predicts southwestward‐propagating fastest growing waves (∼190 km) from 25.5°S to 27.9°S and quasi‐standing most unstable modes (∼230 km) at 22.7°S. These modes have vertical structures roughly consistent with the observed eddy field.

Evidence from the high‐latitude North Atlantic for variations in Antarctic Intermediate water flow during the last deglaciation

We present evidence that the characteristic chemical signature (based on coupled benthic foraminiferal Cd/Ca and δ13C) of Antarctic Intermediate waters (AAIW) penetrated throughout the intermediate depths of the Atlantic basin to the high‐latitude North Atlantic during the abrupt cooling events of the last deglaciation: Heinrich 1 and the Younger Dryas. AAIW may play the dynamic counterpart to the “bipolar seesaw” when near‐freezing salty bottom waters from the Antarctic (AABW) sluggishly ventilate the deep ocean. Our data reinforce the concept that interglacial circulation is stabilized by salinity feedbacks between salty northern sourced deep waters (NADW) and fresh southern sourced waters (AABW and AAIW). Further, the glacial ocean may be susceptible to the more finely balanced relative densities of NADW and AAIW, due to either freshwater input or a reversal of the salinity gradient, such that the ocean is poised for NADW collapse via a negative salinity feedback. The unstable climate of the glacial period and its termination may arise from the closer competition for ubiquity at intermediate depths between northern and southern sourced intermediate waters.

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.

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.

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.

Flow and recirculation of Antarctic Intermediate Water across the Rio Grande Rise

The flow of the low‐salinity Antarctic Intermediate Water (AAIW) at 700–1150 m depth across the Rio Grande Rise and the lower Santos Plateau is studied under the auspices of the World Ocean Circulation Experiment (WOCE) in the context of the Deep Basin Experiment. Our data set consists of several hydrographic sections, a collection of 15 RAFOS float trajectories, and records from 14 moored current meters. The data were gathered during different intervals between 1990 and 1994. The inferred flow field strongly supports a basinwide anticyclonic recirculation cell in the subtropical South Atlantic underneath the wind‐driven gyre. Its center, which appears to be southeast of the Rio Grande Rise, separates the eastward advection of AAIW below the South Atlantic Current from the westward flowing, recirculating AAIW. The two near‐shelf limbs closing the circumference of AAIW flow are formed in the east by the deep Benguela Current, potentially modulated by salty inflow of Indian Ocean Intermediate Water, and in the west by the Brazil Current system. Further important circulation elements are the Brazil‐Falkland (Malvinas) Confluence Zone at 40°S and an unnamed divergence at 28°S close to the 1000 m isobath. The resulting broad southward flow of AAIW augments the share of modified, i.e., saltier, intermediate water in the source region of the South Atlantic Current, while the smaller northward flow marks the source of a narrow equatorward Western Intermediate Boundary Current, ultimately leaving the South Atlantic. This shelf‐trapped jet is clearly documented in hydrographic data from 19°S and in nearby current meter records. The jet contrasts a sluggish flow across this latitude east of 35°W. A continuous flow of AAIW from its subpolar region in the southwestern Argentine Basin all along the western slope toward the equator appears unlikely between 35°S and 25°S.

South Atlantic circulation in a world ocean model

Abstract. The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agreement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather barotropic Antarctic Circumpolar Current (ACC) through the Drake Passage, and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates of the mass transport shed westward at the Agulhas retroflection. Topographic steering of the ACC dominates the structure of flow in the circumpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and fresher Subantarctic surface water (originating in the ACC). The Benguela Current is seen to modify its flow and fate with depth; near the surface it flows north-westwards bifurcating most of its transport northward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the water column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the Benguela Current extension encounters the coast of South America. At intermediate levels, these northward currents trace the flow of Antarctic Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the model's rather coarse resolution, some subtle features of the Brazil-Malvinas Confluence are simulated rather well, including the latitude at which the two currents meet. Conceptual diagrams of the recirculation and interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and isobaric surfaces. This analysis shows how the return path of NADW is partitioned between a cold water route through the Drake Passage (6.5 Sv), a warm water route involving the Agulhas Current sheeding thermocline water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).

South Atlantic circulation in a world ocean model

The circulation in the South Atlantic Ocean has been simulated within a global ocean general circulation model. Preliminary analysis of the modelled ocean circulation in the region indicates a rather close agreement of the simulated upper ocean flows with conventional notions of the large-scale geostrophic currents in the region. The modelled South Atlantic Ocean witnesses the return flow and export of North Atlantic Deep Water (NADW) at its northern boundary, the inflow of a rather barotropic Antarctic Circumpolar Current (ACC) through the Drake Passage, and the inflow of warm saline Agulhas water around the Cape of Good Hope. The Agulhas leakage amounts to 8.7 Sv, within recent estimates of the mass transport shed westward at the Agulhas retroflection. Topographic steering of the ACC dominates the structure of flow in the circumpolar ocean. The Benguela Current is seen to be fed by a mixture of saline Indian Ocean water (originating from the Agulhas Current) and fresher Subantarctic surface water (originating in the ACC). The Benguela Current is seen to modify its flow and fate with depth; near the surface it flows north-westwards bifurcating most of its transport northward into the North Atlantic Ocean (for ultimate replacement of North Atlantic surface waters lost to the NADW conveyor). Deeper in the water column, more of the Benguela Current is destined to return with the Brazil Current, though northward flows are still generated where the Benguela Current extension encounters the coast of South America. At intermediate levels, these northward currents trace the flow of Antarctic Intermediate Water (AAIW) equatorward, though even more AAIW is seen to recirculate poleward in the subtropical gyre. In spite of the model's rather coarse resolution, some subtle features of the Brazil-Malvinas Confluence are simulated rather well, including the latitude at which the two currents meet. Conceptual diagrams of the recirculation and interocean exchange of thermocline, intermediate and deep waters are constructed from an analysis of flows bound between isothermal and isobaric surfaces. This analysis shows how the return path of NADW is partitioned between a cold water route through the Drake Passage (6.5 Sv), a warm water route involving the Agulhas Current sheeding thermocline water westward (2.5 Sv), and a recirculation of intermediate water originating in the Indian Ocean (1.6 Sv).

The Southward Intrusion of North Pacific Intermediate Water along the Mindanao Coast

Abstract A tongue of low salinity intermediate water was observed along the coast of Mindanao during the WEPOCS III experiment in June and July 1988. The tongue, delineated by a discontinuity in θ–S relations, is a southward intrusion of water at 26–27 σθ. It is the Northern Hemisphere counterpart of the northward flow of Antarctic Intermediate Water in the New Guinea Coastal Undercurrent. In the WEPOCS III data, it is seen entering the Celebes Sea near 5°N at the southern tip of Mindanao. Once it passes the southern tip of Mindanao, it circulates within the Celebes Sea. Relatively fresh water at 26.55 σθ is seen continuing to flow toward the Makassar Strait and into the Indonesian Throughflow, although some fraction mixes with intermediate water of equatorial Pacific and South Pacific origin and flows eastward in the northern subsurface countercurrent. The tongue is present in a number of sections along 8°N in the Mindanao Current, but it is often wider than is found in the WEPOCS III sections as measure...

A view of the Canary Basin thermocline circulation in winter

During January and February 1989 the recirculation of the subtropical gyre in the eastern North Atlantic was surveyed with a three‐ship experiment. The analysis of hydrographic measurements and velocity data from a shipboard acoustic Doppler current profiler reveals the synoptic‐scale circulation patterns and water mass distributions in the Canary Basin. The geostrophic transport stream function estimated with a horizontally varying reference level of no motion highlights the major currents in three layers representing the vertical structure of the horizontal circulation. The classical circulation scheme is shown by the stream function in the upper 200 m: the Azores, Canary, and North Equatorial currents. Unlike the deep‐penetrating Azores Current, the Canary Current and the North Equatorial Current are restricted to the upper 200 m. Both carry North Atlantic Central Water along the water mass boundary with South Atlantic Central Water. South Atlantic Central Water flows through the passage between the Cape Verde archipelago and Africa via narrow currents into the area north of 14.5°N. At the southern edge of the subtropical gyre we identify an eastward flow of Antarctic Intermediate Water between 700 and 1200 m.

Southern Ocean circulation and global climate in the Middle Pleistocene (early Brunhes)

Compilation of published data has enabled the reconstruction of the Southern Ocean circulation regime in the Middle Pleistocene between approx. 0.7 and 0.4 m.y. B.P. (early Brunhes). Possible links with northern hemisphere climate are indicated. These data show that Southern Ocean circulation was characterized by: (1) an enhanced Antarctic Circumpolar Current; (2) expansion of Antarctic Surface Water as well as; (3) Antarctic Bottom Water, but by; (4) a reduction of Antarctic Intermediate Water flow at levels of its present depth stratum. It is inferred that at the same time northern hemisphere atmospheric circulation was strongly zonal and displaced to the north. The pattern suggests an upper layer flow in the Atlantic with Gulf Stream waters characterized by lowered T/S values. North Pacific surface waters are concluded to have had higher salinities. In the Southern Ocean a stronger Circumpolar Current can primarily be related to intensified atmospheric circulation associated with an increased ocean to atmosphere heat and moisture flux. Together with enhanced Antarctic Bottom Water formation the latter conditions are indicative of low pycnocline stability. Sea ice formation around Antarctica was probably retarded. Enhanced Southern Ocean convection is assumed to have been linked to a change of the North Atlantic Deep Water T/S signature. A lowering of North Atlantic Deep Water salinity is thought to have originated from lowered T/S values characterizing Gulf Stream waters as well as from increased sinkwater formation in a more ice-free Arctic Ocean. A larger Antarctic ice volume to be ascribed to increased precipitation is proposed to explain lower interglacial sea level stands reported. In addition to generally enhanced convection, larger floating ice shelves may also have favoured Antarctic Bottom Water formation. Interglacial 18O enrichment observed in oxygen isotope records prior to stage 9 (11) is considered to reflect such an increased ice volume on Antarctica. Not only the equatorial region, but also the mid-latitudes of the northern hemisphere are concluded to have had a climatic regime marked by increased moisture starvation.

Vertical structure of the Brazil Current

Recent interest in world climate and interaction of the ocean and atmosphere have led to studies of the meridional fluxes of fresh water and heat in the ocean1,2. Several such studies3,4 have noted the asymmetry of heat fluxes between the North and South Atlantic oceans. In particular, the fluxes across both 24° N and 24° S appear to be northward. Crucial to all of the direct calculation techniques is an accurate estimate of the transport of the western boundary currents. In the North Atlantic, useful measurements in the Florida Current5 over a long period of time are available. For the much less studied South Atlantic no comparable time series has been made. The direct measurements of Brazil Current velocities near 23° S reported here show southward flow of warm water above 400 m depth and northward flow of Antarctic Intermediate Water below that. The upper layer transport was ∼6 × 106 m3 s−1 towards the south offshore of the 200-m isobath, with an indication of comparable flow on the shelf.