And Antarctic Intermediate Water Research Articles

Impacts of Agulhas Current meanders on intermediate water masses along the adjacent continental slope and shelf.

Variability in the Agulhas Current system is dominated by meanders, which constitute cyclonic eddies along the inshore edge of the Current on the southeast coast of South Africa. Few studies have investigated the influence of these meanders on hydrographic variability on the adjacent shelf and slope and to date only a handful have been sampled in situ. This study used available in situ data and GLORYS12v1 model output to investigate the impact of meanders on the distribution of Intermediate waters, namely Red Sea Water (RSW) and Antarctic Intermediate Water (AAIW), as well as mechanisms driving these variations. We focussed on four eddies, sampled in situ during July 1998, April 2010, January–February 2017, and July–August 2017. RSW dominated along the inshore edge of the Agulhas Current in the absence of meanders, but larger proportions of AAIW occurred in the presence of cyclonic eddies. During eddy events, the kinematic steering level was raised above the lower boundary of Intermediate waters, increasing cross-frontal mixing of waters at depths of 800–1800 m. Eddy-induced upwelling of Central and Intermediate waters onto the shelf appeared to be inhibited by bands of strong positive relative vorticity (>0.4 × 10−4 s−1), which likely promoted downwelling conditions inshore of the July 1998, April 2010, and July–August 2017 eddies. Weak positive relative vorticity (<0.2 × 10−4 s−1) inshore of the January–February 2017 eddy was associated with enhanced water mass exchange between the shelf and deeper (>1000 m) ocean. GLORYS12v1 was consistently comparable with satellite and in situ data, and simulated the overall distribution of water masses on the continental shelf and slope despite its inability to reflect the influence of river discharge in nearshore regions during austral summer. The model is thus suitable to investigate the influence of Agulhas Current meanders on the hydrography of South Africa's southeast coast.

Distributions of aluminum, manganese, cobalt, and lead in the western South Pacific: Interplay between the South and North Pacific

Aluminum (Al), manganese (Mn), cobalt (Co), and lead (Pb) are strongly scavenged from seawater. We reported that each element is uniquely related to ocean circulation in the North Pacific (Zheng et al., 2019). Herein, we present the full-depth distributions of these elements in the western South Pacific, which include meridional sections along 170°W (GEOTRACES GP19). We determined dissolved (d) and total dissolvable (td) concentrations using filtered and unfiltered seawater without UV treatment, and we calculated labile particulate (lp) concentrations as the difference between td and d concentrations. This and the previous studies present the basin scale distributions, which enable us to investigate first order processes that drive the biogeochemistry of Al, Mn, Co, and Pb in the Pacific Ocean. The meridional section of dAl along 170°W (GP19)-160°W (GPc06) from 64°S to 54°N indicates that elevated concentrations (maximum 6.1 nmol/kg) occur between 40°S and 10°S from surface to bottom. However, the maxima of lpAl occur at high latitudes. The lpAl/tdAl ratio has a minimum of 0.26 ± 0.12 (ave ± sd, n = 116) in the zone from 30°S to 0°S. Based on these results, we propose a hypothesis that weathering on land has a significant effect on the distribution of Al in the ocean. Intensive weathering on tropical and subtropical islands and Australia forms kaolinite-dominated soils and laterite. This process provides dAl and kaolinite to the ocean. The supply of kaolinite results in kaolinite-dominated sediments that become a major bottom source for dAl. In contrast, strong sources of Mn and Co are continental shelves around the northern boundary. Dissolved Mn and dCo are released from sediments by manganese reduction and carried by intermediate water circulation. In particular, dCo spreads in the North Pacific Intermediate Water (NPIW), Equatorial Pacific Intermediate Water (EqPIW), and Antarctic Intermediate Water (AAIW); 23–59 pmol/kg at a potential density anomaly (σθ) of 27.0. This is partly owing to the uptake of dCo by phytoplankton and remineralization from settling particles. The dPb concentrations are 10 ± 6 pmol/kg (n = 397) in the South Pacific and 30 ± 20 pmol/kg (n = 566) in the North Pacific. The distribution of dPb is characterized by a maximum in the Subtropical Mode Water (SMW) and Central Mode Water (CMW) with σθ∼26 in the North Pacific. These results indicate that Pb is predominantly supplied by anthropogenic aerosols from Asia and Russia to the Pacific Ocean. Because Pb is not actively taken up by phytoplankton, Pb enters mode waters during winter convection and is transported with mode waters.

Multidecadal Sea Level Rise in the Southeast Indian Ocean: The Role of Ocean Salinity Change

AbstractRegional sea level rise in the southeast Indian Ocean (SEIO) exerts growing threats to the surrounding Australian and Indonesian coasts, but the mechanisms of sea level rise have not been firmly established. By analyzing observational datasets and model results, this study investigates multidecadal steric sea level (SSL) rise of the SEIO since the mid-twentieth century, underscoring a significant role of ocean salinity change. The average SSL rising rate from 1960 through 2018 was 7.4 ± 2.4 mm decade−1, and contributions of the halosteric and thermosteric components were ∼42% and ∼58%, respectively. The notable salinity effect arises primarily from a persistent subsurface freshening trend at 400–1000 m. Further insights are gained through the decomposition of temperature and salinity changes into the heaving (vertical displacements of isopycnal surfaces) and spicing (density-compensated temperature and salinity change) modes. The subsurface freshening trend since 1960 is mainly attributed to the spicing mode, reflecting property modifications of the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the southern Indian Ocean. Also noteworthy is a dramatic acceleration of SSL rise (20.3 ± 7.0 mm decade−1) since ∼1990, which was predominantly induced by the thermosteric component (16.3 ± 5.5 mm decade−1) associated with the heaving mode. Enhanced Ekman downwelling by surface winds and radiation forcing linked to global greenhouse gas warming mutually caused the depression of isopycnal surfaces, leading to the accelerated SSL rise through thermosteric effect. This study highlights the complexity of regional sea level rise in a rapidly changing climate, in which the role of ocean salinity is vital and time-varying.

Lagged response of Tropical Atlantic Ocean to cold and fresh water pulse from Antarctic sea ice melting.

The formation of dense water masses at polar regions has been largely influenced by climate changes arising from global warming. In this context, based on ensemble simulations with a coupled model we evaluate the meridional shift of a climate signal (i.e., a cold and fresh water input pulse generated from melting of positive Antarctic sea ice (ASI) extremes) towards the Tropical Atlantic Ocean (TAO). This oceanic signal propagated from Southern Ocean towards the equator through the upper layers due to an increase in its buoyance. Its northward shift has given by the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) flows, that inject cold and fresh mode/intermediate waters from into subtropical basin. The signal has reached low latitudes through the equatorial upwelling and spreads out southwards, through the upper branch of southern subtropical gyre. We concluded that 10 years of coupled simulations was enough time to propagate the climate signal generated by ASI positive extremes melting, which reached TOA around 2 year later. The oceanic connection between Southern Ocean and TAO is indeed established within the timescale analyzed in the study (10 years). Nonetheless, the period needed to completely dissipate the disturbance generated from ASI seems to be longer.

Genesis and Evolution of Ferromanganese Crusts from the Summit of Rio Grande Rise, Southwest Atlantic Ocean

The Rio Grande Rise (RGR) is a large elevation in the Atlantic Ocean and known to host potential mineral resources of ferromanganese crusts (Fe–Mn), but no investigation into their general characteristics have been made in detail. Here, we investigate the chemical and mineralogical composition, growth rates and ages of initiation, and phosphatization of relatively shallow-water (650–825 m) Fe–Mn crusts dredged from the summit of RGR by using computed tomography, X-ray diffraction, 87Sr/86Sr ratios, U–Th isotopes, and various analytical techniques to determine their chemical composition. Fe–Mn crusts from RGR have two distinct generations. The older one has an estimated age of initiation around 48–55 Ma and was extensively affected by post-depositional processes under suboxic conditions resulting in phosphatization during the Miocene (from 20 to 6.8 Ma). As a result, the older generation shows characteristics of diagenetic Fe–Mn deposits, such as low Fe/Mn ratios (mean 0.52), high Mn, Ni, and Li contents and the presence of a 10 Å phyllomanganate, combined with the highest P content among crusts (up to 7.7 wt %). The younger generation is typical of hydrogenetic crusts formed under oxic conditions, with a mean Fe/Mn ratio of 0.75 and mean Co content of 0.66 wt %, and has the highest mean contents of Bi, Nb, Ni, Te, Rh, Ru, and Pt among crusts formed elsewhere. The regeneration of nutrients from local biological productivity in the water column is the main source of metals to crusts, providing mainly metals that regenerate rapidly in the water column and are made available at relatively shallow water depths (Ni, As, V, and Cd), at the expense of metals of slower regeneration (Si and Cu). Additionally, important contributions of nutrients may derive from various water masses, especially the South Atlantic Mode Water and Antarctic Intermediate Water (AAIW). Bulk Fe–Mn crusts from the summit of RGR plateau are generally depleted in metals considered of greatest economic interest in crusts like Co, REE, Mo, Te, and Zr, but are the most enriched in the critical metals Ni and Li compared to other crusts. Further investigations are warranted on Fe–Mn crusts from deeper-water depths along the RGR plateau and surrounding areas, which would less likely be affected by phosphatization.

Variability of bottom carbonate chemistry over the deep coral reefs in the Florida Straits and the impacts of mesoscale processes

Abundant and diverse cold-water coral and fish communities can be found in the deep waters of the Florida Straits, which are believed to be living under suboptimal conditions impacted by increasing oceanic CO2 levels. Yet, little is known regarding the spatial–temporal variability of bottom carbonate chemistry parameters and their dynamic drivers in this area. To address this issue, we present results from numerical simulations of a coupled physical-biogeochemical model for the south Florida shelf and Florida Straits. Our exploratory analysis focuses on two well-known deep-coral habitats: Pourtalès Terrace (200–450 m) and Miami Terrace (270–600 m). Results suggest that bottom waters along the northern/western slope of the Straits are comprised primarily of the North Atlantic Central Water (NWCW) and Antarctic Intermediate Water (AAIW), driven by upwelling associated with the bottom Ekman transport of the Florida Current. Over the Pourtalès Terrace, both the meandering of the Florida Current and mesoscale eddies modulate the upwelling (downwelling) of cold (warm) waters. In contrast, Florida Current makes a sharp turn at the southern end of the Miami Terrace leading to persistent island wakes, frequent occurrences of a transient eddy, and strong upwelling of deep waters toward the platform of the terrace. Passage of the transient eddy often accompanies strong downwelling of warm waters and a return (southward) flow on top of the platform. Overall, bottom water properties including temperature (T), dissolved inorganic carbon (DIC) and total alkalinity (TA) show strong variability on weekly to monthly time-scales over entire Pourtalès Terrace and on the platform of Miami Terrac, mostly driven by physics. In deeper areas (>400 m), bottom water properties are fairly stable with both DIC and TA showing narrow ranges. Interestingly, waters over the southeastern portion of the Pourtalès Terrace show consistently warmer temperature, lower DIC, and higher TA than those on top of this terrace. The aragonite saturation state (Ω) ranges 1.2-2 on top of the Pourtalès Terrace and 1.2-1.7 both on top of Miami Terrace and on the upper slope of Pourtalès Terrace. In the deeper slope areas (>400 m), it is nearly constant at 1.2-1.3. This modeling effort suggests that remote forcing and biogeochemical processes along the transport paths, from the Gulf of Mexico to the Straits, are significant but second-order contributors to the variability of bottom carbonate chemistry. The impacts of benthic biogeochemical processes along the transit paths are not resolved.

The upper, deep, abyssal and overturning circulation in the Atlantic Ocean at 30°S in 2003 and 2011

Mass transports for the thermocline, intermediate, deep and abyssal layers in the Atlantic Ocean, at 30°S and for 2003 and 2011, have been estimated using data from GO-SHIP hydrographic transoceanic sections and applying three inverse models with different constraints. The uppermost layers comprise South Atlantic Central Water (SACW) and Antarctic Intermediate Water (AAIW), with a net northward transport in the range of 12.1–14.7 Sv in 2003 and 11.7–17.7 Sv in 2011, which can be considered as the northward returning limb of the Meridional Overturning Circulation (MOC). The western boundary Brazil Current transports twice as much SACW in 2003 (−20.2 ± 0.7 Sv) than in 2011 (−9.7 ± 0.7 Sv). A poleward current consisting of AAIW and Upper Circumpolar Deep Water (UCDW) flows beneath the Brazil Current. The eastern boundary Benguela Current, characterized by a high mesoscale eddy activity, transports 15.6 ± 0.9 Sv in 2003 and 11.2 ± 0.8 Sv in 2011, east of the Walvis Ridge. In the ocean interior, the northward flow is mainly located east of the Mid Atlantic Ridge (MAR) where Agulhas Rings (ARs), observed in both 2003 and 2011, transport warm and salty water from the Indian to the Atlantic Ocean. For the deep layers, the southward transport of North Atlantic Deep Water (NADW) occurs as the Deep Western Boundary Current and also in the eastern basin. The western and eastern basins transport similar amounts of NADW to the south during both years, although the eastern pathway changes substantially between both years. The total NADW transport, which is also considered the MOC, is in the range 16.3–24.5 Sv in 2003 and 17.1–29.6 Sv in 2011, hence with no significant change.

Regional circulation patterns of Mediterranean Outflow Water near the Iberian and African continental slopes

Abstract. The Mediterranean Outflow Water (MOW) is a dense water mass originated in the Strait of Gibraltar. Downstream of the Gulf of Cádiz, the MOW forms a reservoir region west of the Iberian continental slopes at a buoyant depth of approximately 1000 m. This region plays a key role as the main centre where the MOW is mixed and distributed into the North Atlantic. The seafloor in this area is characterized by the presence of a complex bathymetry with three abyssal plains separated by mountain chains. Although the topographic features do not reach the surface, they influence ocean flows at intermediate and deep ocean layers, conditioning the distribution and circulation of MOW. The Copernicus Marine Environmental Monitoring Service (CMEMS) Iberian–Biscay–Ireland (IBI) ocean reanalysis is used to provide a detailed view of the circulation and mixing processes of MOW near the Iberian and African continental slopes. This work emphasizes the relevance of the complex bathymetric features defining the circulation processes of MOW in this region. The high resolution of the IBI reanalysis allows us to make a description of the mesoscale features forced by the topography. The temperature, salinity, velocity, transport, and vorticity fields are analysed to understand the circulation patterns of MOW. The high-resolution circulation patterns reveal that Horseshoe Basin and the continental slope near Cape Ghir (a.k.a. Cap Rhir or Cabo de Aguer) are key areas controlling the mixing processes of MOW with the surrounding water masses, mainly North Atlantic Central Water (NACW) and Antarctic Intermediate Water (AAIW). The water mass variability is also analysed by means of composite analysis. Results indicate the existence of a variability in the MOW tongue which retracts and expands westwards in opposition to the movement of the underlying North Atlantic Deep Water.

North-south difference of water mass properties across the Lembeh Strait, North Sulawesi, Indonesia

Two field observations were conducted around the Lembeh Strait in September 2015 and 2016, respectively. Evidences indicate that seawater around the Lembeh Strait is consisted of North Pacific Tropical Water (NPTW), North Pacific Intermediate Water (NPIW), North Pacific Tropical Intermediate Water (NPTIW) and Antarctic Intermediate Water (AAIW). Around the Lembeh Strait, there exist some north-south differences in terms of water mass properties. NPTIW is only found in the southern Lembeh Strait. Water mass with the salinity of 34.6 is only detected at 200–240 m between NPTW and NPTIW in the southern Lembeh Strait, and results from the process of mixing between the saltier water transported from the South Pacific Ocean and the lighter water from the North Pacific Ocean and Sulawesi Sea. According to the analysis on mixing layer depth, it is indicated that there exists an onshore surface current in the northern Lembeh Strait and the surface current in the Lembeh Strait is southward. These dramatic differences of water masses demonstrate that the less water exchange has been occurred between the north and south of Lembeh Strait. In 2015, the positive wind stress curl covering the northern Lembeh Strait induces the shoaling of thermocline and deepening of NPIW, which show that the north-south difference of airsea system is possible of inducing north-south differences of seawater properties.

Recent wind-driven change in Subantarctic Mode Water and its impact on ocean heat storage

The subduction and export of Subantarctic Mode Water (SAMW) supplies the upper limb of the overturning circulation and makes an important contribution to global heat, freshwater, carbon and nutrient budgets. Upper ocean heat content has increased since 2006, helping to explain the so-called global warming hiatus between 1998 and 2014, with much of the ocean warming concentrated in extratropical latitudes of the Southern Hemisphere in close association with SAMW and Antarctic Intermediate Water (AAIW)6,7. Here we use Argo observations to assess changes in the thickness, depth and heat content of the SAMW layer. Between 2005 and 2015, SAMW has thickened (3.6 ± 0.3 m yr−1), deepened (2.4 ± 0.2 m yr−1) and warmed (3.9 ± 0.3 W m−2). Wind forcing, rather than buoyancy forcing, is largely responsible for the observed trends in SAMW. Most (84%) of the increase in SAMW heat content is the result of changes in thickness; warming by buoyancy forcing (increased heat flux to the ocean) accounts for the remaining 16%. Projected increases in wind stress curl would drive further deepening of SAMW and increase in heat storage in the Southern Hemisphere oceans.

Regional Impacts of the Westerly Winds on Southern Ocean Mode and Intermediate Water Subduction

AbstractSubduction processes in the Southern Ocean transfer oxygen, heat, and anthropogenic carbon into the ocean interior. The future response of upper-ocean subduction, in the Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) classes, is dependent on the evolution of the combined surface buoyancy forcing and overlying westerly wind stress. Here, the recently observed pattern of a poleward intensification of the westerly winds is divided into its shift and increase components. SAMW and AAIW formation occurs in regional “hot spots” in deep mixed layer zones, primarily in the southeast Indian and Pacific. It is found that the mixed layer depth responds differently to wind stress perturbations across these regional formation zones. An increase only in the westerly winds in the Indian sector steepens isopycnals and increases the local circulation, driving deeper mixed layers and increased subduction. Conversely, in the same region, a poleward shift and poleward intensification of the westerly winds reduces heat loss and increases freshwater input, thus decreasing the mixed layer depth and consequently the associated SAMW and AAIW subduction. In the Pacific sector, all wind stress perturbations lead to increases in heat loss and decreases in freshwater input, resulting in a net increase in SAMW and AAIW subduction. Overall, the poleward shift in the westerly wind stress dominates the SAMW subduction changes, rather than the increase in wind stress. The net decrease in SAMW subduction across all basins would likely decrease anthropogenic carbon sequestration; however, the net AAIW subduction changes across the Southern Ocean are overall minor.

Evolution of Intermediate Water Masses Based on Argo Float Displacements

AbstractThe evolution and dispersion of intermediate water masses in the ocean interior is studied. To this purpose, an empirical statistical model of Lagrangian tracers at a constant depth level is developed. The model follows the transfer operator based on 10-day deep displacements of Argo floats at ~1000 m depth. An asymptotic analysis of the model shows the existence of 10 principal stationary points (the 10 locations attract asymptotically 97% of the tracers). It takes ~1000 years to reach this asymptotic regime relevant for estimating the stationary points. For Lagrangian floats, the concept of attractor needs to be generalized in a statistical sense (versus deterministic), except for a few places in the ocean. In this new framework, a tracer has a likelihood to reach the stationary points, rather than a certainty to reach a single stationary point. The empirical statistical model is used to describe the fate of three intermediate water masses: North Pacific Intermediate Water (NPIW), Mediterranean Water (MW), and Antarctic Intermediate Water (AAIW). These experiments show a dramatic difference in the long-time behavior of NPIW, MW, and AAIW. In the permanent regime, the NPIW concentrates locally (in the Kuroshio recirculation) and the MW remains mainly regional (concentrated in the subtropical gyre of the North Atlantic), whereas the AAIW spreads globally (well mixed throughout the entire ocean).

On the response of subduction in the South Pacific to an intensification of westerlies and heat flux in an eddy permitting ocean model

Based on an eddy permitting ocean general circulation model, the response of water masses to two distinct climate scenarios in the South Pacific is assessed in this paper. Under annually repeating atmospheric forcing that is characterized by different westerlies and associated heat flux, the response of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) is quantitatively estimated. Both SAMW and AAIW are found to be warmer, saltier and denser under intensified westerlies and increased heat loss. The increase in the subduction volume of SAMW and AAIW is about 19.8 Sv (1 Sv = 106 m3 s−1). The lateral induction term plays a dominant role in the changes in the subduction volume due to the deepening of the mixed layer depth (MLD). Furthermore, analysis of the buoyancy budget is used to quantitatively diagnose the reason for the changes in the MLD. The deepening of the MLD is found to be primarily caused by the strengthening of heat loss from the ocean to the atmosphere in the formation region of SAMW and AAIW.

Pliocene-Pleistocene evolution of sea surface and intermediate water temperatures from the southwest Pacific.

Over the last 5 million years, the global climate system has evolved toward a colder mean state, marked by large‐amplitude oscillations in continental ice volume. Equatorward expansion of polar waters and strengthening temperature gradients have been detected. However, the response of the mid latitudes and high latitudes of the Southern Hemisphere is not well documented, despite the potential importance for climate feedbacks including sea ice distribution and low‐high latitude heat transport. Here we reconstruct the Pliocene‐Pleistocene history of both sea surface and Antarctic Intermediate Water (AAIW) temperatures on orbital time scales from Deep Sea Drilling Project Site 593 in the Tasman Sea, southwest Pacific. We confirm overall Pliocene‐Pleistocene cooling trends in both the surface ocean and AAIW, although the patterns are complex. The Pliocene is warmer than modern, but our data suggest an equatorward displacement of the subtropical front relative to present and a poleward displacement of the subantarctic front of the Antarctic Circumpolar Current (ACC). Two main intervals of cooling, from ~3 Ma and ~1.5 Ma, are coeval with cooling and ice sheet expansion noted elsewhere and suggest that equatorward expansion of polar water masses also characterized the southwest Pacific through the Pliocene‐Pleistocene. However, the observed trends in sea surface temperature and AAIW temperature are not identical despite an underlying link to the ACC, and intervals of unusual surface ocean warmth (~2 Ma) and large‐amplitude variability in AAIW temperatures (from ~1 Ma) highlight complex interactions between equatorward displacements of fronts associated with the ACC and/or varying poleward heat transport from the subtropics.

The relation between the Mindanao Current and Mindanao Undercurrent on seasonal scale

The ocean general circulation model for the earth simulator (OFES) products is applied to estimate the transports of the Mindanao Current (MC) and the Mindanao undercurrent (MUC) and explore the relation between them on seasonal scale. In general, the MUC is composed of the lower part of the Southern Pacific Tropical Water (SPTW) and Antarctic Intermediate Water (AAIW). While the deep northward core below 1 500 m is regarded as a portion of MUC. Both salinity and potential density restrictions become more reasonable to estimate the transports of MC/MUC as the properties of water mass having been taken into consideration. The climatological annual mean transport of MC is (37.4±5.81)×106 m3/s while that of MUC is (23.92±6.47)×106 m3/s integrated between 26.5σ θ and 27.7 σ θ , and (17.53±5.45)×106 m3/s integrated between 26.5 σ θ and 27.5 σ θ in the OFES. The variations of MC and MUC have good positive correlation with each other on the seasonal scale: The MC is stronger in spring and weaker in fall, which corresponds well with the MUC, and the correlation coefficient of them is 0.67 in the OFES. The same variations are also appeared in hybrid coordinate ocean model (HYCOM) results. Two sensitive experiments based on HYCOM are conducted to explore the relation between MC and MUC. The MUC (26.5<σ θ <27.7) is strengthening as the MC increases with the enhancement of zonal wind field. It is shown, however, that the main part of the increasement is the deeper northward high potential density water (HPDW), while the AAIW almost remains stable, SPTW decreases, and vice versa.

Impact of oceanic circulation changes on atmospheric δ13CO2

Abstractδ13CO2 measured in Antarctic ice cores provides constraints on oceanic and terrestrial carbon cycle processes linked with millennial‐scale changes in atmospheric CO2. However, the interpretation of δ13CO2 is not straightforward. Using carbon isotope‐enabled versions of the LOVECLIM and Bern3D models, we perform a set of sensitivity experiments in which the formation rates of North Atlantic Deep Water (NADW), North Pacific Deep Water (NPDW), Antarctic Bottom Water (AABW), and Antarctic Intermediate Water (AAIW) are varied. We study the impact of these circulation changes on atmospheric δ13CO2 as well as on the oceanic δ13C distribution. In general, we find that the formation rates of AABW, NADW, NPDW, and AAIW are negatively correlated with changes in δ13CO2: namely, strong oceanic ventilation decreases atmospheric δ13CO2. However, since large‐scale oceanic circulation reorganizations also impact nutrient utilization and the Earth's climate, the relationship between atmospheric levels and ocean ventilation rate is not unequivocal. In both models atmospheric δ13CO2 is very sensitive to changes in AABW formation rates: increased AABW formation enhances the transport of low δ13C waters to the surface and decreases atmospheric δ13CO2. By contrast, the impact of NADW changes on atmospheric δ13CO2 is less robust and might be model dependent. This results from complex interplay between global climate, carbon cycle, and the formation rate of NADW, a water body characterized by relatively high δ13C.

Rapid acidification of mode and intermediate waters in the southwestern Atlantic Ocean

Abstract. Observations along the southwestern Atlantic WOCE A17 line made during the Dutch GEOTRACES-NL programme (2010–2011) were compared with historical data from 1994 to quantify the changes in the anthropogenic component of the total pool of dissolved inorganic carbon (ΔCant). Application of the extended multi-linear regression (eMLR) method shows that the ΔCant from 1994 to 2011 has largely remained confined to the upper 1000 dbar. The greatest changes occur in the upper 200 dbar in the Subantarctic Zone (SAZ), where a maximum increase of 37 μmol kg−1 is found. South Atlantic Central Water (SACW) experienced the highest rate of increase in Cant, at 0.99 ± 0.14 μmol kg−1 yr−1, resulting in a maximum rate of decrease in pH of 0.0016 yr−1. The highest rates of acidification relative to ΔCant, however, were found in Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). The low buffering capacity of SAMW and AAIW combined with their relatively high rates of Cant, increase of 0.53 ± 0.11 and 0.36 ± 0.06 μmol kg−1 yr−1, respectively, has lead to rapid acidification in the SAZ, and will continue to do so whilst simultaneously reducing the chemical buffering capacity of this significant CO2 sink.

The imprint of Southern Ocean overturning on seasonal water mass variability in Drake Passage

Seasonal changes in water mass properties are discussed in thermohaline coordinates from a seasonal climatology and repeat hydrographic sections. The SR1b CTD transects along Drake Passage are used as a case study. The amount of water within temperature and salinity classes and changes therein are used to estimate dia-thermal and dia-haline transformations. These transformations are considered in combination with climatologies of surface buoyancy flux to determine the relative contributions of surface buoyancy fluxes and subsurface mixing to changes in the distribution of water in thermohaline coordinates. The framework developed provides unique insights into the thermohaline circulation of the water masses that are present within Drake Passage, including the erosion of Antarctic Winter Water (AAWW) during the summer months and the interaction between the Circumpolar Deep Waters (CDW) and Antarctic Intermediate Water (AAIW). The results presented are consistent with summertime wind-driven inflation of the CDW layer and deflation of the AAIW layer, and with new AAIW produced in the winter as a mixture of CDW, remnant AAWW, and surface waters. This analysis therefore highlights the role of surface buoyancy fluxes in the Southern Ocean overturning.

Horizontal and vertical distribution of mesozooplankton species richness and composition down to 2,300 m in the southwest Atlantic Ocean

We describe the species richness, distribution and composition of mesozooplankton over the continental shelf and slope, and in the water masses in the Campos Basin, southwest Atlantic Ocean. We analyzed the mesozooplankton from two oceanographic cruises (rainy and dry seasons, 2009) with samples taken in five different water masses from the surface to 2,300 m depth. In the Subsurface Water (SS), in both sampling periods, more species were recorded over the slope (rainy: 100; dry: 128) than the continental shelf (rainy: 97; dry: 104). Over the slope, species richness decreased with increasing depth: the highest values were observed in the South Atlantic Central Water (SACW), and the lowest values in the North Atlantic Deep Water (NADW), in both sampling periods. We recorded 262 species in 10 groups (Hydrozoa, Siphonophora, Ctenophora, Branchiopoda, Copepoda, Euphausiacea, Decapoda, Chaetognatha, Appendicularia e Thaliacea), with 13 new occurrences for the southwest Atlantic. Copepoda was the group with the highest species richness, containing 138 species. In both periods, the samples from SS, SACW and Antarctic Intermediate Water (AAIW)/Upper Circumpolar Deep Water (UCDW) were clustered in different faunistic zones, based on species composition. This study confirmed that zooplankton richness in the southwest Atlantic Ocean is underestimated, and suggests that additional efforts must be directed toward a better understanding of this fairly unknown region.

Dissolved cadmium in the Southern Ocean: Distribution, speciation, and relation to phosphate

We report isotope dilution analyses of dissolved cadmium (Cd) and electrochemical Cd speciation measurements in the Atlantic sector of the Southern Ocean. Bioavailable inorganic Cd is &gt; 100 times higher in near‐surface waters south of the Polar Front compared to the Subantarctic Zone because of upwelling and reduced complexation by organic Cd ligands. To trace local changes in the relation between Cd and P, we examine the deviations from a linear deep‐water Cd vs. P relation (Cd*), and find that changes in Cd* coincide with the position of frontal systems and covary with primary productivity and total dissolved Mn and Fe concentrations. These covariations agree with potential local changes in phytoplankton Cd uptake rates, resulting from differences in the availability of Cd, Zn, Mn, and Fe. A band of negative Cd* values is associated with formation of Subantarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW). In contrast to SAMW, which may export low Cd : P ratios from the Southern Ocean, the Cd : P ratios in AAIW increase by mixing with underlying Upper Circumpolar Deep Water before being exported from the Southern Ocean. Deep waters show constant Cd : P ratios, and both elements behave conservatively with end‐member mixing between deep waters of the Weddell Gyre, the Antarctic Circumpolar Current, and inflowing North Atlantic Deep Water. Overall, our results support the hypothesis that the kink in the global Cd vs. P relation is largely caused by high Cd : P uptake ratios in the trace‐nutrient—limited Southern Ocean.