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Mean salinities, temperatures, neutral densities g, and dissolved oxygen with standard deviations in parentheses on the salinity minimum from the 3 cruises.
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New data are presented from 24°S in the South Atlantic in an investigation of the decadal variability of the intermediate and thermocline water masses at this latitude. Variation of salinity on neutral density surfaces is investigated with three transatlantic, full-depth hydrographic sections from 1958, 1983, and 2009. The thermocline is seen to fr...
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... The thermocline waters most affected by long-term warming signals are the mode and intermediate waters since they are formed at the surface in both the subpolar and subtropical regions and then penetrate the South Atlantic subtropical gyre at depth [144][145][146][147] . A recent study found that about 1.5 × 10 22 J of heat was converging in the South Atlantic in the σ θ = 26.0-27.0 ...
Since the inception of the international South Atlantic Meridional Overturning Circulation initiative in the 21st century, substantial advances have been made in observing and understanding the Southern Hemisphere component of the Atlantic Meridional Overturning Circulation (AMOC). Here we synthesize insights gained into overturning flows, interocean exchanges, and water mass distributions and pathways in the South Atlantic. The overturning circulation in the South Atlantic uniquely carries heat equatorward and exports freshwater poleward and consists of two strong overturning cells. Density and pressure gradients, winds, eddies, boundary currents, and interocean exchanges create an energetic circulation in the subtropical and tropical South Atlantic Ocean. The relative importance of these drivers varies with the observed latitude and time scale. AMOC, interocean exchanges, and climate changes drive ocean warming at all depths, upper ocean salinification, and freshening in the deep and abyssal ocean in the South Atlantic. Long-term sustained observations are critical to detect and understand these changes and their impacts. The overturning circulation in the South Atlantic Ocean is driven by winds, pressure and density gradients, interocean exchanges, and eddies that vary spatially and temporally. A synthesis of observations reveals that waters that engage in this circulation are experiencing pronounced warming.
... For decades, the Antarctic Intermediate water (AAIW) has experienced warming off the coast off Brazil. From 1958 to 2009, the temperature has increased to about 0.23°C (McCarthy et al., 2011). ...
Recent oceanographic observations have identified significant changes of intermediate water masses characterized by increased temperatures, lowered pH and deoxygenation. In order to improve our understanding as to how these changes may impact deep-sea ecosystems one important strategy is to reconstruct past oceanic conditions. Here we examine the applicability of the scleractinian cold-water coral Solenosmilia variabilis as a marine archive for the reconstructions of past intermediate water mass temperatures by using Lithium (Li)/Magnesium (Mg) ratios. In particular, our study addresses 1) the calibration of Li/Mg ratios against in-situ temperature data, 2) the reconstruction of past intermediate water mass temperatures using scleractinian coral fossil samples from the Brazilian continental margin and 3) the identification of intraspecies variability within the coral microstructure. Results showed that Li/Mg ratios measured in the skeletons of S. variabilis fit into existing Li/Mg-T calibrations of other cold-water scleractinian. Furthermore, the coral microstructure exhibits interspecies variability of Li/Ca and Mg/Ca ratios were also similar to what has been observed in other cold-water scleractinian corals, suggesting a similar biomineralization control on the incorporation of Li and Mg into the skeleton. However, the Li/Mg based temperature reconstruction using fossil samples resulted in unexpectedly high variations >10°C, which might not be solely related to temperature variations of the intermediate water mass over the last 160 ka on the Brazilian continental margin. We speculate that such temperature variability may be caused by vertical movements of the aragonite saturation horizon and the associated seawater pH changes, which in turn influence the incorporation of Li and Mg into the coral skeleton. Based on these results it is recommended that future studies investigating past oceanic conditions need to consider the carbonate system parameters and how they might impact the mechanisms of Li and Mg being incorporated into skeletons of cold-water coral species such as S. variabilis.
... Moreover, variations in the properties and the spreading of the water masses on interannual and decadal time scales can affect the strength of the mixing processes in both lateral and vertical directions. Remarkably, in the North Atlantic Ocean, AAIW became warmer and saltier in the last decades (Arbic & Brechner Owens, 2001;Fu et al., 2018;Sarafanov et al., 2007;Schmidtko & Johnson, 2012), while it became warmer and lighter in the South Atlantic Ocean (Arbic & Brechner Owens, 2001;McCarthy et al., 2011;Schmidtko & Johnson, 2012). The South Atlantic trend has been attributed to warming trends near the AAIW formation region and to the decadal variability of the Agulhas Leakage (Hummels et al., 2015;Lübbecke et al., 2015;Schmidtko & Johnson, 2012). ...
... In contrast, the trend in the North Atlantic appeared to coincide with a slower northward and westward spreading of AAIW in this region (Fu et al., 2018(Fu et al., , 2019. Another difference between the trends in the two hemispheres is that the salinity trends of AAIW are not uniformly positive in the South Atlantic, while this is the case in the North Atlantic (Curry et al., 2003;Durack & Wijffels, 2010;Fu et al., 2018;McCarthy et al., 2011;Schmidtko & Johnson, 2012). ...
Because of its pronounced fresh signature, the properties of the northward‐flowing Antarctic Intermediate Water (AAIW) affect the Atlantic Meridional Overturning Circulation. Hence, understanding modifications of AAIW along its path is important. Here, we analyze AAIW changes along its path in the Caribbean Sea and assess whether vertical fluxes from background turbulence and from double‐diffusive mixing in thermohaline staircases can explain these variations. We deduce the occurrence rate of staircases (7%) and estimate the flux ratio (γ=αFTβFS=0.8) $(\gamma =\frac{\alpha {F}_{T}}{\beta {F}_{S}}=0.8)$ from Argo float profiles. In combination with vertical fluxes from background turbulence, these values are used in a steady‐state advection‐diffusion model to estimate the effective diffusivity of salt that arises from double diffusion (KSdd=O(10−5m2s−1)) $({K}_{S}^{dd}=\mathcal{O}(1{0}^{-5}\hspace*{.5em}{\mathrm{m}}^{2}\hspace*{.5em}{\mathrm{s}}^{-1}))$. This value for KSdd ${K}_{S}^{dd}$ is similar to observed values (Schmitt, 2005, https://doi.org/10.1126/science.1108678), implying the observed modification of AAIW in the Caribbean Sea may be attributable primarily to vertical mixing in the region itself.
... Furthermore, to smooth out some of the higher frequency variability (i.e. mesoscale eddies and internal waves), the investigation of halocline variation should be along neutral density surfaces (McCarthy et al., 2011;McDougall, 1987). The neutral density, denoted by γ n , is a function of salinity, in situ temperature (°C), pressure (db), and latitude/longitude position. ...
... Although long-term warming and salinification trends have been assessed, it remains unclear whether AAIW properties also vary on interannual timescales since different works have achieved contradictory trends.McCarthy et al. (2011) showed that the AAIW core salinity in the South Atlantic varies on interannual timescales with a peak-to-trough magnitude of 0.06 and attributes that to westward propagation of planetary waves. Differently,Curry et al. (2003) andYao et al. (2017) have observed a basin-scale freshening trend in the AAIW percolating the southwestern Atlan ...
With an approximate length of 8500 km between latitudes 34.5°S and 6°N, the ocean area adjacent to Brazil - including the economic exclusive zone - is known as the “Blue Amazon”, with an equivalent area of approximately 3.5 million km², but with much less discrete oceanographic observations if compared to other ocean regions in the last 50 years. This study introduces a hydrographic data not yet fully integrated with the main oceanographic databases (WOD & GLODAP), including an unprecedented set of 28 oceanographic expeditions from the Brazilian Navy and PIRATA-BR between the years 1998 and 2018, performing a total of 783 oceanographic stations with Rosette/CTD systems at varying depths between the continental shelf and the deep ocean (up to 5800 m deep) that were not analyzed on such a broad scale before. The data obtained at all oceanographic stations were reprocessed according to the international standard GO-SHIP in order to investigate the vertical thermohaline structure of the water column, as well as to analyze the salinity variability along several neutral density surfaces corresponding to water masses interfaces throughout the last two decades. Finally, the observed results depict a salinification trend in the upper layer (+0.01993; +0.00186 g/kg . yr-1 ), a dipole of freshening-salinification trends in central (+0.00062; -0.00231 g/kg . yr-1 ) and intermediate waters (-0.00155; +0.00042 g/kg . yr-1 ), and a freshening trend in deep waters (-0.00043; -0.0016 g/kg . yr-1 ), which diminishes the spatio-temporal aliasing of available discrete data and secures even more robustness for inferences in the Southwestern Atlantic Ocean.
... AAIW found in the Atlantic Ocean has contrasting properties across the basin (Piola and Georgi, 1982;McCarthy et al., 2011;Rusciano et al., 2012). Atlantic AAIW originates from two different source waters (see Rusciano et al., 2012 for a detailed discussion): Subantarctic Mode ...
The variability of the Atlantic meridional overturning circulation (AMOC) has considerable impacts on the global climate system. Past studies have shown that changes in the South Atlantic control the stability of the AMOC and drive an important part of its variability. That is why significant resources have been invested in a South (S)AMOC observing system. In January 2017, the RV Maria S. Merian conducted the first GO‐SHIP hydrographic transect along the SAMOC‐Basin Wide Array (SAMBA) line at 34.5°S in the South Atlantic. This paper presents estimates of meridional volume, freshwater (MFT), and heat (MHT) transports through the line using the slow varying geostrophic density field and direct velocity observations. An upper and an abyssal overturning cell are identified with a strength of 15.64 ± 1.39 Sv and 2.4 ± 1.6 Sv, respectively. The net northward MHT is 0.27 ± 0.10 PW, increasing by 0.12 PW when we remove the observed mesoscale eddies with a climatology derived from the Argo floats data set. We attribute this change to an anomalous predominance of cold core eddies during the cruise period. The highest velocities are observed in the western boundary, within the Brazil and the Deep Western Boundary currents. These currents appear as a continuous deep jet located 150 km off the slope squeezed between two cyclonic eddies. The zonal changes in water masses properties and velocity denote the imprint of exchange pathways with both the Southern and the Indian oceans.
... Due to mixing with surrounding waters on its way northward, AAIW gradually loses the salinity minimum signature, which can be traced up to about 20°N (Tsuchiya, 1989). When AAIW reaches the tropical North Atlantic, its oxygen level has strongly reduced contributing to the lower part the oxygen minimum zone (OMZ) near the eastern boundary (e.g., Brandt et al., 2010Brandt et al., , 2015Stramma et al., 2008) A number of studies have investigated the salinity and temperature changes in the intermediate layer of the Atlantic Ocean (Arbic & Owens, 2001;Fu et al., 2018;McCarthy et al., 2011;Sarafanov et al., 2007;Schmidtko & Johnson, 2012). Consistently, these studies revealed that during the last decades, AAIW became warmer and lighter in the South Atlantic, while it became warmer and more saline in the tropical North Atlantic. ...
Interannual variability of Antarctic Intermediate Water (AAIW) in the tropical North Atlantic is investigated using the GECCO2 ocean state estimate and Argo data. AAIW salinity variability near the western boundary is highly correlated with the transport along the western boundary on interannual timescales. Northward propagating anomalies are associated with the western boundary transport variability that, to some extent, is related to the large‐scale wind stress curl forcing by means of the Sverdrup balance. AAIW anomalies also propagate westward with the speed of baroclinic Rossby waves, indicating that the displacement of the meridional salinity gradient by westward propagation of baroclinic Rossby waves plays a role in the variability of AAIW characteristics. Slower eastward spreading of AAIW anomalies is identified on decadal timescales likely associated with the advection of salinity anomalies by weak eastward current bands. Understanding the observed interannual and decadal variability of AAIW salinity is important to properly interpret salinity changes reported in response to changes in the hydrological cycle.
... Theoretical and observational studies of the mechanisms controlling the MOC in the Atlantic have suggested that the MOC is presently bistable, with on and off states, and that MOC stability is controlled by interocean exchange between the South Atlantic and Southern Oceans (e.g., Dijkstra, 2007;Drijfhout et al., 2011;Garzoli et al., 2013;Huisman et al., 2010). While continuous, daily, observations of the MOC have been ongoing for 13+ years in the North Atlantic at 26.5°N (e.g., Frajka-Williams et al., 2016;Kanzow et al., 2007;Smeed et al., 2018) sections using either expendable bathythermograph (XBT) probes (e.g., Dong et al., 2009;Garzoli & Baringer, 2007) or full-depth conductivity-temperature-depth profiles (e.g., Bryden et al., 2011;Lumpkin & Speer, 2007;McCarthy et al., 2011), and to indirect estimates combining satellite observations with irregularly spaced (in time and space) hydrographic data (e.g., Dong et al., 2015;Majumder et al., 2016). While these studies have greatly improved our understanding of the MOC structure and dynamics (e.g., Buckley & Marshall, 2016;, the daily MOC observations at 26.5°N have illustrated the importance of continuous observations to avoid aliasing highly energetic short-period variations (e.g., Kanzow et al., 2010). ...
Six years of simultaneous moored observations near the western and eastern boundaries of the South Atlantic are combined with satellite winds to produce a daily time series of the basin-wide meridional overturning circulation (MOC) volume transport at 34.5°S. The results demonstrate that barotropic and baroclinic signals at both boundaries cause significant transport variations, and as such must be concurrently observed. The data, spanning ~20 months during 2009–2010 and ~4 years during 2013–2017, reveal a highly energetic MOC record with a temporal standard deviation of 8.3 Sv, and strong variations at time scales ranging from a few days to years (peak-to-peak range = 54.6 Sv). Seasonal transport variations are found to have both semiannual (baroclinic) and annual (Ekman and barotropic) timescales. Interannual MOC variations result from both barotropic and baroclinic changes, with density profile changes at the eastern boundary having the largest impact on the year-to-year variations.
... However, a weakening of the Atlantic Meridional Overturning Circulation (AMOC) and therefore the transport of warm and salty water from the subtropical Atlantic Ocean into the subpolar Atlantic, could imprint as a cooling and freshening trend on isopycnals as shown in model analyses (Pardaens et al., 2008). Further, in the South Atlantic different salinity changes have been reported in AAIW, with a freshening reported between 1985 and 1999 (Curry et al., 2003) and a contrary salinisation between 1983(McCarthy et al., 2011. Fluctuations in the transport of saltier AAIW from the Indian Ocean in to the South Atlantic has been suggested as a source of variability that could produce this difference (McCarthy et al., 2011). ...
... Further, in the South Atlantic different salinity changes have been reported in AAIW, with a freshening reported between 1985 and 1999 (Curry et al., 2003) and a contrary salinisation between 1983(McCarthy et al., 2011. Fluctuations in the transport of saltier AAIW from the Indian Ocean in to the South Atlantic has been suggested as a source of variability that could produce this difference (McCarthy et al., 2011). More recently, westward propagating salinity anomalies have been highlighted as a potential pathway for the transport of saltier water, but the exact nature of these anomalies is unclear (McCarthy et al., 2012b). ...
This thesis develops and applies novel methods to understand water mass variability and change in the global ocean. A water mass framework is developed that determines the diathermal and diahaline transformations of water between water mass classes from the temporal variations in their volumetric distribution. Water masses are defined in terms of their temperature and salinity. This reveals the roles of air--sea fluxes, mixing and transport changes. The second chapter explores the drivers of interannual variability in the overturning circulation in the North Atlantic subtropical gyre using the water mass framework. Variations in the volumetric distribution of water masses reveal that transport anomalies at the gyre boundaries predominantly set the volume and heat budget and that these transport anomalies are governed by Ekman pumping over the gyre. In the third and fourth chapters of this thesis the water mass framework is applied to observations of temperature and salinity in the Southern Ocean. Seasonal variations in the distribution of water masses reveal the imprint of the Southern Ocean overturning. This highlights the importance of seasonally varying air-sea fluxes in the formation of intermediate water at the expense of deep water, winter water and surface water. This reveals a diabatic pathway for the upwelling and conversion of deep water into intermediate water. Deep water is first cooled and freshened during the winter by mixing with overlying winter water triggered by a cabbeling instability. Sea ice-melt and surface heating then warm and freshen this seasonally formed water mass to create intermediate water during the summer months. These results suggest that the process of cabbeling could be a rate determining step in the global overturning circulation and the upwelling of deep waters. The fifth chapter of this thesis explores an alternative method to determine a volumetric distribution using individual Argo profiles. The volumetric distribution determined using this profile based estimate is compared to the distribution calculated using a geographically interpolated dataset. This comparison reveals that the interpolation scheme used to geographically grid Argo appears to artificially mix water masses toward the centre of the distribution.
... The combined effect of these two mechanisms, i.e., oxygen supply via ocean circulation and oxygen demanded from ocean biogeochemistry, delimits the geographical boundaries of OMZs. In addition, observed and predicted changes that are linked to global warming suggest that the intermediate waters of the Atlantic Ocean are becoming less oxygenated over time (e.g., Garcia et al., 1998;Arbic and Brechner Owens, 2001;Keeling and Garcia, 2002;McCarthy et al., 2011). Repeated hydrographic surveys have indicated that the upper 3000 m of the worldʼs oceans have warmed over the past six decades (e.g., Domingues et al., 2008;Levitus et al., 2001Levitus et al., , 2005Levitus et al., , 2009). ...
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 ± 0.67 m yr−1 for the ETSA and 3.37 ± 0.73 m yr−1 for the ETNA), the mean oxygen concentration has decreased (– 0.12 ± 0.03 μmol kg−1 yr−1 for the ETSA and − 0.17 ± 0.05 μmol kg−1 yr−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 ± 0.71 m yr−1, a decrease in the mean oxygen concentration of − 0.18 ± 0.04 μmol kg−1 yr−1 and an increase in the mean temperature of 0.010 ± 0.005 °C yr−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.
... Their positions are 101 presented in Fig. 2. The first occupation was collected with 72 stations in 2003 by the R/V 102 Mirai (Japan, [Kawano et al., 2004]), the other was in 2011 with 81 stations by the Ronald H. 103 Brown (United States, [Feely et al., 2011]) 104 To reduce the effect of dynamic process in ocean interior (i.e. mesoscale eddies and 110 internal waves), the investigation of halocline variation would be along neutral density 111 surface [G McCarthy et al., 2011; McDougall, 1987]. The layer of AAIW is defined using 112 neutral density (γ n , unit: kg/m 3 ) [Jackett and McDougall, 1997] instead of potential density, 113 with the upper and lower boundaries of 27.1γ n and 27.6γ n [Goes et al., 2014], respectively. ...
Basin-scaled freshening of Antarctic Intermediate Water (AAIW) is reported to have dominated South Atlantic Ocean during period from 2005 to 2014, as shown by the gridded monthly means Argo (Array for Real-time Geostrophic Oceanography) data. The relevant investigation was also revealed by two transatlantic occupations of repeated section along 30° S, from World Ocean Circulation Experiment Hydrographic Program. Freshening of the AAIW was compensated by the opposing salinity increase of thermocline water, indicating the contemporaneous hydrological cycle intensification. This was illustrated by the precipitation less evaporation change in the Southern Hemisphere from 2000 to 2014, with freshwater input from atmosphere to ocean surface increasing in the subpolar high-precipitation region and vice versa in the subtropical high-evaporation region. Against the background of hydrological cycle augment, the decreased transport of Agulhas Leakage (AL) was proposed to be one of the contributors for the associated freshening of AAIW. This indirectly estimated variability of AL, reflected by the weakening of wind stress over the South Indian Ocean since the beginning of 2000s, facilitates the freshwater input from source region and partly contributes to the observed freshened AAIW. Both of our mechanical analysis is qualitative, but this work would be helpful to validate and test predictably coupled sea-air model simulations.
