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Andrew J. Weaver - One of the best experts on this subject based on the ideXlab platform.
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A Region of Enhanced Northward Antarctic Intermediate Water Transport in a Coupled Climate Model
Journal of Physical Oceanography, 2003Co-Authors: Oleg A. Saenko, Andrew J. Weaver, Matthew H. EnglandAbstract:Abstract A global coupled model is used to examine pathways of freshWater transport in the Southern Ocean. On the background of a strong zonal freshWater transport along the pathway of the Antarctic Circumpolar Current (ACC), there are meridional freshWater flows distributed nonuniformly around the globe, including in the upper ocean. The analysis does not support a simple two-dimensional scheme of Antarctic Intermediate Water (AAIW) formation, according to which the fresh AAIW forms uniformly around the circumpolar ocean. Rather, a more complex three-dimensional picture of the freshWater transport in the Southern Ocean is revealed, with enhanced AAIW formation in the southeast Pacific Ocean both north and south of the Drake Passage latitudes. Freshened by intense precipitation and surface Waters from around Antarctica, the ACC transports freshWater from the northwest to the southeast toward Drake Passage. There, a fraction of this freshWater is transported southward across 60°S with the subsurface ACC an...
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NOTES AND CORRESPONDENCE A Region of Enhanced Northward Antarctic Intermediate Water Transport in a Coupled Climate Model
2003Co-Authors: Oleg A. Saenko, Andrew J. Weaver, Matthew H. EnglandAbstract:A global coupled model is used to examine pathways of freshWater transport in the Southern Ocean. On the background of a strong zonal freshWater transport along the pathway of the Antarctic Circumpolar Current (ACC), there are meridional freshWater flows distributed nonuniformly around the globe, including in the upper ocean. The analysis does not support a simple two-dimensional scheme of Antarctic Intermediate Water (AAIW) formation, according to which the fresh AAIW forms uniformly around the circumpolar ocean. Rather, a more complex three-dimensional picture of the freshWater transport in the Southern Ocean is revealed, with enhanced AAIW formation in the southeast Pacific Ocean both north and south of the Drake Passage latitudes. Freshened by intense precipitation and surface Waters from around Antarctica, the ACC transports freshWater from the northwest to the southeast toward Drake Passage. There, a fraction of this freshWater is transported southward across 608S with the subsurface ACC and the eddy-induced flow. West of the Antarctic Peninsula, the freshWater subducts to Intermediate depths and turns northward, following the ACC and contributing to the formation of AAIW. This analysis supports previous results of enhanced subduction localized to the southern tip of South America.
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Importance of wind‐driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model
Geophysical Research Letters, 2001Co-Authors: Oleg A. Saenko, Andrew J. WeaverAbstract:An ocean-atmosphere-sea ice model is used to show the importance of wind-driven sea ice motion in the formation of Antarctic Intermediate Water (AAIW). The model is able to reasonably simulate a tongue of low salinity AAIW even when the direct momentum transfer from wind to the ocean is neglected, provided that the wind stress is applied to sea ice. In contrast, when the wind stress exclusively drives the ocean, the model fails to capture the properties of AAIW. The growth and subsequent offshore transport of sea ice acts as a freshWater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics are also shown to be important in the simulation of a local salinity minimum at Intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea.
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importance of wind driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model
Geophysical Research Letters, 2001Co-Authors: Oleg A. Saenko, Andrew J. WeaverAbstract:An ocean-atmosphere-sea ice model is used to show the importance of wind-driven sea ice motion in the formation of Antarctic Intermediate Water (AAIW). The model is able to reasonably simulate a tongue of low salinity AAIW even when the direct momentum transfer from wind to the ocean is neglected, provided that the wind stress is applied to sea ice. In contrast, when the wind stress exclusively drives the ocean, the model fails to capture the properties of AAIW. The growth and subsequent offshore transport of sea ice acts as a freshWater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics are also shown to be important in the simulation of a local salinity minimum at Intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea.
Yuzhu You - One of the best experts on this subject based on the ideXlab platform.
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Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic.
Deep Sea Research Part II: Topical Studies in Oceanography, 2005Co-Authors: Ismael Núñez-riboni, Michel Ollitrault, Olaf Boebel, Yuzhu You, Philip L. Richardson, Russ DavisAbstract: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.
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Lagrangian circulation of Antarctic Intermediate Water in the subtropical South Atlantic
Deep Sea Research Part II: Topical Studies in Oceanography, 2005Co-Authors: Ismael Núñez-riboni, Michel Ollitrault, Olaf Boebel, Yuzhu You, Philip L. Richardson, Russ DavisAbstract:International audienceThis 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
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Quantitative estimate of Antarctic Intermediate Water contributions from the Drake Passage and the southwest Indian Ocean to the South Atlantic
Journal of Geophysical Research, 2002Co-Authors: Yuzhu YouAbstract:[1] Recently obtained World Ocean Circulation Experiment (WOCE) bottle sections and a pre-WOCE bottle data set are used in a Water mass mixing model. The mixing scheme comprises three Intermediate Water sources: Antarctic Intermediate Water (AAIW) from the northern Drake Passage, a combination source of the Indian Ocean Intermediate Waters entering from south of Africa, and a transformed end-member of the former two sources. I call them dAAIW, iAAIW, and aAAIW, respectively. The dAAIW originates from the southeast South Pacific, enters the South Atlantic in the northern Drake Passage, and is modified in the Falkland Current loop. The iAAIW is a combination of the Indian Ocean sources including Red Sea Intermediate Water, Indonesian Intermediate Water, and AAIW formed locally in the south central Indian Ocean and transformed dAAIW that has returned following a loop through the Indian Ocean. The aAAIW is a transformed end-member of a mixture of dAAIW and iAAIW located in the eastern tropical South Atlantic, characterized by an oxygen minimum and nutrient maxima. Although aAAIW is not an import source like dAAIW and iAAIW, it spans property fields to extrema as a result of Water mass mixing and transformation processes and therefore must be included in the basin-wide Water mass mixing scheme. The study is performed on five neutral surfaces that encompass the AAIW layer from 700 to 1200 dbar in the subtropical latitudes with a distance of about 100 dbar between a pair of surfaces. Four conservative variables of potential temperature, salinity, initial phosphate (PO4o), and NO and one conservative dynamical tracer fN2 (where f is the Coriolis frequency and N2 is the squared buoyancy frequency) are used as input information to the mixing model. The model-derived mixing fraction gives a quantitative description of AAIW sources when they are mapped onto neutral surfaces. The contoured pattern of mixing fraction shows Water mass spreading paths, thus implying circulation and ventilation of AAIW in the South Atlantic. Results show that dAAIW is a dominant Water mass and iAAIW is about 30–60% of dAAIW in the subtropical latitudes. With the mixing proportion of AAIW sources derived from the mixing model the geostrophic volume transport and dianeutral upwelling transport can be separated into the individual contributions from each AAIW source. It is found that the percentage of the transport contributed by dAAIW and iAAIW to the South Atlantic is almost constant at 64 ± 2% and 36 ± 2%, respectively. Meridional transport in the subtropical latitudes between 30° and 10°S by dAAIW and iAAIW (referenced to 2000 dbar) has a mean of 4.26 Sv northward (1 Sv = 106 m3 s−1) shared between dAAIW at 2.70 Sv (63%) and iAAIW at 1.56 Sv (37%). The mean zonal transport in the western and eastern South Atlantic between 40° and 3°S is −5.13 Sv westward, shared between dAAIW at −3.38 Sv (66%) and iAAIW at −1.75 Sv (34%). The dianeutral upwelling transport across the uppermost neutral surface σN = 27.25 in the northwest South Atlantic (north of 30°S and west of 10°W) is 2.26 Sv shared between dAAIW at 1.40 Sv (62%) and iAAIW at 0.86 Sv (38%).
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Dianeutral mixing, transformation and transport of Antarctic Intermediate Water in the South Atlantic Ocean
Deep Sea Research Part II: Topical Studies in Oceanography, 1999Co-Authors: Yuzhu YouAbstract:Recently obtained World Ocean Circulation Experiment (WOCE) sections combined with a specially prepared pre-WOCE South Atlantic data set are used to study the dianeutral (across neutral surface) mixing and transport achieving Antarctic Intermediate Water (AAIW) being transformed to be part of the North Atlantic Deep Water (NADW) return cell. Five neutral surfaces are mapped, encompassing the AAIW from 700 to 1100 db at the subtropical latitudes. Coherent and significant dianeutral upwelling is found in the western boundary near the Brazil coast north of the separation point (about 25°S) between the anticyclonic subtropical and cyclonic south equatorial gyres. The magnitude of dianeutral upwelling transport is 10-3 Sv (1 Sv=106 m3 s-1) for 1°×1° square area. It is found that the AAIW sources from the southwestern South Atlantic and southwestern Indian Ocean do not rise significantly into the Benguela Current. Instead, they contribute to the NADW return formation by dianeutral upwelling into the South Equatorial Current. In other words, the AAIW sources cannot obtain enough heat/buoyancy to rise until they return to the western boundary region but north of the separation point. The basin-wide integration of dianeutral transport shows net upward transports, ranging from 0.25 to 0.6 Sv, across the lower and upper boundary of AAIW north of 40°S. This suggests that the equatorward AAIW is a slow rising Water on a basin average. Given one order of uncertainty in evaluating the along-neutral-surface and dianeutral diffusivities from the assumed values, K=103 m2 s-1 and D=10-5 m2 s-1, the integrated dianeutral transport has an error band of about 10–20%. The relatively weak integrated dianeutral upwelling transport compared with AAIW in other oceans implies much stronger lateral advection of AAIW in the South Atlantic. Mapped Turner Angle in diagnosing the double-diffusion processes shows that the salty Central Water can flux salt down to the upper half of AAIW layer through salt-fingering. Therefore, the northward transition of AAIW can gain salt either through along-neutral-surface advection and diffusion or through salt fingering from the Central Water and heat through either along-neutral-surface advection and diffusion or dianeutral upwelling. Cabbeling and thermobaricity are found significant in the Antarctic frontal zone and contribute to dianeutral downwelling with velocity as high as −1.5×10-7 m s-1. A schematic AAIW circulation in the South Atlantic suggests that dianeutral mixing plays an essential role in transforming AAIW into NADW return formation.
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Dianeutral mixing and transformation of Antarctic Intermediate Water in the Indian Ocean
Journal of Geophysical Research: Oceans, 1998Co-Authors: Yuzhu YouAbstract:Transformation of Antarctic Intermediate Water (AAIW) is achieved through one or more processes of epineutral advection, epineutral diffusion, dianeutral advection, and dianeutral diffusion. This paper points to the importance of dianeutral mixing in achieving the AAIW Water-mass transformation in the Indian Ocean. Six neutral surfaces were mapped to span the Intermediate Water of the Indian Ocean between 580 and 1500 m at the reference cast. South of the Antarctic frontal zone, AAIW shows a diffusive tongue in the meridional Turner angle (after J. Stewart Turner) sections. On its equatorward transition, the transformed AAIW is characterized by a tongue of doubly stable conditions extending a great distance to as far as about 5°S. A maximum downwelling dianeutral velocity of −2×10−7 m s−1 due to cabbeling is found in the Antarctic frontal zone, which is 3 orders larger than that in the subtropical gyre. Thermobaricity acts similarly to cabbeling in the Antarctic frontal zone and contributes a maximum downwelling dianeutral velocity of −1×10−7 m s−1 but mainly arises in upwelling north of the frontal zone. A strong downwelling dianeutral velocity of −2×10−7 m s−1 contributed by vertical turbulent mixing occurs on the upper two neutral surfaces south of the frontal zone. With assumed constant epineutral diffusivity K of 103 m2 s−1 and dianeutral diffusivity D of 10−5 m2 s−1, an area-mean net dianeutral upwelling velocity of 0.11×10−7 m s−1 is found north of 32°S across the lowermost neutral surface σθ=27.66. It means a net upward volume transport of 0.6 Sv (1 Sv=106 m3 s−1). This weak but net upwelling transport roughly corresponds to a net 0.5 Sv transported downward, with a downwelling dianeutral velocity of −0.25×10−7 m s−1 across the same neutral surface south of 45°S. Toward the core of AAIW, the net dianeutral velocity increases to 0.15×10−7 m s−1 across the“27.37” neutral surface. The corresponding net dianeutral transport increases to 0.8 Sv. You [1996] has estimated a net dianeutral upwelling transport of 1.4 Sv across the lower thermocline north of 32°S, which suggests about the same amount of volume transport upward across the upper Intermediate layer. However, south of 45°S a much stronger area-mean net downwelling dianeutral velocity of −2.63×10−7 m s−1 is found across the uppermost neutral surface σθ=27.1, indicative of a net 5.4 Sv transported downward. That the downwelling transport across the uppermost surface is 10 times larger than that across the lowermost surface south of 45°S has a strong implication for the equatorward advection of AAIW between the uppermost and lowermost neutral surfaces. When both the epineutral and dianeutral diffusivities are increased by 1 order of magnitude to K=104 m2 s−1 and D=10−4 m2 s−1, the above estimated integrated total dianeutral velocity and transport would increase by almost 10 times. However, when the diffusivities are decreased by 1 order of magnitude to K=102 m2 s−1 and D=10−6 m2 s−1, the integrated total dianeutral velocity and transport would decrease by only less than one time.
Oleg A. Saenko - One of the best experts on this subject based on the ideXlab platform.
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A Region of Enhanced Northward Antarctic Intermediate Water Transport in a Coupled Climate Model
Journal of Physical Oceanography, 2003Co-Authors: Oleg A. Saenko, Andrew J. Weaver, Matthew H. EnglandAbstract:Abstract A global coupled model is used to examine pathways of freshWater transport in the Southern Ocean. On the background of a strong zonal freshWater transport along the pathway of the Antarctic Circumpolar Current (ACC), there are meridional freshWater flows distributed nonuniformly around the globe, including in the upper ocean. The analysis does not support a simple two-dimensional scheme of Antarctic Intermediate Water (AAIW) formation, according to which the fresh AAIW forms uniformly around the circumpolar ocean. Rather, a more complex three-dimensional picture of the freshWater transport in the Southern Ocean is revealed, with enhanced AAIW formation in the southeast Pacific Ocean both north and south of the Drake Passage latitudes. Freshened by intense precipitation and surface Waters from around Antarctica, the ACC transports freshWater from the northwest to the southeast toward Drake Passage. There, a fraction of this freshWater is transported southward across 60°S with the subsurface ACC an...
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NOTES AND CORRESPONDENCE A Region of Enhanced Northward Antarctic Intermediate Water Transport in a Coupled Climate Model
2003Co-Authors: Oleg A. Saenko, Andrew J. Weaver, Matthew H. EnglandAbstract:A global coupled model is used to examine pathways of freshWater transport in the Southern Ocean. On the background of a strong zonal freshWater transport along the pathway of the Antarctic Circumpolar Current (ACC), there are meridional freshWater flows distributed nonuniformly around the globe, including in the upper ocean. The analysis does not support a simple two-dimensional scheme of Antarctic Intermediate Water (AAIW) formation, according to which the fresh AAIW forms uniformly around the circumpolar ocean. Rather, a more complex three-dimensional picture of the freshWater transport in the Southern Ocean is revealed, with enhanced AAIW formation in the southeast Pacific Ocean both north and south of the Drake Passage latitudes. Freshened by intense precipitation and surface Waters from around Antarctica, the ACC transports freshWater from the northwest to the southeast toward Drake Passage. There, a fraction of this freshWater is transported southward across 608S with the subsurface ACC and the eddy-induced flow. West of the Antarctic Peninsula, the freshWater subducts to Intermediate depths and turns northward, following the ACC and contributing to the formation of AAIW. This analysis supports previous results of enhanced subduction localized to the southern tip of South America.
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Importance of wind‐driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model
Geophysical Research Letters, 2001Co-Authors: Oleg A. Saenko, Andrew J. WeaverAbstract:An ocean-atmosphere-sea ice model is used to show the importance of wind-driven sea ice motion in the formation of Antarctic Intermediate Water (AAIW). The model is able to reasonably simulate a tongue of low salinity AAIW even when the direct momentum transfer from wind to the ocean is neglected, provided that the wind stress is applied to sea ice. In contrast, when the wind stress exclusively drives the ocean, the model fails to capture the properties of AAIW. The growth and subsequent offshore transport of sea ice acts as a freshWater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics are also shown to be important in the simulation of a local salinity minimum at Intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea.
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importance of wind driven sea ice motion for the formation of Antarctic Intermediate Water in a global climate model
Geophysical Research Letters, 2001Co-Authors: Oleg A. Saenko, Andrew J. WeaverAbstract:An ocean-atmosphere-sea ice model is used to show the importance of wind-driven sea ice motion in the formation of Antarctic Intermediate Water (AAIW). The model is able to reasonably simulate a tongue of low salinity AAIW even when the direct momentum transfer from wind to the ocean is neglected, provided that the wind stress is applied to sea ice. In contrast, when the wind stress exclusively drives the ocean, the model fails to capture the properties of AAIW. The growth and subsequent offshore transport of sea ice acts as a freshWater conduit from near-shore regions, where AABW is formed, to subpolar regions, where AAIW is formed. Sea ice dynamics are also shown to be important in the simulation of a local salinity minimum at Intermediate depths in the southern Indian Ocean and a local salinity maximum in the western Weddell Sea.
Bernadette M. Sloyan - One of the best experts on this subject based on the ideXlab platform.
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comparison of subAntarctic mode Water and Antarctic Intermediate Water formation rates in the south pacific between ncar ccsm4 and observations
Geophysical Research Letters, 2014Co-Authors: Bernadette M. Sloyan, Corinne Hartin, Rana A. Fine, Igor KamenkovichAbstract:Average formation rates for SubAntarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the South Pacific are calculated from the National Center for Atmospheric Research Community Climate System Model version 4 (NCAR-CCSM4), using chlorofluorocarbon inventories (CFC-12). When compared to observations, CCSM4 accurately simulates the southeast Pacific as the main formation region for SAMW and AAIW. Formation rates for SAMW in CCSM4 are 3.4 sverdrup (Sv), about half of the observational rate, due in part to shallow mixed layers, a thinner SAMW layer, and insufficient meridional transport. A formation rate of 8.1 Sv for AAIW in CCSM4 is higher than observations due to higher inventories in the southwest and central Pacific and surface concentrations within CCSM4. Also, a lack of data in the southwest Pacific may bias the observational rate low. This model-observation comparison is useful for understanding the uptake and transport of other gases, e.g., CO2 by the model.
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Comparison of SubAntarctic Mode Water and Antarctic Intermediate Water formation rates in the South Pacific between NCAR‐CCSM4 and observations
Geophysical Research Letters, 2014Co-Authors: Corinne Hartin, Rana A. Fine, Igor Kamenkovich, Bernadette M. SloyanAbstract:Average formation rates for SubAntarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) in the South Pacific are calculated from the National Center for Atmospheric Research Community Climate System Model version 4 (NCAR-CCSM4), using chlorofluorocarbon inventories (CFC-12). When compared to observations, CCSM4 accurately simulates the southeast Pacific as the main formation region for SAMW and AAIW. Formation rates for SAMW in CCSM4 are 3.4 sverdrup (Sv), about half of the observational rate, due in part to shallow mixed layers, a thinner SAMW layer, and insufficient meridional transport. A formation rate of 8.1 Sv for AAIW in CCSM4 is higher than observations due to higher inventories in the southwest and central Pacific and surface concentrations within CCSM4. Also, a lack of data in the southwest Pacific may bias the observational rate low. This model-observation comparison is useful for understanding the uptake and transport of other gases, e.g., CO2 by the model.
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Formation rates of SubAntarctic mode Water and Antarctic Intermediate Water within the South Pacific
Deep Sea Research Part I: Oceanographic Research Papers, 2011Co-Authors: Corinne Hartin, Lynne D. Talley, Teresa K. Chereskin, Bernadette M. Sloyan, Rana A. Fine, James D. HappellAbstract:The formation of SubAntarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW) significantly contributes to the total uptake and storage of anthropogenic gases, such as CO2 and chlorofluorocarbons (CFCs), within the world’s oceans. SAMW and AAIW formation rates in the South Pacific are quantified based on CFC-12 inventories using hydrographic data from WOCE, CLIVAR, and data collected in the austral winter of 2005. This study documents the first wintertime observations of CFC-11 and CFC-12 saturations with respect to the 2005 atmosphere in the formation region of the southeast Pacific for SAMW and AAIW. SAMW is 94% and 95% saturated for CFC-11 and CFC-12, respectively, and AAIW is 60% saturated for both CFC-11 and CFC-12. SAMW is defined from the SubAntarctic Front to the equator between potential densities 26.80–27.06 kg m � 3 , and AAIW is
Cristiano Mazur Chiessi - One of the best experts on this subject based on the ideXlab platform.
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Antarctic Intermediate Water circulation in the South Atlantic over the past 25,000 years
Paleoceanography, 2016Co-Authors: Jacob N W Howe, Delia W Oppo, Alexander M Piotrowski, Kuo-fang Huang, Stefan Mulitza, Cristiano Mazur Chiessi, Jurek BlusztajnAbstract:Antarctic Intermediate Water is an essential limb of the Atlantic meridional overturning circulation that redistributes heat and nutrients within the Atlantic Ocean. Existing reconstructions have yielded conflicting results on the history of Antarctic Intermediate Water penetration into the Atlantic across the most recent glacial termination. In this study we present leachate, foraminiferal, and detrital neodymium isotope data from three Intermediate-depth cores collected from the southern Brazil margin in the South Atlantic covering the past 25 kyr. These results reveal that strong chemical leaching following decarbonation does not extract past seaWater neodymium composition in this location. The new foraminiferal records reveal no changes in seaWater Nd isotopes during abrupt Northern Hemisphere cold events at these sites. We therefore conclude that there is no evidence for greater incursion of Antarctic Intermediate Water into the South Atlantic during either the Younger Dryas or Heinrich Stadial 1. We do, however, observe more radiogenic Nd isotope values in the Intermediate-depth South Atlantic during the mid-Holocene. This radiogenic excursion coincides with evidence for a southward shift in the Southern Hemisphere westerlies that may have resulted in a greater entrainment of radiogenic Pacific-sourced Water during Intermediate Water production in the Atlantic sector of the Southern Ocean. Our Intermediate-depth records show similar values to a deglacial foraminiferal Nd isotope record from the deep South Atlantic during the Younger Dryas but are clearly distinct during the Last Glacial Maximum and Heinrich Stadial 1, demonstrating that the South Atlantic remained chemically stratified during Heinrich Stadial 1.
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Holocene changes in Antarctic Intermediate Water flow strength in the Southwest Atlantic
Palaeogeography Palaeoclimatology Palaeoecology, 2016Co-Authors: Ines Voigt, Alberto R. Piola, Cristiano Mazur Chiessi, Rüdiger HenrichAbstract:Abstract 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 ~ 7 ka 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.
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a submarine canyon as a climate archive interaction of the Antarctic Intermediate Water with the mar del plata canyon southwest atlantic
Marine Geology, 2013Co-Authors: Ines Voigt, Alberto R. Piola, Ruediger Henrich, Benedict Preu, Till J J Hanebuth, Tilmann Schwenk, Cristiano Mazur ChiessiAbstract:Abstract The Mar del Plata Canyon is located at the continental margin off northern Argentina in a key Intermediate and deep-Water oceanographic setting. In this region, strong contour currents shape the continental margin by eroding, transporting and depositing sediments. These currents generate various depositional and erosive features which together are described as a Contourite Depositional System (CDS). The Mar del Plata Canyon intersects the CDS, and does not have any obvious connection to the shelf or to an onshore sediment source. Here we present the sedimentary processes that act in the canyon and show that continuous Holocene sedimentation is related to Intermediate-Water current activity. The Holocene deposits in the canyon are strongly bioturbated and consist mainly of the terrigenous “sortable silt” fraction (10–63 μm) without primary structures, similarly to drift deposits. We propose that the Mar del Plata Canyon interacts with an Intermediate-depth nepheloid layer generated by the northward-flowing Antarctic Intermediate Water (AAIW). This interaction results in rapid and continuous deposition of coarse silt sediments inside the canyon with an average sedimentation rate of 160 cm/kyr during the Holocene. We conclude that the presence of the Mar del Plata Canyon decreases the transport capacity of AAIW, in particular of its deepest portion that is associated with the nepheloid layer, which in turn generates a change in the contourite deposition pattern around the canyon. Since sedimentation processes in the Mar del Plata Canyon indicate a response to changes of AAIW contour-current strength related to Late Glacial/Holocene variability, the sediments deposited within the canyon are a great climate archive for paleoceanographic reconstructions. Moreover, an additional involvement of (hemi) pelagic sediments indicates episodic productivity events in response to changes in upper ocean circulation possibly associated with Holocene changes in intensity of El Nino/Southern Oscillation.
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A submarine canyon as a climate archive — Interaction of the Antarctic Intermediate Water with the Mar del Plata Canyon (Southwest Atlantic)
Marine Geology, 2013Co-Authors: Ines Voigt, Alberto R. Piola, Ruediger Henrich, Benedict Preu, Till J J Hanebuth, Tilmann Schwenk, Cristiano Mazur ChiessiAbstract:Abstract The Mar del Plata Canyon is located at the continental margin off northern Argentina in a key Intermediate and deep-Water oceanographic setting. In this region, strong contour currents shape the continental margin by eroding, transporting and depositing sediments. These currents generate various depositional and erosive features which together are described as a Contourite Depositional System (CDS). The Mar del Plata Canyon intersects the CDS, and does not have any obvious connection to the shelf or to an onshore sediment source. Here we present the sedimentary processes that act in the canyon and show that continuous Holocene sedimentation is related to Intermediate-Water current activity. The Holocene deposits in the canyon are strongly bioturbated and consist mainly of the terrigenous “sortable silt” fraction (10–63 μm) without primary structures, similarly to drift deposits. We propose that the Mar del Plata Canyon interacts with an Intermediate-depth nepheloid layer generated by the northward-flowing Antarctic Intermediate Water (AAIW). This interaction results in rapid and continuous deposition of coarse silt sediments inside the canyon with an average sedimentation rate of 160 cm/kyr during the Holocene. We conclude that the presence of the Mar del Plata Canyon decreases the transport capacity of AAIW, in particular of its deepest portion that is associated with the nepheloid layer, which in turn generates a change in the contourite deposition pattern around the canyon. Since sedimentation processes in the Mar del Plata Canyon indicate a response to changes of AAIW contour-current strength related to Late Glacial/Holocene variability, the sediments deposited within the canyon are a great climate archive for paleoceanographic reconstructions. Moreover, an additional involvement of (hemi) pelagic sediments indicates episodic productivity events in response to changes in upper ocean circulation possibly associated with Holocene changes in intensity of El Nino/Southern Oscillation.