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  • Ocean circulation  (2)
  • Paleoclimate  (2)
  • Inverse modeling
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  • 1
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    On the Abyssal circulation in the glacial Atlantic (2010)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2008. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 38 (2008): 2014-2037, doi:10.1175/2008JPO3895.1.
    Description: An inverse method is used to evaluate the information contained in sediment data for the Atlantic basin during the Last Glacial Maximum (defined here as the time interval 18–21 kyr before present). The data being considered are an updated compilation of the isotopic ratios 18O/16O (δ18O) and 13C/12C (δ13C) of fossil shells of benthic foraminifera (bottom-dwelling organisms). First, an estimate of the abyssal circulation in the modern Atlantic is obtained, which is consistent with (i) climatologies of temperature and salinity of the World Ocean Circulation Experiment, (ii) observational estimates of volume transport at specific locations, and (iii) the statements of a finite-difference geostrophic model. Second, estimates of water properties (δ18O of equilibrium calcite or δ18Oc and δ13C of dissolved inorganic carbon or δ13CDIC) derived from sediment data are combined with this circulation estimate to test their consistency with the modern flow. It is found that more than approximately 80% of water property estimates (δ18Oc or δ13CDIC) are compatible with the modern flow given their uncertainties. The consistency of glacial δ13CDIC estimates with the modern flow could be rejected after two assumptions are made: (i) the uncertainty in these estimates is ±0.1‰ (this uncertainty includes errors in sediment core chronology and oceanic representativity of benthic δ13C, which alone appears better than this value on average); and (ii) δ13CDIC in the glacial deep Atlantic was dominated by a balance between water advection and organic C remineralization. Measurements of δ13C on benthic foraminifera are clearly useful, but the current uncertainties in the distribution and budget of δ13CDIC in the glacial Atlantic must be reduced to increase the power of the test.
    Description: Support for this work comes from the U.S. National Science Foundation.
    Keywords: Abyssal circulation ; Atlantic Ocean ; Paleoclimate
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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  • 2
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    Inferring surface water equilibrium calcite δ18O during the last deglacial period from benthic foraminiferal records : implications for ocean circulation (2016)
    John Wiley & Sons
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Geophysical Union, 2015. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 30 (2015): 1470-1489, doi:10.1002/2014PA002743.
    Description: The ocean circulation modifies mixed layer (ML) tracer signals as they are communicated to the deep ocean by advection and mixing. We develop and apply a procedure for using tracer signals observed “upstream” (by planktonic foraminifera) and “downstream” (by benthic foraminifera) to constrain how tracer signals are modified by the intervening circulation and, by extension, to constrain properties of that circulation. A history of ML equilibrium calcite δ18O (δ18Oc) spanning the last deglaciation is inferred from a least-squares fit of eight benthic foraminiferal δ18Oc records to Green's function estimated for the modern ocean circulation. Disagreements between this history and the ML history implied by planktonic records would indicate deviations from the modern circulation. No deviations are diagnosed because the two estimates of ML δ18Oc agree within their uncertainties, but we suggest data collection and modeling procedures useful for inferring circulation changes in future studies. Uncertainties of benthic-derived ML δ18Oc are lowest in the high-latitude regions chiefly responsible for ventilating the deep ocean; additional high-resolution planktonic records constraining these regions are of particular utility. Benthic records from the Southern Ocean, where data are sparse, appear to have the most power to reduce uncertainties in benthic-derived ML δ18Oc. Understanding the spatiotemporal covariance of deglacial ML δ18Oc will also improve abilities of δ18Oc records to constrain deglacial circulation.
    Description: 2016-05-12
    Keywords: Oxygen isotopes ; Inverse modeling ; Deglaciation ; Tracers ; Ocean circulation ; Green's function
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
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    A global glacial ocean state estimate constrained by upper-ocean temperature proxies (2018)
    American Meteorological Society
    Details
    Publication Date: 2022-05-25
    Description: Author Posting. © American Meteorological Society, 2018. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Climate 31 (2018): 8059-8079, doi:10.1175/JCLI-D-17-0769.1.
    Description: We use the method of least squares with Lagrange multipliers to fit an ocean general circulation model to the Multiproxy Approach for the Reconstruction of the Glacial Ocean Surface (MARGO) estimate of near sea surface temperature (NSST) at the Last Glacial Maximum (LGM; circa 23–19 thousand years ago). Compared to a modern simulation, the resulting global, last-glacial ocean state estimate, which fits the MARGO data within uncertainties in a free-running coupled ocean–sea ice simulation, has global-mean NSSTs that are 2°C lower and greater sea ice extent in all seasons in both the Northern and Southern Hemispheres. Increased brine rejection by sea ice formation in the Southern Ocean contributes to a stronger abyssal stratification set principally by salinity, qualitatively consistent with pore fluid measurements. The upper cell of the glacial Atlantic overturning circulation is deeper and stronger. Dye release experiments show similar distributions of Southern Ocean source waters in the glacial and modern western Atlantic, suggesting that LGM NSST data do not require a major reorganization of abyssal water masses. Outstanding challenges in reconstructing LGM ocean conditions include reducing effects from model biases and finding computationally efficient ways to incorporate abyssal tracers in global circulation inversions. Progress will be aided by the development of coupled ocean–atmosphere–ice inverse models, by improving high-latitude model processes that connect the upper and abyssal oceans, and by the collection of additional paleoclimate observations.
    Description: DEA was supported by a NSF Graduate Research Fellowship and NSF Grant OCE-1060735. OM acknowledges support from the NSF. GF was supported by NASA Award 1553749 and Simons Foundation Award 549931.
    Keywords: Ocean ; Abyssal circulation ; Sea surface temperature ; Paleoclimate ; Inverse methods ; Ocean models
    Repository Name: Woods Hole Open Access Server
    Type: Article
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  • 4
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    On the observability of oceanic gyres (2014)
    American Meteorological Society
    Details
    Publication Date: 2022-05-26
    Description: Author Posting. © American Meteorological Society, 2014. This article is posted here by permission of American Meteorological Society for personal use, not for redistribution. The definitive version was published in Journal of Physical Oceanography 44 (2014): 2498–2523, doi:10.1175/JPO-D-13-0183.1.
    Description: This study examines the observability of a stratified ocean in a square flat basin on a midlatitude beta plane. Here, “observability” means the ability to establish, in a finite interval of time, the time-dependent ocean state given density observations over the same interval and with no regard for errors. The dynamics is linearized and hydrostatic, so that the motion can be decomposed into normal modes and the observability analysis is simplified. An observability Gramian (a symmetric matrix) is determined for the flows in an inviscid interior, in frictional boundary layers, and in a closed basin. Its properties are used to establish the condition for complete observability and to identify optimal data locations for each of these flows. It is found that complete observability of an oceanic interior in time-dependent Sverdrup balance requires that the observations originate from the westernmost location at each considered latitude. The degree of observability increases westward due to westward propagation of long baroclinic Rossby waves: data collected in the west are more informative than data collected in the east. Likewise, the best locations for observing variability in the western (eastern) boundary layer are near (far from) the boundary. The observability of a closed basin is influenced by the westward propagation and the boundaries. Optimal data locations that are identified for different resolutions (0.01 to 1 yr) and lengths of data records (0.2 to 20 yr) show a variable influence of the planetary vorticity gradient. Data collected near the meridional boundaries appear always less informative, from the viewpoint of basin observability, than data collected away from these boundaries.
    Description: This work was supported by the U.S. National Science Foundation.
    Description: 2015-03-01
    Keywords: Circulation/ Dynamics ; Ocean circulation ; Rossby waves ; Mathematical and statistical techniques ; Inverse methods ; Variability ; Oceanic variability
    Repository Name: Woods Hole Open Access Server
    Type: Article
    Format: application/pdf
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