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  • 1
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    American Meteorological Society
    Publication Date: 2017-01-07
    Description: Author Posting. © American Meteorological Society, 2009. 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 39 (2009): 2910-2925, doi:10.1175/2009JPO4139.1.
    Description: The propagation of Rossby waves on a midlatitude β plane is investigated in the presence of density diffusion with the aid of linear hydrostatic theory. The search for wave solutions in a vertically bounded medium subject to horizontal (vertical) diffusion leads to an eigenvalue problem of second (fourth) order. Exact solutions of the problem are obtained for uniform background stratification (N), and approximate solutions are constructed for variable N using the Wentzel–Kramers–Brillouin method. Roots of the eigenvalue relations for free waves are found and discussed. The barotropic wave of adiabatic theory is also a solution of the eigenvalue problem as this is augmented with density diffusion in the horizontal or vertical direction. The barotropic wave is undamped as fluid parcels in the wave move only horizontally and are therefore insensitive to the vortex stretching induced by mixing. On the other hand, density diffusion modifies the properties of baroclinic waves of adiabatic theory. In the presence of horizontal diffusion the baroclinic modes are damped but their vertical structure remains unaltered. The ability of horizontal diffusion to damp baroclinic waves stems from its tendency to counteract the deformation of isopycnal surfaces caused by the passage of these waves. The damping rate increases (i) linearly with horizontal diffusivity and (ii) nonlinearly with horizontal wavenumber and mode number. In the presence of vertical diffusion the baroclinic waves suffer both damping and a change in vertical structure. In the long-wave limit the damping is critical (wave decay rate numerically equal to wave frequency) and increases as the square roots of vertical diffusivity and zonal wavenumber. Density diffusion in the horizontal or vertical direction reduces the amplitude of the phase speed of westward-propagating waves. Observational estimates of eddy diffusivities suggest that horizontal and vertical mixing strongly attenuates baroclinic waves in the ocean but that vertical mixing is too weak to notably modify the vertical structure of the gravest modes.
    Description: This work was supported by the U.S. National Science Foundation.
    Keywords: Rossby waves ; Extratropics ; Buoyancy ; Mixing
    Repository Name: Woods Hole Open Access Server
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  • 2
    Publication Date: 2017-01-05
    Description: Author Posting. © The Author(s), 2010. This is the author's version of the work. It is posted here by permission of Springer-Verlag for personal use, not for redistribution. The definitive version was published in Climate Dynamics 6 (2010): 763-779, doi:10.1007/s00382-010-0790-6.
    Description: An ocean-atmosphere-sea ice model is developed to explore the time-dependent response of climate to Milankovitch forcing for the time interval 5-3 Myr BP. The ocean component is a zonally averaged model of the circulation in five basins (Arctic, Atlantic, Indian, Pacific, and Southern Oceans). The atmospheric component is a one-dimensional (latitudinal) energy balance model, and the sea-ice component is a thermodynamic model. Two numerical experiments are conducted. The first experiment does not include sea ice and the Arctic Ocean; the second experiment does. Results from the two experiments are used to investigate (i) the response of annual mean surface air and ocean temperatures to Milankovitch forcing, and (ii) the role of sea ice in this response. In both experiments, the response of air temperature is dominated by obliquity cycles at most latitudes. On the other hand, the response of ocean temperature varies with latitude and depth. Deep water formed between 45°N-65°N in the Atlantic Ocean mainly responds to precession. In contrast, deep water formed south of 60°S responds to obliquity when sea ice is not included. Sea ice acts as a time-integrator of summer insolation changes such that annual mean sea-ice conditions mainly respond to obliquity. Thus, in the presence of sea ice, air temperature changes over the sea ice are amplified, and temperature changes in deep water of southern origin are suppressed since water below sea ice is kept near the freezing point.
    Description: This work was supported by an NSERC Discovery Grant awarded to L.A.M. We also thank GEC3 for a Network Grant.
    Repository Name: Woods Hole Open Access Server
    Type: Preprint
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  • 3
    Publication Date: 2017-01-04
    Description: Author Posting. © The Author(s), 2012. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Geochimica et Cosmochimica Acta 90 (2012):126–148, doi:10.1016/j.gca.2012.05.009.
    Description: The ability of paired measurements of thorium isotope activity and particle concentration to constrain rate constants of sorption reactions and particle dynamics in the ocean is examined. This study is motivated by GEOTRACES and other sampling programs where Th and particle data are gathered in various oceanic environments. Our approach relies on inversions with a model of trace metal and particle cycling in the water column. First, the model is used to simulate vertical profiles of (i) the activity of three Th isotopes (228,230,234Th) in the dissolved phase, small suspended particles, and large sinking particles, and (ii) the concentration of small and large particles. The simulated profiles are then subsampled and corrupted with noise to generate a pseudo data set. These data are combined with the model with arbitrary values of rate constants of Th adsorption, Th desorption, particle sinking, particle remineralization, and particle (dis)aggregation in an effort to recover the actual values used to generate the data. Inversions are performed using a least-squares technique with varying assumptions about data noise, data sampling, and model errors. We find that accurate and precise recovery of rate parameters is possible when all data have a relative error of less than 20%, vertical sampling is dense enough to resolve activity and concentration gradients, and model errors are negligible. Estimating cycling rates from data with larger errors and (or) at locations where model assumptions are not tenable would remain challenging. On the other hand, the paired data set would improve significantly the relative precision of rate parameters compared to that of prior estimates (⩾100%), even with current data uncertainties and significant model errors. Based on these results, we advocate the joint measurement of all three Th isotopes, 228Ra, and particles collected by in situ filtration within GEOTRACES and other sampling programs targeted at the study of particle processes in the ocean.
    Repository Name: Woods Hole Open Access Server
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  • 4
    Publication Date: 2017-01-05
    Description: Author Posting. © American Geophysical Union, 2011. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 26 (2011): PA1212, doi:10.1029/2010PA002022.
    Description: Records of 231Pa/230Th from Atlantic sediments have been interpreted to reflect changes in ocean circulation during the geologic past. Such interpretations should be tested with due regard to the limited spatial coverage of 231Pa/230Th data and the uncertainties in our current understanding of the behavior of both nuclides in the ocean. Here an inverse method is used to evaluate the information contained in 231Pa/230Th compilations for the Holocene, Last Glacial Maximum (LGM), and Heinrich Event 1 (H1). An estimate of the abyssal circulation in the modern Atlantic Ocean is obtained by combining hydrographic observations and dynamical constraints. Then sediment 231Pa/230Th data for each time interval are combined with an advection-scavenging model in order to determine their (in)consistency with the modern circulation estimate. We find that the majority of sediment 231Pa/230Th data for the Holocene, LGM, or H1 can be brought into consistency with the modern circulation if plausible assumptions are made about the large-scale distribution of 231Pa and about model uncertainties. Moreover, the adjustments in the data needed to reach compatibility with a hypothetical state of no flow (no advection) are positively biased for each time interval, suggesting that the 231Pa/230Th data (including that for H1) are more consistent with a persistence of some circulation than with no circulation. Our study does not imply that earlier claims of a circulation change during the LGM or H1 are inaccurate, but that these claims cannot be given a rigorous basis given the current uncertainties involved in the analysis of the 231Pa/230Th data.
    Description: O.M. acknowledges the support from the U.S. National Science Foundation. J.F.M. acknowledges support from the U.S. National Science Foundation and the Comer Research and Education Foundation.
    Keywords: Pa-231/Th-230 ; Meridional overturning circulation ; Inverse method ; Heinrich Event
    Repository Name: Woods Hole Open Access Server
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  • 5
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    American Meteorological Society
    Publication Date: 2017-01-05
    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
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  • 6
    Publication Date: 2017-01-04
    Description: Author Posting. © American Geophysical Union, 2010. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Paleoceanography 25 (2010): PA4222, doi:10.1029/2010PA001936.
    Description: Observations and an ocean box model are combined in order to test the adequacy of the freshwater forcing hypothesis to explain abrupt climate change given the uncertainties in the parameterization of vertical buoyancy transport in the ocean. The combination is carried out using Bayesian stochastic inversion, which allows us to infer changes in the mass balance of Northern Hemisphere (NH) ice sheets and in the meridional transports of mass and heat in the Atlantic Ocean that would be required to explain Dansgaard-Oeschger Interstadials (DOIs) from 30 to 39 kyr B.P. The mean sea level changes implied by changes in NH ice sheet mass balance agree in amplitude and timing with reconstructions from the geologic record, which gives some support to the freshwater forcing hypothesis. The inversion suggests that the duration of the DOIs should be directly related to the growth of land ice. Our results are unaffected by uncertainties in the representation of vertical buoyancy transport in the ocean. However, the solutions are sensitive to assumptions about physical processes at polar latitudes.
    Description: This material is based upon work supported by the National Science Foundation under grant OCE‐0402363 and Department of Energy grant DE‐FG02‐08ER64619.
    Keywords: Inversion ; MOC ; Abrupt ; Sea level ; Coral ; Mixing
    Repository Name: Woods Hole Open Access Server
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  • 7
    Publication Date: 2017-01-05
    Description: Author Posting. © Sears Foundation for Marine Research, 2011. This article is posted here by permission of Sears Foundation for Marine Research for personal use, not for redistribution. The definitive version was published in Journal of Marine Research 69 (2011): 603-645, doi:10.1357/002224011799849417.
    Description: Freshwater is released along a wall of a basin containing salt water and rotating anticlockwise. The freshwater source is located near the surface between the center of the cylindrical basin and a corner along the wall. Experiments are performed with different discharge rates and the same rotation rate. The freshwater initially forms a bulge near the source, and then a buoyant gravity current bends to the right and flows along the wall toward the periphery of the basin. Separation of the current at the corner is never observed. The salinity front along the wall moves persistently away from the wall with a time scale greatly exceeding the rotation period. Its movement is compared to numerical solutions of a two-layer theory, where friction in the Ekman layer straddling the layer interface is the sole ageostrophic effect. The theory shows that the depth of the interface (h) satisfies a nonlinear diffusion equation. The symmetric part of the diffusion tensor causes light fluid to move down the gradient of h and represents the effect of vertical friction. The associated diffusivity reaches a maximum at h/δ = π/2, where δ is the Ekman layer depth. The antisymmetric part of the diffusion tensor causes light fluid to move perpendicularly to ∇h and represents the effect of geostrophic motion. The associated diffusivity increases monotonically with h/δ and greatly exceeds the diffusivity of the symmetric part if h/δ is of order of one or more. Comparison of numerical solutions with experimental data supports the theory.
    Description: This study was supported by the Ocean and Climate Change Institute at WHOI.
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  • 8
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    Unknown
    American Meteorological Society
    Publication Date: 2017-01-04
    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
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  • 9
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2010. 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 23 (2010): 4841–4855, doi:10.1175/2010JCLI3273.1.
    Description: A 1-Myr-long time-dependent solution of a zonally averaged ocean–atmosphere model subject to Milankovitch forcing is examined to gain insight into long-term changes in the planetary-scale meridional moisture flux in the atmosphere. The model components are a one-dimensional (latitudinal) atmospheric energy balance model with an active hydrological cycle and an ocean circulation model representing four basins (Atlantic, Indian, Pacific, and Southern Oceans). This study finds that the inclusion of an active hydrological cycle does not significantly modify the responses of annual-mean air and ocean temperatures to Milankovitch forcing found in previous integrations with a fixed hydrological cycle. Likewise, the meridional overturning circulation of the North Atlantic Ocean is not significantly affected by hydrological changes. Rather, it mainly responds to precessionally driven variations of ocean temperature in subsurface layers (between 70- and 500-m depth) of this basin. On the other hand, annual and zonal means of evaporation rate and meridional flux of moisture in the atmosphere respond notably to obliquity-driven changes in the meridional gradient of annual-mean insolation. Thus, when obliquity is decreased (increased), the meridional moisture flux in the atmosphere is intensified (weakened). This hydrological response is consistent with deuterium excess records from polar ice cores, which are characterized by dominant obliquity cycles.
    Description: A. A. thanks the Global Environmental and Climate Change Centre of McGill University for a Network Grant that made possible an enriching twoweek stay at WHOI during June 2007. O. M. acknowledges support from theU.S.National Science Foundation. Support from a Canadian NSERC Discovery Grant awarded to L.A.M. is gratefully acknowledged.
    Keywords: Forcing ; Moisture ; Fluxes ; Ocean models ; Coupled models ; Southern Ocean ; Pacific Ocean ; Atlantic Ocean ; Indian Ocean
    Repository Name: Woods Hole Open Access Server
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  • 10
    Publication Date: 2017-01-04
    Description: Author Posting. © American Meteorological Society, 2007. 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 37 (2007): 394-407, doi:10.1175/jpo3018.1.
    Description: The ability of paleoceanographic tracers to constrain rates of transport is examined using an inverse method to combine idealized observations with a geostrophic model. Considered are the spatial distribution, accuracy, and types of tracers required to constrain changes in meridional transport within an idealized single-hemisphere basin. Measurements of density and radioactive tracers each act to constrain rates of transport. Conservative tracers, while not of themselves able to inform regarding rates of transport, improve constraints when coupled with density or radioactive observations. It is found that the tracer data would require an accuracy one order of magnitude better than is presently available for paleo-observations to conclusively rule out factor-of-2 changes in meridional transport, even when assumed available over the entire model domain. When data are available only at the margins and bottom of the model, radiocarbon is unable to constrain transport while density remains effective only when a reference velocity level is assumed. The difficulty in constraining the circulation in this idealized model indicates that placing firm bounds on past meridional transport rates will prove challenging.
    Description: The first author is supported by the NOAA Postdoctoral Program in Climate and Global Change and GG by the National Ocean Partnership Program (ECCO). Author OM acknowledges support from the National Science Foundation.
    Keywords: Tracers ; Transport ; Paleoclimatology ; Ocean models
    Repository Name: Woods Hole Open Access Server
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