How long to oceanic tracer and proxy equilibrium
TL;DR: In this paper, a global ocean circulation model, forced to least-square consistency with modern data, is used to find lower bounds for the time taken by surface-injected passive tracers to reach equilibrium.
About: This article is published in Quaternary Science Reviews.The article was published on 2008-04-01. It has received 90 citations till now. The article focuses on the topics: Thermohaline circulation & North Atlantic Deep Water.
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TL;DR: In this article, the authors assess the timing and spatial variability of nannoplankton extinction by analysing nannofossil counts in Cretaceous/Palaeogene boundary sections from all of the main ocean basins.
Abstract: Calcareous nannoplankton, a large group of marine autotrophs that produce carbonate skeletons, were decimated to less than 10% of species during the Cretaceous/Palaeogene boundary mass extinction, 65 million years ago. Although the mass extinction followed an impact event, the exact cause of the nannoplankton mortality is not well understood. Here we assess the timing and spatial variability of nannoplankton extinction by analysing nannofossil counts in Cretaceous/Palaeogene boundary sections from all of the main ocean basins. We find that extinction rates were higher in the Northern Hemisphere oceans, and diversity remained low for 310,000 years. In contrast, Southern Hemisphere oceans showed lower extinction rates, and a nearly immediate recovery of normal nannoplankton populations. We propose that the oblique, northward impact concentrated ejected particulates into the Northern Hemisphere, blocking sunlight and suppressing photosynthesis. Increased rates of extinction and a prolonged recovery would then be associated with the greatest concentration of particulates. We speculate that metal poisoning from fallout of the particulates may have exacerbated and extended the nannoplankton crisis in the Northern Hemisphere, and thereby slowed the recovery of the Northern Hemisphere marine food web. Over 90% of calcareous nannoplankton species disappeared during the Cretaceous–Palaeogene mass extinction, which occurred after an impact event. Palaeontological analyses show that the extinction was most pronounced in the Northern Hemisphere oceans, possibly as a result of an increased concentration of particulates created by the impact in the north.
105 citations
TL;DR: In this paper, the authors present an inverse framework to determine the mean age and its upper and lower bounds given GlobalOceanDataAnalysisProject(GLODAP) radiocarbonobservations, and show that the potential range of ages increases with the number of constituents or sources that are included in the analysis.
Abstract: A number of previous observational studies have found that the waters of the deep Pacific Ocean have an age, or elapsed time since contact with the surface, of 700‐1000 yr. Numerical models suggest ages twice as old. Here, the authors present an inverse framework to determine the mean age and its upper and lower bounds givenGlobalOceanDataAnalysisProject(GLODAP) radiocarbonobservations, andthey show that the potential range of ages increases with the number of constituents or sources that are included in the analysis. The inversion requires decomposing the World Ocean into source waters, which is obtained here using the total matrix intercomparison (TMI) method at up to 2 83 28 horizontal resolution with 11 113 surface sources. The authors find that the North Pacific at 2500-m depth can be no younger than 1100 yr old, which is older than some previous observational estimates. Accounting for the broadness of surface regions where waters originate leads to a reservoir-age correction of almost 100 yr smaller than would be estimated with a two or three water-mass decomposition and explains some of the discrepancy with previous observational studies. A best estimate of mean age is also presented using the mixing history along circulation pathways. Subject to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200‐1500 yr old, which is more in line with existing numerical model results.
102 citations
Cites background or result from "How long to oceanic tracer and prox..."
...Previous measures of the time scale of circulation, such as replacement times (Stuiver et al. 1983; Primeau and Holzer 2006; Broecker et al. 2007) or equilibration times (Wunsch and Heimbach 2008), may be rather different than the mean age of the ocean and their relationship is usually not trivial....
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...…to the caveats that inference of the mixing history would benefit from further observations and that radiocarbon cannot rule out the presence of extremely old waters from exotic sources, the deep North Pacific waters are 1200–1500 yr old, which is more in line with existing numerical model results....
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TL;DR: In this paper, the authors compare benthic δ18O records from 20 Atlantic sites and 14 Pacific sites to evaluate systematic differences in the timing of terminations in benthi18O and find evidence of ∼4000-year Pacific δ-18O lags at approximately 128 ka and 330 ka, suggesting that stratigraphic correlation of δ 18O has the potential to generate age model errors of several thousand years during terminations.
Abstract: [1] Benthic δ18O is often used as a stratigraphic tool to place marine records on a common age model and as a proxy for the timing of ice volume/sea level change. However, Skinner and Shackleton (2005) found that the timing of benthic δ18O change at the last termination differed by 3900 years between one Atlantic site and one Pacific site. These results suggest that benthic δ18O change may not always accurately record the timing of deglaciation. We compare benthic δ18O records from 20 Atlantic sites and 14 Pacific sites to evaluate systematic differences in the timing of terminations in benthic δ18O. Analysis of sedimentation rates derived from the alignment of benthic δ18O suggests a statistically significant Atlantic lead over Pacific benthic δ18O change during the last six terminations. We estimate an average Pacific benthic δ18O lag of 1600 years for Terminations 1–5, slightly larger than the delay expected from ocean mixing rates given that most glacial meltwater probably enters the North Atlantic. We additionally find evidence of ∼4000-year Pacific δ18O lags at approximately 128 ka and 330 ka, suggesting that stratigraphic correlation of δ18O has the potential to generate age model errors of several thousand years during terminations. A simple model demonstrates that these lags can be generated by diachronous temperature changes and do not require slower circulation rates. Most importantly, diachronous benthic δ18O responses must be taken into account when comparing Atlantic and Pacific benthic δ18O records or when using benthic δ18O records as a proxy for the timing of ice volume change.
95 citations
Centre national de la recherche scientifique1, Scripps Institution of Oceanography2, Geophysical Fluid Dynamics Laboratory3, Fisheries and Oceans Canada4, Woods Hole Oceanographic Institution5, Max Planck Society6, National Center for Atmospheric Research7, University of Exeter8, Geophysical Institute, University of Bergen9, Bjerknes Centre for Climate Research10, University of Kiel11, University of Paris12, Columbia University13, Goddard Institute for Space Studies14
TL;DR: In this paper, the authors explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models contributes to model-to-model differences in the simulated fields.
Abstract: . During the fifth phase of the Coupled Model Intercomparison Project (CMIP5) substantial efforts were made to systematically assess the skill of Earth system models. One goal was to check how realistically representative marine biogeochemical tracer distributions could be reproduced by models. In routine assessments model historical hindcasts were compared with available modern biogeochemical observations. However, these assessments considered neither how close modeled biogeochemical reservoirs were to equilibrium nor the sensitivity of model performance to initial conditions or to the spin-up protocols. Here, we explore how the large diversity in spin-up protocols used for marine biogeochemistry in CMIP5 Earth system models (ESMs) contributes to model-to-model differences in the simulated fields. We take advantage of a 500-year spin-up simulation of IPSL-CM5A-LR to quantify the influence of the spin-up protocol on model ability to reproduce relevant data fields. Amplification of biases in selected biogeochemical fields (O2, NO3, Alk-DIC) is assessed as a function of spin-up duration. We demonstrate that a relationship between spin-up duration and assessment metrics emerges from our model results and holds when confronted with a larger ensemble of CMIP5 models. This shows that drift has implications for performance assessment in addition to possibly aliasing estimates of climate change impact. Our study suggests that differences in spin-up protocols could explain a substantial part of model disparities, constituting a source of model-to-model uncertainty. This requires more attention in future model intercomparison exercises in order to provide quantitatively more correct ESM results on marine biogeochemistry and carbon cycle feedbacks.
81 citations
Cites background from "How long to oceanic tracer and prox..."
...The equilibration of ocean biogeochemical tracers across the entire water column amounts to several thousands of years (e.g., Heinze et al., 1999; Wunsch and Heimbach, 2008) and depends on the state of background ocean circulation as well as the turbulent mixing and eddy stirring parameterizations (e....
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...Modeling studies of paleo-oceanographic passive tracers such as δ18O or114C (Duplessy et al., 1991), or global ocean passive tracers (Wunsch and Heimbach, 2008), as well as more recently available modern global-scale data compilations (e.g., Key et al., 2004; Sarmiento and Gruber, 2006) and…...
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...…of sinking organic matter spans days to perhaps several months, the associated impact on deep water chemistry accumulates over several decades to centuries as zones of differential remineralization are mixed across water masses and follows the ocean circulation (Wunsch and Heimbach, 2008)....
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...Model Dev., 9, 1827–1851, 2016 Wunsch and Heimbach, 2007, 2008)....
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..., 1991), or global ocean passive tracers (Wunsch and Heimbach, 2008), as well as more recently available modern global-scale data compilations (e....
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TL;DR: In this article, a box inversion method is developed to explore the contributions of all surface locations to the ocean interior, as well as the degree to which the observed tracer fields can be explained by a steady-state circulation with unchanging surface-boundary conditions.
Abstract: Ocean tracer distributions have long been used to decompose the deep ocean into constituent water masses, but previous inverse methods have generally been limited to just a few water masses that have been defined by a subjective choice of static property combinations Through air–sea interaction and upper-ocean processes, all surface locations are potential sources of distinct tracer properties, and thus it is natural to define a distinct water type for each surface site Here, a new box inversion method is developed to explore the contributions of all surface locations to the ocean interior, as well as the degree to which the observed tracer fields can be explained by a steady-state circulation with unchanging surface-boundary conditions The total matrix intercomparison (TMI) method is a novel way to invert observations to solve for the pathways connecting every surface point to every interior point In the limiting case that the circulation is steady and that five conservative tracers are perf
70 citations
References
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TL;DR: The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible, except that the horizontal resolution is T62 (about 210 km) as discussed by the authors.
Abstract: The NCEP and NCAR are cooperating in a project (denoted “reanalysis”) to produce a 40-year record of global analyses of atmospheric fields in support of the needs of the research and climate monitoring communities. This effort involves the recovery of land surface, ship, rawinsonde, pibal, aircraft, satellite, and other data; quality controlling and assimilating these data with a data assimilation system that is kept unchanged over the reanalysis period 1957–96. This eliminates perceived climate jumps associated with changes in the data assimilation system. The NCEP/NCAR 40-yr reanalysis uses a frozen state-of-the-art global data assimilation system and a database as complete as possible. The data assimilation and the model used are identical to the global system implemented operationally at the NCEP on 11 January 1995, except that the horizontal resolution is T62 (about 210 km). The database has been enhanced with many sources of observations not available in real time for operations, provided b...
28,145 citations
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31 Dec 1959
TL;DR: In this paper, a classic account describes the known exact solutions of problems of heat flow, with detailed discussion of all the most important boundary value problems, including boundary value maximization.
Abstract: This classic account describes the known exact solutions of problems of heat flow, with detailed discussion of all the most important boundary value problems.
21,807 citations
TL;DR: In this article, a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics, including a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized.
Abstract: If model parameterizations of unresolved physics, such as the variety of upper ocean mixing processes, are to hold over the large range of time and space scales of importance to climate, they must be strongly physically based. Observations, theories, and models of oceanic vertical mixing are surveyed. Two distinct regimes are identified: ocean mixing in the boundary layer near the surface under a variety of surface forcing conditions (stabilizing, destabilizing, and wind driven), and mixing in the ocean interior due to internal waves, shear instability, and double diffusion (arising from the different molecular diffusion rates of heat and salt). Mixing schemes commonly applied to the upper ocean are shown not to contain some potentially important boundary layer physics. Therefore a new parameterization of oceanic boundary layer mixing is developed to accommodate some of this physics. It includes a scheme for determining the boundary layer depth h, where the turbulent contribution to the vertical shear of a bulk Richardson number is parameterized. Expressions for diffusivity and nonlocal transport throughout the boundary layer are given. The diffusivity is formulated to agree with similarity theory of turbulence in the surface layer and is subject to the conditions that both it and its vertical gradient match the interior values at h. This nonlocal “K profile parameterization” (KPP) is then verified and compared to alternatives, including its atmospheric counterparts. Its most important feature is shown to be the capability of the boundary layer to penetrate well into a stable thermocline in both convective and wind-driven situations. The diffusivities of the aforementioned three interior mixing processes are modeled as constants, functions of a gradient Richardson number (a measure of the relative importance of stratification to destabilizing shear), and functions of the double-diffusion density ratio, Rρ. Oceanic simulations of convective penetration, wind deepening, and diurnal cycling are used to determine appropriate values for various model parameters as weak functions of vertical resolution. Annual cycle simulations at ocean weather station Papa for 1961 and 1969–1974 are used to test the complete suite of parameterizations. Model and observed temperatures at all depths are shown to agree very well into September, after which systematic advective cooling in the ocean produces expected differences. It is argued that this cooling and a steady salt advection into the model are needed to balance the net annual surface heating and freshwater input. With these advections, good multiyear simulations of temperature and salinity can be achieved. These results and KPP simulations of the diurnal cycle at the Long-Term Upper Ocean Study (LOTUS) site are compared with the results of other models. It is demonstrated that the KPP model exchanges properties between the mixed layer and thermocline in a manner consistent with observations, and at least as well or better than alternatives.
3,756 citations
TL;DR: In this paper, a subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models.
Abstract: A subgrid-scale form for mesoscale eddy mixing on isopycnal surfaces is proposed for use in non-eddy-resolving ocean circulation models. The mixing is applied in isopycnal coordinates to isopycnal layer thickness, or inverse density gradient, as well as to passive scalars, temperature and salinity. The transformation of these mixing forms to physical coordinates is also presented.
3,107 citations
"How long to oceanic tracer and prox..." refers methods in this paper
...The underlying numerical code is that of Marshall et al. (1997) as modified by the ECCO projects in the interim, and includes the Large et al. (1994) mixed layer formulation, and the Gent and McWilliams (1990) eddy-flux parameterization....
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TL;DR: A preconditioner is used which, in the hydrostatic limit, is an exact integral of the Poisson operator and so leads to a single algorithm that seamlessly moves from nonhydrostatic to hydrostatic limits, competitive with the fastest ocean climate models in use today.
Abstract: The numerical implementation of an ocean model based on the incompressible Navier Stokes equations which is designed for studies of the ocean circulation on horizontal scales less than the depth of the ocean right up to global scale is described. A "pressure correction" method is used which is solved as a Poisson equation for the pressure field with Neumann boundary conditions in a geometry as complicated as that of the ocean basins. A major objective of the study is to make this inversion, and hence nonhydrostatic ocean modeling, efficient on parallel computers. The pressure field is separated into surface, hydrostatic, and nonhydrostatic components. First, as in hydrostatic models, a two-dimensional problem is inverted for the surface pressure which is then made use of in the three-dimensional inversion for the nonhydrostatic pressure. Preconditioned conjugate-gradient iteration is used to invert symmetric elliptic operators in both two and three dimensions. Physically motivated preconditioners are designed which are efficient at reducing computation and minimizing communication between processors. Our method exploits the fact that as the horizontal scale of the motion becomes very much larger than the vertical scale, the motion becomes more and more hydrostatic and the three- dimensional Poisson operator becomes increasingly anisotropic and dominated by the vertical axis. Accordingly, a preconditioner is used which, in the hydrostatic limit, is an exact integral of the Poisson operator and so leads to a single algorithm that seamlessly moves from nonhydrostatic to hydrostatic limits. Thus in the hydrostatic limit the model is "fast," competitive with the fastest ocean climate models in use today based on the hydrostatic primitive equations. But as the resolution is increased, the model dynamics asymptote smoothly to the Navier Stokes equations and so can be used to address small- scale processes. A "finite-volume" approach is employed to discretize the model in space in which property fluxes are defined normal to faces that delineate the volumes. The method makes possible a novel treatment of the boundary in which cells abutting the bottom or coast may take on irregular shapes and be "shaved" to fit the boundary. The algorithm can conveniently exploit massively parallel computers and suggests a domain decomposition which allocates vertical columns of ocean to each processing unit. The resulting model, which can handle arbitrarily complex geometry, is efficient and scalable and has been mapped on to massively parallel multiprocessors such as the Connection Machine (CM5) using data-parallel FORTRAN and the Massachusetts Institute of Technology data-flow machine MONSOON using the implicitly parallel language Id. Details of the numerical implementation of a model which has been designed for the study of dynamical processes in the ocean from the convective, through the geostrophic eddy, up to global scale are set out. The "kernel" algorithm solves the incompressible Navier Stokes equations on the sphere, in a geometry as complicated as that of the ocean basins with ir- regular coastlines and islands. (Here we use the term "Navier Stokes" to signify that the full nonhydrostatic equations are being employed; it does not imply a particular constitutive relation. The relevant equations for modeling the full complex- ity of the ocean include, as here, active tracers such as tem- perature and salt.) It builds on ideas developed in the compu- tational fluid community. The numerical challenge is to ensure that the evolving velocity field remains nondivergent. Most
2,315 citations
"How long to oceanic tracer and prox..." refers methods in this paper
...The underlying numerical code is that of Marshall et al. (1997) as modified by the ECCO projects in the interim, and includes the Large et al. (1994) mixed layer formulation, and the Gent and McWilliams (1990) eddy-flux parameterization....
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