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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Journal Article•
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.
409 citations
TL;DR: In this paper, an evaluation of the historical record of volcanic eruptions shows that subaerial volcanism increases globally by two to six times above background levels between 12-ka and 7-ka, during the last deglaciation.
263 citations
Cites background from "How long to oceanic tracer and prox..."
..., 2007), or longer (Wunsch and Heimbach, 2008), and we assign wide bounds on τ of 300–2000 yr....
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...Estimates of the equilibration time scale, τ, range from ~300 yr (Archer, 2005) to ~1800 yr (Montenegro et al., 2007), or longer (Wunsch and Heimbach, 2008), and we assign wide bounds on τ of 300–2000 yr....
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Journal Article•
TL;DR: This work presents a stricter approach to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change, and reveals a climate sensitivity over the past 65 million years of 0.3–1.9 at 95% or 68% probability.
Abstract: Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W−1 m2) of 0.3–1.9 or 0.6–1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2–4.8 K per doubling of atmospheric CO2, which agrees with IPCC estimates.
233 citations
TL;DR: In this article, a stricter approach was proposed to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change, which revealed a climate sensitivity (in K W -1 m 2) of 0.3-1.9 or 0.6 -1.3 at 95% or 68% probability, respectively.
Abstract: Many palaeoclimate studies have quantified pre-anthropogenic climate change to calculate climate sensitivity (equilibrium temperature change in response to radiative forcing change), but a lack of consistent methodologies produces a wide range of estimates and hinders comparability of results. Here we present a stricter approach, to improve intercomparison of palaeoclimate sensitivity estimates in a manner compatible with equilibrium projections for future climate change. Over the past 65 million years, this reveals a climate sensitivity (in K W -1 m 2) of 0.3-1.9 or 0.6-1.3 at 95% or 68% probability, respectively. The latter implies a warming of 2.2-4.8 K per doubling of atmospheric CO 2, which agrees with IPCC estimates. © 2012 Macmillan Publishers Limited. All rights reserved.
228 citations
TL;DR: In this paper, the authors used a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in Southern Ocean wind stress and global diapycnal mixing.
Abstract: This study uses a sector configuration of an ocean general circulation model to examine the sensitivity of circumpolar transport and meridional overturning to changes in Southern Ocean wind stress and global diapycnal mixing. At eddy-permitting, and finer, resolution, the sensitivity of circumpolar transport to forcing magnitude is drastically reduced. At sufficiently high resolution, there is little or no sensitivity of circumpolar transport to wind stress, even in the limit of no wind. In contrast, the meridional overturning circulation continues to vary with Southern Ocean wind stress, but with reduced sensitivity in the limit of high wind stress. Both the circumpolar transport and meridional overturning continue to vary with diapycnal diffusivity at all model resolutions. The circumpolar transport becomes less sensitive to changes in diapycnal diffusivity at higher resolution, although sensitivity always remains. In contrast, the overturning circulation is more sensitive to change in diapycnal...
195 citations
Cites background from "How long to oceanic tracer and prox..."
...This is short with respect to the spin up time of the ocean, which may extend to multimillennial timescales (Wunsch and Heimbach 2008; Allison et al. 2011; Jones et al. 2011)....
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References
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TL;DR: In this article, the authors explored analytical and numerical solutions for the light they cast on understanding a fluid flow and showed that boundary green functions emerge as the fundamental physical/mathematical link between interior tracer distributions and surface and other boundary variations.
Abstract: [1] Transient tracers and the closely related “age” tracers exhibit a rich physical and mathematical structure even for problems of one space dimension. This richness tends to make interpretation of observations, which are inevitably thin in both space and time, difficult, in contrast to the situation in modeling studies. At least six different timescales and corresponding space scales can appear in one-dimensional problems. In higher dimensions the number of scales increases. Several examples of analytical and numerical solutions are explored for the light they cast on understanding a fluid flow. Boundary Green functions emerge as the fundamental physical/mathematical link between interior tracer distributions and surface and other boundary variations. With transient tracers in inverse calculations one should normally use the underlying tracer distributions to attempt to solve for fundamental fluid properties, such as the mixing coefficients, rather than ambiguous “ventilation” times, which among other problems, may be determined only by the detection threshold and are often mainly functions of the tracer decay constant rather than of fluid properties. Tracers that are transient only through stochastic boundary conditions show that large-scale space/time patterns can emerge in the tracer field, having little or no clear connection to the underlying fluid flow.
44 citations
"How long to oceanic tracer and prox..." refers background in this paper
...Although it seems not widely recognized, a number of papers (e.g., Haine and Hall, 2002; Wunsch, 2002; Waugh et al., 2003) have shown that tracer ages are functions of the tracer properties including, particularly, the radioactive decay constants....
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...Often the distributions measured in cores are interpreted as depicting a system at tracer equilibrium—in the specific sense that the system has become globally uniform and, as also unchanging over extended periods....
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...(18) of Wunsch (2002) has many misprints, and we take the opportunity to both correct it, and to display a qualitatively useful analytical solution....
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39 citations
TL;DR: This paper showed that ocean temperatures in the California Current led ice volume change by 10,000 to 15,000 years, which is consistent with the leading theory of ice age cycles and supports earlier results from a terrestrial record at Devils Hole.
Abstract: At the end of an ice age, climate changes across the globe, but this response is not uniform. In some parts of the world, oceanic and atmospheric temperatures started to rise well before continental ice sheets began to melt. In his Perspective, [Lea][1] highlights the report by [ Herbert et al .][2], who show that ocean temperatures in the California Current led ice volume change by 10,000 to 15,000 years. The data support earlier results from a terrestrial record at Devils Hole, helping to reconcile them with the leading theory of ice age cycles.
[1]: http://www.sciencemag.org/cgi/content/full/293/5527/59
[2]: http://www.sciencemag.org/cgi/content/short/293/5527/71
21 citations