Seasonal and mesoscale variability of oceanic transport of anthropogenic CO₂

Estimates of the ocean's large-scale transport of anthropogenic CO₂ are based on one-time hydrographic sections, but the temporal variability of this transport has not been investigated. The aim of this study is to evaluate how the seasonal and mesoscale variability affect data-based estimates of anthropogenic CO₂ transport. To diagnose this variability, we made a global anthropogenic CO₂ simulation using an eddy-permitting version of the coupled ocean sea-ice model ORCA-LIM. As for heat transport, the seasonally varying transport of anthropogenic CO₂ is largest within 20° of the equator and shows secondary maxima in the subtropics. Ekman transport generally drives most of the seasonal variability, but the contribution of the vertical shear becomes important near the equator and in the Southern Ocean. Mesoscale variabilty contributes to the annual-mean transport of both heat and anthropogenic CO₂ with strong poleward transport in the Southern Ocean and equatorward transport in the tropics. This "rectified" eddy transport is largely baroclinic in the tropics and barotropic in the Southern Ocean due to a larger contribution from standing eddies. Our analysis revealed that most previous hydrographic estimates of meridional transport of anthropogenic CO₂ are severely biased because they neglect temporal fluctuations due to non-Ekman velocity variations. In each of the three major ocean basins, this bias is largest near the equator and in the high southern latitudes. In the subtropical North Atlantic, where most of the hydrographic-based estimates have been focused, this uncertainty represents up to 20% and 30% of total meridional transport of heat and CO₂. Generally though, outside the tropics and Southern Ocean, there are only small variations in meridional transport due to seasonal variations in tracer fields and time variations in eddy transport. For the North Atlantic, eddy variability accounts for up to 10% and 15% of the total transport of heat and CO₂. This component is not accounted for in coarse-resolution hydrographic surveys

A modeling sensitivity study of the influence of the Atlantic meridional overturning circulation on neodymium isotopic composition at the Last Glacial Maximum

Using a simple parameterisation that resolves the first order global Nd isotopic composition (hereafter expressed as ε<sub>Nd</sub> in an Ocean Global Circulation Model, we have tested the impact of different circulation scenarios on the ε<sub>Nd</sub> in the Atlantic for the Last Glacial Maximum (LGM), relative to a modern control run. Three different LGM freshwater forcing experiments are performed to test for variability in the ε<sub>Nd</sub> oceanic distribution as a function of ocean circulation. Highly distinct representations of the ocean circulation are generated in the three simulations, which drive significant differences in ε<sub>Nd</sub>, particularly in deep waters of the western part of the basin. However, at the LGM, the Atlantic is more radiogenic than in the modern control run, particularly in the Labrador basin and in the Southern Ocean. A fourth experiment shows that changes in Nd sources and bathymetry drive a shift in the ε<sub>Nd</sub> signature of the basin that is sufficient to explain the changes in the ε<sub>Nd</sub> signature of the northern end-member (NADW or GNAIW glacial equivalent) in our LGM simulations. All three of our LGM circulation scenarios show good agreement with the existing intermediate depth ε<sub>Nd</sub> paleo-data. This study cannot indicate the likelihood of a given LGM oceanic circulation scenario, even if simulations with a prominent water mass of southern origin provide the most conclusive results. Instead, our modeling results highlight the need for more data from deep and bottom waters from western Atlantic, where the ε<sub>Nd</sub> change in the three LGM scenarios is the most important (up to 3 ε<sub>Nd</sub>. This would also aid more precise conclusions concerning the evolution of the northern end-member ε<sub>Nd</sub> signature, and thus the potential use of ε<sub>Nd</sub> as a tracer of past oceanic circulation

Modelling of the anthropogenic tritium transient and its decay product helium-3 in the Mediterranean Sea using a high-resolution regional model

International audienceThis numerical study provides the first simulation of the anthropogenic tritium invasion and its decay product helium-3 (3 He) in the Mediterranean Sea. The simulation covers the entire tritium (3 H) transient generated by the atmospheric nuclear weapons tests performed in the 1950s and early 1960s and is run till 2011. Tritium, helium-3 and their derived age estimates are particularly suitable for studying intermediate and deep-water ventilation and spreading of water masses at intermediate/deep levels. The simulation is made using a high-resolution regional model NEMO (Nucleus for European Modelling of the Ocean), in a regional configuration for the Mediterranean Sea called MED12, forced at the surface with prescribed tritium evolution derived from observations. The simulation is compared to measurements of tritium and helium-3 performed along large-scale transects in the Mediterranean Sea during the last few decades on cruises of R/V Meteor: M5/6, M31/1, M44/4, M51/2, M84/3, and R/V Poseidon: 234. The results show that the input function used for the tritium generates a realistic distribution of the main hydrographic features of the Mediterranean Sea circulation. In the eastern basin, the results highlight the weak formation of Adriatic Deep Water in the model, which explains its weak contribution to the Eastern Mediterranean Deep Water (EMDW) in the Ionian sub-basin. It produces a realistic representation of the Eastern Mediterranean Transient (EMT) signal, simulating a deep-water formation in the Aegean sub-basin at the beginning of 1993, with a realistic timing of deep-water renewal in the eastern basin

A global scavenging and circulation ocean model of thorium-230 and protactinium-231 with improved particle dynamics (NEMO–ProThorP 0.1)

In this paper we set forth a 3-D ocean model of the radioactive trace isotopes 230Th and 231Pa. The interest arises from the fact that these isotopes are extensively used for investigating particle transport in the ocean and reconstructing past ocean circulation. The tracers are reversibly scavenged by biogenic and lithogenic particles.Our simulations of 230Th and 231Pa are based on the NEMO–PISCES ocean biogeochemistry general circulation model, which includes biogenic particles, namely small and big particulate organic carbon, calcium carbonate and biogenic silica. Small and big lithogenic particles from dust deposition are included in our model as well. Their distributions generally compare well with the small and big lithogenic particle concentrations from recent observations from the GEOTRACES programme, except for boundary nepheloid layers for which, as of today, there are no non-trivial prognostic models available on a global scale. Our simulations reproduce 230Th and 231Pa dissolved concentrations: they compare well with recent GEOTRACES observations in many parts of the ocean. Particulate 230Th and 231Pa concentrations are significantly improved compared to previous studies, but they are still too low because of missing particles from nepheloid layers. Our simulation reproduces the main characteristics of the 231Pa∕230Th ratio observed in the sediments and supports a moderate affinity of 231Pa to biogenic silica as suggested by recent observations relative to 230Th.Future model development may further improve understanding, especially when this will include a more complete representation of all particles, including different size classes, manganese hydroxides and nepheloid layers. This can be done based on our model as its source code is readily available.</p

Quantifying the roles of ocean circulation and biogeochemistry in governing ocean carbon-13 and atmospheric carbon dioxide at the last glacial maximum

We use a state-of-the-art ocean general circulation and biogeochemistry model to examine the impact of changes in ocean circulation and biogeochemistry in governing the change in ocean carbon-13 and atmospheric CO2 at the last glacial maximum (LGM). We examine 5 different realisations of the ocean's overturning circulation produced by a fully coupled atmosphere-ocean model under LGM forcing and suggested changes in the atmospheric deposition of iron and phytoplankton physiology at the LGM. Measured changes in carbon-13 and carbon-14, as well as a qualitative reconstruction of the change in ocean carbon export are used to evaluate the results. Overall, we find that while a reduction in ocean ventilation at the LGM is necessary to reproduce carbon-13 and carbon-14 observations, this circulation results in a low net sink for atmospheric CO2. In contrast, while biogeochemical processes contribute little to carbon isotopes, we propose that most of the change in atmospheric CO2 was due to such factors. However, the lesser role for circulation means that when all plausible factors are accounted for, most of the necessary CO2 change remains to be explained. This presents a serious challenge to our understanding of the mechanisms behind changes in the global carbon cycle during the geologic past

Global Ocean Sediment Composition and Burial Flux in the Deep Sea

Quantitative knowledge about the burial of sedimentary components at the seafloor has wide-ranging implications in ocean science, from global climate to continental weathering. The use of 230Th-normalized fluxes reduces uncertainties that many prior studies faced by accounting for the effects of sediment redistribution by bottom currents and minimizing the impact of age model uncertainty. Here we employ a recently compiled global data set of 230Th-normalized fluxes with an updated database of seafloor surface sediment composition to derive atlases of the deep-sea burial flux of calcium carbonate, biogenic opal, total organic carbon (TOC), nonbiogenic material, iron, mercury, and excess barium (Baxs). The spatial patterns of major component burial are mainly consistent with prior work, but the new quantitative estimates allow evaluations of deep-sea budgets. Our integrated deep-sea burial fluxes are 136 Tg C/yr CaCO3, 153 Tg Si/yr opal, 20Tg C/yr TOC, 220 Mg Hg/yr, and 2.6 Tg Baxs/yr. This opal flux is roughly a factor of 2 increase over previous estimates, with important implications for the global Si cycle. Sedimentary Fe fluxes reflect a mixture of sources including lithogenic material, hydrothermal inputs and authigenic phases. The fluxes of some commonly used paleo-productivity proxies (TOC, biogenic opal, and Baxs) are not well-correlated geographically with satellite-based productivity estimates. Our new compilation of sedimentary fluxes provides detailed regional and global information, which will help refine the understanding of sediment preservation

Skill metrics for confronting global upper ocean ecosystem-biogeochemistry models against field and remote sensing data

Author Posting. © Elsevier B.V., 2009. 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 Journal of Marine Systems 76 (2009): 95-112, doi:10.1016/j.jmarsys.2008.05.015.We present a generalized framework for assessing the skill of global upper ocean ecosystem-biogeochemical models against in-situ field data and satellite observations. We illustrate the approach utilizing a multi-decade (1979-2004) hindcast experiment conducted with the Community Climate System Model (CCSM-3) ocean carbon model. The CCSM-3 ocean carbon model incorporates a multi-nutrient, multi-phytoplankton functional group ecosystem module coupled with a carbon, oxygen, nitrogen, phosphorus, silicon, and iron biogeochemistry module embedded in a global, threedimensional ocean general circulation model. The model is forced with physical climate forcing from atmospheric reanalysis and satellite data products and time-varying atmospheric dust deposition. Data-based skill metrics are used to evaluate the simulated time-mean spatial patterns, seasonal cycle amplitude and phase, and subannual to interannual variability. Evaluation data include: sea surface temperature and mixed layer depth; satellite derived surface ocean chlorophyll, primary productivity, phytoplankton growth rate and carbon biomass; large-scale climatologies of surface nutrients, pCO2, and air-sea CO2 and O2 flux; and time-series data from the Joint Global Ocean Flux Study (JGOFS). Where the data is sufficient, we construct quantitative skill metrics using: model-data residuals, time-space correlation, root mean square error, and Taylor diagrams.This work was supported in part by grants from the NSF/ONR National Ocean Partnership Program (N000140210370), the NASA Ocean Biology and Biogeochemistry Program (NNX07AL80G), and the NSF Center for Microbial Oceanography Research and Education (C-MORE)

Inverse estimates of the oceanic sources and sinks of natural CO2 and the implied oceanic carbon transport

Author Posting. © American Geophysical Union, 2007. This article is posted here by permission of American Geophysical Union for personal use, not for redistribution. The definitive version was published in Global Biogeochemical Cycles 21 (2007): , doi:10.1029/2006GB002751.We use an inverse method to estimate the global-scale pattern of the air-sea flux of natural CO2, i.e., the component of the CO2 flux due to the natural carbon cycle that already existed in preindustrial times, on the basis of ocean interior observations of dissolved inorganic carbon (DIC) and other tracers, from which we estimate ΔC gasex , i.e., the component of the observed DIC that is due to the gas exchange of natural CO2. We employ a suite of 10 different Ocean General Circulation Models (OGCMs) to quantify the error arising from uncertainties in the modeled transport required to link the interior ocean observations to the surface fluxes. The results from the contributing OGCMs are weighted using a model skill score based on a comparison of each model's simulated natural radiocarbon with observations. We find a pattern of air-sea flux of natural CO2 characterized by outgassing in the Southern Ocean between 44°S and 59°S, vigorous uptake at midlatitudes of both hemispheres, and strong outgassing in the tropics. In the Northern Hemisphere and the tropics, the inverse estimates generally agree closely with the natural CO2 flux results from forward simulations of coupled OGCM-biogeochemistry models undertaken as part of the second phase of the Ocean Carbon Model Intercomparison Project (OCMIP-2). The OCMIP-2 simulations find far less air-sea exchange than the inversion south of 20°S, but more recent forward OGCM studies are in better agreement with the inverse estimates in the Southern Hemisphere. The strong source and sink pattern south of 20°S was not apparent in an earlier inversion study, because the choice of region boundaries led to a partial cancellation of the sources and sinks. We show that the inversely estimated flux pattern is clearly traceable to gradients in the observed ΔC gasex , and that it is relatively insensitive to the choice of OGCM or potential biases in ΔC gasex . Our inverse estimates imply a southward interhemispheric transport of 0.31 ± 0.02 Pg C yr−1, most of which occurs in the Atlantic. This is considerably smaller than the 1 Pg C yr−1 of Northern Hemisphere uptake that has been inferred from atmospheric CO2 observations during the 1980s and 1990s, which supports the hypothesis of a Northern Hemisphere terrestrial sink.This research was financially supported by the National Aeronautics and Space Administration under grant NAG5-12528. N. G. also acknowledges support by the National Science Foundation (OCE-0137274). Climate and Environmental Physics, Bern, acknowledges support by the European Union through the Integrated Project CarboOcean and the Swiss National Science Foundation

Biogeochemical protocols and diagnostics for the CMIP6 Ocean Model Intercomparison Project (OMIP)

The Ocean Model Intercomparison Project (OMIP) focuses on the physics and biogeochemistry of the ocean component of Earth system models participating in the sixth phase of the Coupled Model Intercomparison Project (CMIP6). OMIP aims to provide standard protocols and diagnostics for ocean models, while offering a forum to promote their common assessment and improvement. It also offers to compare solutions of the same ocean models when forced with reanalysis data (OMIP simulations) vs. when integrated within fully coupled Earth system models (CMIP6). Here we detail simulation protocols and diagnostics for OMIP's biogeochemical and inert chemical tracers. These passive-tracer simulations will be coupled to ocean circulation models, initialized with observational data or output from a model spin-up, and forced by repeating the 1948–2009 surface fluxes of heat, fresh water, and momentum. These so-called OMIP-BGC simulations include three inert chemical tracers (CFC-11, CFC-12, SF6) and biogeochemical tracers (e.g., dissolved inorganic carbon, carbon isotopes, alkalinity, nutrients, and oxygen). Modelers will use their preferred prognostic BGC model but should follow common guidelines for gas exchange and carbonate chemistry. Simulations include both natural and total carbon tracers. The required forced simulation (omip1) will be initialized with gridded observational climatologies. An optional forced simulation (omip1-spunup) will be initialized instead with BGC fields from a long model spin-up, preferably for 2000 years or more, and forced by repeating the same 62-year meteorological forcing. That optional run will also include abiotic tracers of total dissolved inorganic carbon and radiocarbon, CTabio and 14CTabio, to assess deep-ocean ventilation and distinguish the role of physics vs. biology. These simulations will be forced by observed atmospheric histories of the three inert gases and CO2 as well as carbon isotope ratios of CO2. OMIP-BGC simulation protocols are founded on those from previous phases of the Ocean Carbon-Cycle Model Intercomparison Project. They have been merged and updated to reflect improvements concerning gas exchange, carbonate chemistry, and new data for initial conditions and atmospheric gas histories. Code is provided to facilitate their implementation