Bibliography - A. Gnanadesikan

1. Little, C. M., Daniel Goldberg, A. Gnanadesikan, and Michael Oppenheimer, 2012: On the coupled response to ice-shelf basal melting. Journal of Glaciology, 58(208), doi:10.3189/2012JoG11J037 203-215
   [ Abstract ]

   Ice-shelf basal melting is tightly coupled to ice-shelf morphology. Ice shelves, in turn, are coupled to grounded ice via their influence on compressive stress at the grounding line ('ice-shelf buttressing'). Here, we examine this interaction using a local parameterization that relates the basal melt rate to the ice-shelf thickness gradient. This formulation permits a closed-form solution for a steady-state ice tongue. Time-dependent numerical simulations reveal the spatial and temporal evolution of ice-shelf/ice-stream systems in response to changes in ocean temperature, and the influence of morphology-dependent melting on grounding-line retreat.We find that a rapid (<1 year) re-equilibration in upstream regions of ice shelves establishes a spatial pattern of basal melt rates (relative to the grounding line) that persists over centuries. Coupling melting to ice-shelf shape generally, but not always, increases grounding-line retreat rates relative to a uniform distribution with the same areaaverage melt rate. Because upstream ice-shelf thickness gradients and retreat rates increase nonlinearly with thermal forcing, morphology-dependent melting is more important to the response of weakly buttressed, strongly forced ice streams grounded on beds that slope upwards towards the ocean (e.g. those in the Amundsen Sea).

2. Downes, S. M., A. Gnanadesikan, Stephen M. Griffies, and Jorge Sarmiento, 2011: Water mass exchange in the Southern Ocean in coupled climate models. Journal of Physical Oceanography, American Meteorological Society, doi:10.1175/2011JPO4586.1
   [ Abstract ]

   We estimate water mass transformation rates resulting from surface buoyancy fluxes and interior diapycnal fluxes in the region south of 30°S in the ECCO model based state estimation and three free-running coupled climate models. The meridional transport of deep and intermediate waters across 30°S agrees well between models and observationally based estimates in the Atlantic Ocean, but not in the Indian and Pacific where the model based estimates are much smaller. Associated with this, in the models about half the southward flowing deep water is converted into lighter waters and half to denser bottom waters, whereas the observationally-based estimates convert most of the inflowing deep water to bottom waters. In the models, both Antarctic Intermediate Water (AAIW) and Antarctic Bottom Water (AABW) are formed primarily via an interior diapycnal transformation rather than being transformed at the surface via heat or freshwater fluxes. Given the small vertical diffusivity specified in the models in this region, we conclude that other processes such as cabbeling and thermobaricity must be playing an important role in water mass transformation. Finally, in the models, the largest contribution of the surface buoyancy fluxes in the Southern Ocean is to convert Upper Circumpolar Deep Water (UCDW) and Antarctic Intermediate Water (AAIW) into lighter Sub-Antarctic Mode Water (SAMW) and Antarctic Intermediate Water (AAIW).

3. Galbraith, E. D., E. Y. Kwon, A. Gnanadesikan, K. B. Rodgers, Stephen M. Griffies, D. Bianchi, Jorge Sarmiento, J. P. Dunne, J. Simeon, R. D. Slater, Andrew T. Wittenberg, and I. Held, 2011: Climate Variability and Radiocarbon in the CM2Mc Earth System Model. Journal of Climate, American Meteorological Society, doi:10.1175/2011JCLI3919.1
   [ Abstract ]

   The distribution of radiocarbon (¹⁴C) in the ocean and atmosphere has fluctuated on timescales ranging from seasons to millennia. It is thought that these fluctuations partly reflect variability in the climate system, offering a rich potential source of information to help understand mechanisms of past climate change. Here, a long simulation with a new, coupled model is used to explore the mechanisms that redistribute ¹⁴C within the Earth system on inter-annual to centennial timescales. The model, CM2Mc, is a lower-resolution version of the Geophysical Fluid Dynamics Laboratory's CM2M model, uses no flux adjustments, and incorporates a simple prognostic ocean biogeochemistry model including ¹⁴C. The atmospheric ¹⁴C and radiative boundary conditions are held constant, so that the oceanic distribution of ¹⁴C is only a function of internal climate variability. The simulation displays previously-described relationships between tropical sea surface ¹⁴C and the model-equivalents of the El Niño Southern Oscillation and Indonesian Throughflow. Sea surface ¹⁴C variability also arises from fluctuations in the circulations of the subarctic Pacific and Southern Ocean, including North Pacific decadal variability, and episodic ventilation events in the Weddell Sea that are reminiscent of the Weddell Polynya of 1974-1976. Interannual variability in the air-sea balance of ¹⁴C is dominated by exchange within the belt of intense Southern Westerly winds, rather than at the convective locations where the surface ¹⁴C is most variable. Despite significant interannual variability, the simulated impact on air-sea exchange is an order of magnitude smaller than the recorded atmospheric ¹⁴C variability of the past millennium. This result partly reflects the importance of variability in the production rate of ¹⁴C in determining atmospheric ¹⁴C, but may also reflect an underestimate of natural climate variability, particularly in the Southern Westerly winds.

4. Rodgers, K. B., S. E. Mikaloff-Fletcher, D. Bianchi, C. Beaulieu, E. D. Galbraith, A. Gnanadesikan, A. G. Hogg, D. Iudicone, B. R. Lintner, T. Naegler, P. J. Reimer, Jorge Sarmiento, and R. D. Slater, 2011: Interhemispheric gradient of atmospheric radiocarbon reveals natural variability of Southern Ocean winds. Climate of the Past, European Geosciences Union, 7, doi:10.5194/cp-7-1123-2011 1123-1138
   [ Abstract ]

   Tree ring Δ¹⁴C data (Reimer et al., 2004; McCormac et al., 2004) indicate that atmospheric Δ¹⁴C varied on multi-decadal to centennial timescales, in both hemispheres, over the period between AD950 and 1830. The Northern and Southern Hemispheric Δ¹⁴C records display similar variability, but from the data alone is it not clear whether these variations are driven by the production of ¹⁴C in the stratosphere (Stuiver and Quay, 1980) or by perturbations to exchanges between carbon reservoirs (Siegenthaler et al., 1980). As the sea-air flux of ¹⁴CO₂ has a clear maximum in the open ocean regions of the Southern Ocean, relatively modest perturbations to the winds over this region drive significant perturbations to the interhemispheric gradient. In this study, model simulations are used to show that Southern Ocean winds are likely a main driver of the observed variability in the interhemispheric gradient over AD950-1830, and further, that this variability may be larger than the Southern Ocean wind trends that have been reported for recent decades (notably 1980-2004). This interpretation also implies that there may have been a significant weakening of the winds over the Southern Ocean within a few decades of AD1375, associated with the transition between the Medieval Climate Anomaly and the Little Ice Age. The driving forces that could have produced such a shift in the winds at the Medieval Climate Anomaly to Little Ice Age transition remain unknown. Our process-focused suite of perturbation experiments with models raises the possibility that the current generation of coupled climate and earth system models may underestimate the natural background multi-decadal- to centennial-timescale variations in the winds over the Southern Ocean.

5. Bianchi, D., Jorge Sarmiento, A. Gnanadesikan, R. M. Key, P. Schlosser, and R. Newton, 2010: Low helium flux from the mantle inferred from simulations of oceanic helium. Earth and Planetary Science Letters, (297), doi:10.1016/j.epsl.2010.06.037 Elsevier
   [ Abstract ]

   The high ³He/⁴He isotopic ratio of oceanic helium relative to the atmosphere has long been recognized as the signature of mantle ³He outgassing from the Earth's interior. The outgassing flux of helium is frequently used to normalize estimates of chemical fluxes of elements from the solid Earth, and provides a strong constraint to models of mantle degassing. Here we use a suite of ocean general circulation models and helium isotope data obtained by the World Ocean Circulation Experiment to constrain the flux of helium from the mantle to the oceans. Our results suggest that the currently accepted flux is overestimated by a factor of 2. We show that a flux of 527±102mol year⁻¹ is required for ocean general circulation models that produce distributions of ocean ventilation tracers such as radiocarbon and chlorofluorocarbons that match observations. This new estimate calls for a reevaluation of the degassing fluxes of elements that are currently tied to the helium fluxes, including noble gases and carbon dioxide.

6. Gnanadesikan, A., K. A. Emanuel, Gabriel A. Vecchi, Whit G. Anderson, and R. Hallberg, July 2010: How Ocean color can steer Pacific tropical cyclones. Geophysical Research Letters, (InPress),
   [ Abstract ]

   Because ocean color alters the absorption of sunlight, it can produce changes in sea surface temperatures with further impacts on atmospheric circulation. These changes can project onto fields previously recognized to alter the distribution of tropical cyclones. If the North Pacific subtropical gyre contained no absorbing and scattering materials, the result would be to reduce subtropical cyclone activity in the subtropical Northwest Pacific by 2/3, while concentrating cyclone tracks along the equator. Predicting tropical cyclone activity using coupled models may thus require consideration of the details of how heat moves into the upper thermocline as well as biogeochemical cycling.

7. Palter, J. B., Jorge Sarmiento, A. Gnanadesikan, J. Simeon, and R. D. Slater, 2010: Fueling export production: nutrient return pathways from the deep ocean and their dependence on the Meridional Overturning Circulation. Biogeosciences, 7, doi:10.5194/bg-7-3549-2010 3549–3568
   [ Abstract ]

   In the Southern Ocean, mixing and upwelling in the presence of heat and freshwater surface fluxes transform subpycnocline water to lighter densities as part of the upward branch of the Meridional Overturning Circulation (MOC). One hypothesized impact of this transformation is the restoration of nutrients to the global pycnocline, without which biological productivity at low latitudes would be significantly reduced. Here we use a novel set of modeling experiments to explore the causes and consequences of the Southern Ocean nutrient return pathway. Specifically, we quantify the contribution to global productivity of nutrients that rise from the ocean interior in the Southern Ocean, the northern high latitudes, and by mixing across the low latitude pycnocline. In addition, we evaluate how the strength of the Southern Ocean winds and the parameterizations of subgridscale processes change the dominant nutrient return pathways in the ocean. Our results suggest that nutrients upwelled from the deep ocean in the Antarctic Circumpolar Current and subducted in Subantartic Mode Water support between 33 and 75% of global export production between 30^◦ S and 30^◦ N. The high end of this range results from an ocean model in which the MOC is driven primarily by windinduced Southern Ocean upwelling, a configuration favored due to its fidelity to tracer data, while the low end results from an MOC driven by high diapycnal diffusivity in the pycnocline. In all models, nutrients exported in the SAMW layer are utilized and converted rapidly (in less than 40 years) to remineralized nutrients, explaining previous modeling results that showed little influence of the drawdown of SAMW surface nutrients on atmospheric carbon concentrations.

8. Sarmiento, Jorge, R. D. Slater, J. P. Dunne, A. Gnanadesikan, and M. R. Hiscock, 2010: Efficiency of small scale carbon mitigation by patch iron fertilization. Biogeosciences, 7, doi:10.5194/bg-7-3593-2010 3593–3624
   [ Abstract ]

   While nutrient depletion scenarios have long shown that the high-latitude High Nutrient Low Chlorophyll (HNLC) regions are the most effective for sequestering atmospheric carbon dioxide, recent simulations with prognostic biogeochemical models have suggested that only a fraction of the potential drawdown can be realized. We use a global ocean biogeochemical general circulation model developed at GFDL and Princeton to examine this and related issues. We fertilize two patches in the North and Equatorial Pacific, and two additional patches in the Southern Ocean HNLC region north of the biogeochemical divide and in the Ross Sea south of the biogeochemical divide. We evaluate the simulations using observations from both artificial and natural iron fertilization experiments at nearby locations. We obtain by far the greatest response to iron fertilization at the Ross Sea site, where sea ice prevents escape of sequestered CO₂ during the wintertime, and the CO₂ removed from the surface ocean by the biological pump is carried into the deep ocean by the circulation. As a consequence, CO₂ remains sequestered on century time-scales and the efficiency of fertilization remains almost constant no matter how frequently iron is applied as long as it is confined to the growing season. The second most efficient site is in the Southern Ocean. The North Pacific site has lower initial nutrients and thus a lower efficiency. Fertilization of the Equatorial Pacific leads to an expansion of the suboxic zone and a striking increase in denitrification that causes a sharp reduction in overall surface biological export production and CO₂ uptake. The impacts on the oxygen distribution and surface biological export are less prominent at other sites, but nevertheless still a source of concern. The century time scale retention of iron in this model greatly increases the long-term biological response to iron addition as compared with simulations in which the added iron is rapidly scavenged from the ocean.

9. Rodgers, K. B., R. M. Key, A. Gnanadesikan, Jorge Sarmiento, O. Aumont, L. Bopp, S. C. Doney, J. P. Dunne, D. M. Glover, A. Ishida, M. Ishii, and A. R. Jacobson, et al., 2009: Using altimetry to help explain patchy changes in hydrographic carbon measurements. Journal of Geophysical Research – Oceans, doi:10.1029/2008JC005183 114
   [ Abstract ]

   Here we use observations and ocean models to identify mechanisms driving large seasonal to interannual variations in dissolved inorganic carbon (DIC) and dissolved oxygen (O₂) in the upper ocean. We begin with observations linking variations in upper ocean DIC and O₂ inventories with changes in the physical state of the ocean. Models are subsequently used to address the extent to which the relationships derived from short-timescale (six months to two years) repeat measurements are representative of variations over larger spatial and temporal scales. The main new result is that convergence and divergence (column stretching) attributed to baroclinic Rossby waves can make a first-order contribution to DIC and O₂ variability in the upper ocean. This results in a close correspondence between natural variations in DIC and O₂ column inventory variations and sea surface height (SSH) variations over much of the ocean. Oceanic Rossby wave activity is an intrinsic part of the natural variability in the climate system and is elevated even in the absence of significant interannual variability in climate mode indices. The close correspondence between SSH and both DIC and O₂ column inventories for many regions suggests that SSH changes (inferred from satellite altimetry) may prove useful in reducing uncertainty in separating natural and anthropogenic DIC signals (using measurements from CLIVAR’s CO₂/Repeat Hydrography program).

10. Sarmiento, Jorge, R. D. Slater, J. P. Dunne, A. Gnanadesikan, and M. R. Hiscock, 2009: Efficiency of Small Scale Carbon Mitigation by Patch Iron Fertilization. Biogeosciences, http://www.biogeosciences-discuss.net/6/10381/2009/bgd-6-10381-2009.html, (6), 10381-10446
    [ Abstract ]

    While nutrient depletion scenarios have long shown that the high-latitude High Nutrient Low Chlorophyll (HNLC) regions are the most effective for sequestering atmospheric carbon dioxide, recent simulations with prognostic biogeochemical models have suggested that only a fraction of the potential drawdown can be realized. We use a global ocean biogeochemical general circulation model developed at GFDL and Princeton to examine this and related issues. We fertilize two patches in the North and Equatorial Pacific, and two additional patches in the Southern Ocean HNLC region north of the biogeochemical divide and in the Ross Sea south of the biogeochemical divide. We obtain by far the greatest response to iron fertilization at the Ross Sea site. Here the CO2 remains sequestered on century time-scales and the efficiency of fertilization remains almost constant no matter how frequently iron is applied as long as it is confined to the growing season. The second most efficient site is in the Southern Ocean. Here the biological response to iron fertilization is comparable to the Ross Sea, but the enhanced biological uptake of CO2 is more spread out in the vertical and thus less effective at leading to removal of CO2 from the atmosphere. The North Pacific site has lower initial nutrients and thus a lower efficiency. Fertilization of the Equatorial Pacific leads to an expansion of the suboxic zone and a striking increase in denitrification that causes a sharp reduction in overall surface biological export production and CO2 uptake. The impacts on the oxygen distribution and surface biological export are less prominent at other sites, but nevertheless still a source of concern. The century time scale retention of iron in these models greatly increases the long-term biological response to iron addition as compared with models in which the added iron is rapidly scavenged from the ocean.

11. Marinov, I., A. Gnanadesikan, Jorge Sarmiento, J. R. Toggweiler, M. Follows, and B. K. Mignone, 2008: Impact of oceanic circulation on biological Carbon Storage in the ocean and atmospheric pCO₂. Global Biogeochemical Cycles, 22(GB3007), doi:10.1029/2007GB002958
    [ Abstract ]

    We use both theory and ocean biogeochemistry models to examine the role of the soft-tissue biological pump in controlling atmospheric CO₂. We demonstrate that atmospheric CO₂ can be simply related to the amount of inorganic carbon stored in the ocean by the soft-tissue pump, which we term (OCSsoft). OCSsoft is linearly related to the inventory of remineralized nutrient, which in turn is just the total nutrient inventory minus the preformed nutrient inventory. In a system where total nutrient is conserved, atmospheric CO₂ can thus be simply related to the global inventory of preformed nutrient. Previous model simulations have explored how changes in the surface concentration of nutrients in deepwater formation regions change the global preformed nutrient inventory. We show that changes in physical forcing such as winds, vertical mixing, and lateral mixing can shift the balance of deepwater formation between the North Atlantic (where preformed nutrients are low) and the Southern Ocean (where they are high). Such changes in physical forcing can thus drive large changes in atmospheric CO₂, even with minimal changes in surface nutrient concentration. If Southern Ocean deepwater formation strengthens, the preformed nutrient inventory and thus atmospheric CO₂ increase. An important consequence of these new insights is that the relationship between surface nutrient concentrations, biological export production, and atmospheric CO₂ is more complex than previously predicted. Contrary to conventional wisdom, we show that OCSsoft can increase and atmospheric CO₂ decrease, while surface nutrients show minimal change and export production decreases.

12. Marinov, I., M. Follows, A. Gnanadesikan, Jorge Sarmiento, and R. D. Slater, 2008: How does ocean biology affect atmospheric pCO₂: Theory and models. Journal of Geophysical Research, 113(C07032), doi:10.1029/2007JC004598
    [ Abstract ]

    This paper examines the sensitivity of atmospheric pCO₂ to changes in ocean biology that result in drawdown of nutrients at the ocean surface. We show that the global inventory of preformed nutrients is the key determinant of atmospheric pCO₂ and the oceanic carbon storage due to the soft-tissue pump (OCSsoft). We develop a new theory showing that under conditions of perfect equilibrium between atmosphere and ocean, atmospheric pCO₂ can be written as a sum of exponential functions of OCSsoft. The theory also demonstrates how the sensitivity of atmospheric pCO₂ to changes in the soft-tissue pump depends on the preformed nutrient inventory and on surface buffer chemistry. We validate our theory against simulations of nutrient depletion in a suite of realistic general circulation models (GCMs). The decrease in atmospheric pCO₂ following surface nutrient depletion depends on the oceanic circulation in the models. Increasing deep ocean ventilation by increasing vertical mixing or Southern Ocean winds increases the atmospheric pCO₂ sensitivity to surface nutrient forcing. Conversely, stratifying the Southern Ocean decreases the atmospheric CO₂ sensitivity to surface nutrient depletion. Surface CO₂ disequilibrium due to the slow gas exchange with the atmosphere acts to make atmospheric pCO₂ more sensitive to nutrient depletion in high-ventilation models and less sensitive to nutrient depletion in low-ventilation models. Our findings have potentially important implications for both past and future climates.

13. Dunne, J. P., Jorge Sarmiento, and A. Gnanadesikan, 2007: A synthesis of global particle export from the surface ocean and cycling through the ocean interior and on the seafloor. Global Biogeochemical Cycles, 21(GB4006), doi:10.1029/2006GB002907
    [ Abstract ]

    We present a new synthesis of the oceanic cycles of organic carbon, silicon, and calcium carbonate. Our calculations are based on a series of algorithms starting with satellite-based primary production and continuing with conversion of primary production to sinking particle flux, penetration of particle flux to the deep sea, and accumulation in sediments. Regional and global budgets from this synthesis highlight the potential importance of shelves and near-shelf regions for carbon burial. While a high degree of uncertainty remains, this analysis suggests that shelves, less than 50 m water depths accounting for 2% of the total ocean area, may account for 48% of the global flux of organic carbon to the seafloor. Our estimates of organic carbon and nitrogen flux are in generally good agreement with previous work while our estimates for CaCO₃ and SiO₂ fluxes are lower than recent work. Interannual variability in particle export fluxes is found to be relatively small compared to intra-annual variability over large domains with the single exception of the dominating role of El Nin˜o-Southern Oscillation variability in the central tropical Pacific. Comparison with available sediment-based syntheses of benthic remineralization and burial support the recent theory of mineral protection of organic carbon flux through the deep ocean, pointing to lithogenic material as an important carrier phase of organic carbon to the deep seafloor. This work suggests that models which exclude the role of lithogenic material would underestimate the penetration of POC to the deep seafloor by approximately 16–51% globally, and by a much larger fraction in areas with low productivity. Interestingly, atmospheric dust can only account for 31% of the total lithogenic flux and 42% of the lithogenically associated POC flux, implying that a majority of this material is riverine or directly erosional in origin.

14. Marinov, I., A. Gnanadesikan, J. R. Toggweiler, and Jorge Sarmiento, 2006: The Southern Ocean biogeochemical divide. Nature, 441, doi:10.1038/nature04883 964-967
    [ Abstract ]

    Modeling studies have demonstrated that the nutrient and carbon cycles in the Southern Ocean play a central role in setting the air-sea balance of CO₂ and global biological production. Box model studies first pointed out that an increase in nutrient utilization in the high latitudes results in a strong decrease in the atmospheric carbon dioxide partial pressure (pCO₂ ). This early research led to two important ideas: high latitude regions are more important in determining atmospheric (pCO₂ than low latitudes, despite their much smaller area, and nutrient utilization and atmospheric (pCO₂ are tightly linked. Subsequent general circulation model simulations show that the Southern Ocean is the most important high latitude region in controlling preindustrial atmospheric CO₂ because it serves as a lid to a larger volume of the deep ocean. Other studies point out the crucial role of the Southern Ocean in the uptake and storage of anthropogenic carbon dioxide and in controlling global biological production. Here we probe the system to determine whether certain regions of the Southern Ocean are more critical than others for air-sea CO₂ balance and the biological export production, by increasing surface nutrient drawdown in an ocean general circulation model. We demonstrate that atmospheric CO₂ and global biological export production are controlled by different regions of the Southern Ocean. The air–sea balance of carbon dioxide is controlled mainly by the biological pump and circulation in the Antarctic deep-water formation region, whereas global export production is controlled mainly by the biological pump and circulation in the Subantarctic intermediate and mode water formation region. The existence of this biogeochemical divide separating the Antarctic from the Subantarctic suggests that it may be possible for climate change or human intervention to modify one of these without greatly altering the other.

15. Mignone, B. K., A. Gnanadesikan, Jorge Sarmiento, and R. D. Slater, 2006: Central role of southern hemisphere winds and eddies in modulating the oceanic uptake of anthropogenic carbon. Geophysical Research Letters, 33(L01604), doi:10.1029/2005GL024464
    [ Abstract ]

    Although the world ocean is known to be a major sink of anthropogenic carbon dioxide, the exact processes governing the magnitude and regional distribution of carbon uptake remain poorly understood. Here we show that Southern Hemisphere winds, by altering the Ekman volume transport out of the Southern Ocean, strongly control the regional distribution of anthropogenic uptake in an ocean general circulation model, while winds and isopycnal thickness mixing together, by altering the volume of light, actively-ventilated ocean water, exert strong control over the absolute magnitude of anthropogenic uptake. These results are provocative in suggesting that climate-mediated changes in pycnocline volume may ultimately control changes in future carbon uptake.

16. Dunne, J. P., R. A. Armstrong, A. Gnanadesikan, and Jorge Sarmiento, 2005: Empirical and mechanistic models for the particle export ratio. Global Biogeochemical Cycles, 19(GB40226), doi:10.1029/2004GB002390
    [ Abstract ]

    We present new empirical and mechanistic models for predicting the export of organic carbon out of the surface ocean by sinking particles. To calibrate these models, we have compiled a synthesis of field observations related to ecosystem size structure, primary production and particle export from around the globe. The empirical model captures 61% of the observed variance in the ratio of particle export to primary production (the pe ratio) using sea-surface temperature and chlorophyll concentrations (or primary productivity) as predictor variables. To describe the mechanisms responsible for pe-ratio variability, we present size-based formulations of phytoplankton grazing and sinking particle export, combining them into an alternative, mechanistic model. The formulation of grazing dynamics, using simple power laws as closure terms for small and large phytoplankton, reproduces 74% of the observed variability in phytoplankton community composition wherein large phytoplankton augment small ones as production increases. The formulation for sinking particle export partitions a temperature-dependent fraction of small and large phytoplankton grazing into sinking detritus. The mechanistic model also captures 61% of the observed variance in pe ratio, with large phytoplankton in high biomass and relatively cold regions leading to more efficient export. In this model, variability in primary productivity results in a biomass-modulated switch between small and large phytoplankton pathways.

17. Orr, J. C., V. J. Fabry, O. Aumont, L. Bopp, S. C. Doney, R. A. Feely, A. Gnanadesikan, N. Gruber, A. Ishida, F. Joos, R. M. Key, and K. Lindsay, et al., 2005: Anthropogenic ocean acidification over the 21st century and its impact on marine calcifying organisms. Nature, 437, doi:10.1038/nature04095 681-686
    [ Abstract ]

    Today’s surface ocean is saturated with respect to calcium carbonate, but increasing atmospheric carbon dioxide concentrations are reducing ocean pH and carbonate ion concentrations, and thus the level of calcium carbonate saturation. Experimental evidence suggests that if these trends continue, key marine organisms—such as corals and some plankton—will have difficulty maintaining their external calcium carbonate skeletons. Here we use 13 models of the ocean–carbon cycle to assess calcium carbonate saturation under the IS92a ‘business-as-usual’ scenario for future emissions of anthropogenic carbon dioxide. In our projections, Southern Ocean surface waters will begin to become undersaturated with respect to aragonite, a metastable form of calcium carbonate, by the year 2050. By 2100, this undersaturation could extend throughout the entire Southern Ocean and into the subarctic Pacific Ocean. When live pteropods were exposed to our predicted level of undersaturation during a two-day shipboard experiment, their aragonite shells showed notable dissolution. Our findings indicate that conditions detrimental to high-latitude ecosystems could develop within decades, not centuries as suggested previously.

18. Doney, S. C., K. Lindsay, K. Caldeira, J.-M. Campin, H. Drange, J. C. Dutay, M. Follows, Y. Gao, A. Gnanadesikan, N. Gruber, A. Ishida, and F. Joos, et al., 2004: Evaluating global ocean carbon models: The importance of realistic physics. Global Biogeochemical Cycles, 18(GB3017), doi:10.1029/2003GB002150
    [ Abstract ]

    A suite of standard ocean hydrographic and circulation metrics are applied to the equilibrium physical solutions from 13 global carbon models participating in phase 2 of the Ocean Carbon-cycle Model Intercomparison Project (OCMIP-2). Model-data comparisons are presented for sea surface temperature and salinity, seasonal mixed layer depth, meridional heat and freshwater transport, 3-D hydrographic fields, and meridional overturning. Considerable variation exists among the OCMIP-2 simulations, with some of the solutions falling noticeably outside available observational constraints. For some cases, model-model and model-data differences can be related to variations in surface forcing, subgrid-scale parameterizations, and model architecture. These errors in the physical metrics point to significant problems in the underlying model representations of ocean transport and dynamics, problems that directly affect the OCMIP predicted ocean tracer and carbon cycle variables (e.g., air-sea CO₂ flux, chlorofluorocarbon and anthropogenic CO₂ uptake, and export production). A substantial fraction of the large model-model ranges in OCMIP-2 biogeochemical fields (±25–40%) represents the propagation of known errors in model physics. Therefore the model-model spread likely overstates the uncertainty in our current understanding of the ocean carbon system, particularly for transport-dominated fields such as the historical uptake of anthropogenic CO₂. A full error assessment, however, would need to account for additional sources of uncertainty such as more complex biological-chemical-physical interactions, biases arising from poorly resolved or neglected physical processes, and climate change.

19. Gnanadesikan, A., J. P. Dunne, I. Aavatsmark, R. M. Key, Jorge Sarmiento, R. D. Slater, and P. S. Swathi, 2004: Oceanic ventilation and biogeochemical cycling: Understanding the physical mechanisms that produce realistic distributions of tracers and productivity. Global Biogeochemical Cycles, 18(GB4010), doi:10.1029/2003GB002097
    [ Abstract ]

    Differing models of the ocean circulation support different rates of ventilation, which in turn produce different distributions of radiocarbon, oxygen, and export production. We examine these fields within a suite of general circulation models run to examine the sensitivity of the circulation to the parameterization of subgridscale mixing and surface forcing. We find that different models can explain relatively high fractions of the spatial variance in some fields such as radiocarbon, and that newer estimates of the rate of biological cycling are in better agreement with the models than previously published estimates. We consider how different models achieve such agreement and show that they can accomplish this in different ways. For example, models with high vertical diffusion move young surface waters into the Southern Ocean, while models with high winds move more young North Atlantic water into this region. The dependence on parameter values is not simple. Changes in the vertical diffusion coefficient, for example, can produce major changes in advective fluxes. In the coarse-resolution models studied here, lateral diffusion plays a major role in the tracer budget of the deep ocean, a somewhat worrisome fact as it is poorly constrained both observationally and theoretically.

20. Matsumoto, K., Jorge Sarmiento, R. M. Key, O. Aumont, J. L. Bullister, K. Caldeira, J.-M. Campin, S. C. Doney, H. Drange, J. C. Dutay, Y. Gao, A. Gnanadesikan, and N. Gruber, et al., 2004: Evaluation of ocean carbon cycle models with data-based metrics. Geophysical Research Letters, 31(L07303), doi:10.1029/2003GL018970
    [ Abstract ]

    New radiocarbon and chlorofluorocarbon-11 data from the World Ocean Circulation Experiment are used to assess a suite of 19 ocean carbon cycle models. We use the distributions and inventories of these tracers as quantitative metrics of model skill and find that only about a quarter of the suite is consistent with the new databased metrics. This should serve as a warning bell to the larger community that not all is well with current generation of ocean carbon cycle models. At the same time, this highlights the danger in simply using the available models to represent the state-of-the-art modeling without considering the credibility of each model.

21. Mignone, B. K., Jorge Sarmiento, R. D. Slater, and A. Gnanadesikan, 2004: Sensitivity of sequestration efficiency to mixing processes in the global ocean. Energy, 29(9-10), doi:10.1016/j.energy.2004.03.080 1467-1478
    [ Abstract ]

    A number of large-scale sequestration strategies have been considered to help mitigate rising levels of atmospheric carbon dioxide (CO₂). Here, we use an ocean general circulation model (OGCM) to evaluate the efficiency of one such strategy currently receiving much attention, the direct injection of liquid CO₂ into selected regions of the abyssal ocean. We find that currents typically transport the injected plumes quite far before they are able to return to the surface and release CO₂ through air–sea gas exchange. When injected at sufficient depth (well within or below the main thermocline), most of the injected CO₂ outgasses in high latitudes (mainly in the Southern Ocean) where vertical exchange is most favored. Virtually all OGCMs that have performed similar simulations confirm these global patterns, but regional differences are significant, leading efficiency estimates to vary widely among models even when identical protocols are followed. In this paper, we make a first attempt at reconciling some of these differences by performing a sensitivity analysis in one OGCM, the Princeton Modular Ocean Model. Using techniques we have developed to maintain both the modeled density structure and the absolute magnitude of the overturning circulation while varying important mixing parameters, we estimate the sensitivity of sequestration efficiency to the magnitude of vertical exchange within the low-latitude pycnocline. Combining these model results with available tracer data permits us to narrow the range of model behavior, which in turn places important constraints on sequestration efficiency.

22. Gnanadesikan, A., Jorge Sarmiento, and R. D. Slater, 2003: Effects of patchy ocean fertilization on atmospheric carbon dioxide and biological production. Global Biogeochemical Cycles, 17(2), doi:10.1029/2002GB001940
    [ Abstract ]

    Increasing oceanic productivity by fertilizing nutrient-rich regions with iron has been proposed as a mechanism to offset anthropogenic emissions of carbon dioxide. Earlier studies examined the impact of large-scale fertilization of vast reaches of the ocean for long periods of time. We use an ocean general circulation model to consider more realistic scenarios involving fertilizing small regions (a few hundred kilometers on a side) for limited periods of time (of order 1 month). A century after such a fertilization event, the reduction of atmospheric carbon dioxide is between 2% and 44% of the initial pulse of organic carbon export to the abyssal ocean. The fraction depends on how rapidly the surface nutrient and carbon fields recover from the fertilization event. The modeled recovery is very sensitive to the representation of biological productivity and remineralization. Direct verification of the uptake would be nearly impossible since changes in the air-sea flux due to fertilization would be much smaller than those resulting from natural spatial variability. Because of the sensitivity of the uptake to the long-term fate of the iron and organic matter, indirect verification by measurement of the organic matter flux would require high vertical resolution and long-term monitoring. Finally, the downward displacement of the nutrient profile resulting from an iron-induced productivity spurt may paradoxically lead to a long-term reduction in biological productivity. In the worst-case scenario, removing 1 ton of carbon from the atmosphere for a century is associated with a 30-ton reduction in biological export of carbon.

23. Toggweiler, J. R., A. Gnanadesikan, and S. Carson, 2003: Representation of the carbon cycle in box models and GCMs: 1. Solubility pump. Global Biogeochemical Cycles, 17(1), doi:10.1029/2001GB001401
    [ Abstract ]

    Bacastow [1996], Broecker et al. [1999], and Archer et al. [2000] have called attention recently to the fact that box models and general circulation models (GCMs) represent the thermal partitioning of CO₂ between the warm surface ocean and cold deep ocean in different ways. They attribute these differences to mixing and circulation effects in GCMs that are not resolved in box models. The message that emerges from these studies is that box models have overstated the importance of the ocean’s polar regions in the carbon cycle. A reduced role for the polar regions has major implications for the mechanisms put forth to explain glacial - interglacial changes in atmospheric CO₂. In parts 1 and 2 of this paper, a new analysis of the ocean’s carbon pumps is carried out to examine these findings. This paper, part 1, shows that unresolved mixing and circulation effects in box models are not the main reason for box model-GCM differences. The main factor is very different kinds of restrictions on gas exchange in polar areas. Polar outcrops in GCMs are much smaller than in box models, and they are assumed to be ice covered in an unrealistic way. This finding does not support a reduced role for the ocean’s polar regions in the cycling of organic carbon, the subject taken up in part 2.

24. Toggweiler, J. R., R. Murnane, S. Carson, A. Gnanadesikan, and Jorge Sarmiento, 2003: Representation of the carbon cycle in box models and GCMs: 2. Organic pump. Global Biogeochemical Cycles, 17(1), doi:10.1029/2001GB001841
    [ Abstract ]

    Box models of the ocean/atmosphere CO₂ system rely on mechanisms at polar outcrops to alter the strength of the ocean’s organic carbon pump. GCM-based carbon system models are reportedly less sensitive to the same processes. Here we separate the carbon pumps in a three-box model and the GCM-based Princeton Ocean Biogeochemistry Model to show how the organic pumps operate in the two kinds of models. The organic pumps are found to be quite different in two respects. Deep water in the three-box model is relatively well equilibrated with respect to the pCO₂ of the atmosphere while deep water in the GCM tends to be poorly equilibrated. This makes the organic pump inherently stronger in the GCM than in the three-box model. The second difference has to do with the role of polar nutrient utilization. The organic pump in the GCM is shown to have natural upper and lower limits that are set by the initial PO₄ concentrations in the deep water formed in the North Atlantic and Southern Ocean. The strength of the organic pump can swing between these limits in response to changes in deep-water formation that alter the mix of northern and southern deep water. Thus, unlike the situation in the three-box model, the organic pump in the GCM can become weaker or stronger without changes in polar nutrient utilization.

25. Gnanadesikan, A., R. D. Slater, N. Gruber, and Jorge Sarmiento, 2002: Oceanic vertical exchange and new production: A comparison between models and observations. Deep Sea Research II, 49(1-3), doi:10.1016/S0967-0645(01)00107-2 363-401
    [ Abstract ]

    This paper explores the relationship between large-scale vertical exchange and the cycling of biologically active nutrients within the ocean. It considers how the parameterization of vertical and lateral mixing effects estimates of newproducti on (defined as the net uptake of phosphate). A baseline case is run with low vertical mixing in the pycnocline and a relatively lowlate ral diffusion coefficient. The magnitude of the diapycnal diffusion coefficient is then increased within the pycnocline, within the pycnocline of the Southern Ocean, and in the top 50 m; while the lateral diffusion coefficient is increased throughout the ocean. It is shown that it is possible to change lateral and vertical diffusion coefficients so as to preserve the structure of the pycnocline while changing the pathways of vertical exchange and hence the cycling of nutrients. Comparisons between the different models reveal that new production is very sensitive to the level of vertical mixing within the pycnocline, but only weakly sensitive to the level of lateral and upper ocean diffusion. The results are compared with two estimates of new production based on ocean color and the annual cycle of nutrients. On a global scale, the observational estimates are most consistent with the circulation produced with a low diffusion coefficient within the pycnocline, resulting in a new production of around 10 GtC yr¹: On a regional level, however, large differences appear between observational and model based estimates. In the tropics, the models yield systematically higher levels of new production than the observational estimates. Evidence from the Eastern Equatorial Pacific suggests that this is due to both biases in the data used to generate the observational estimates and problems with the models. In the North Atlantic, the observational estimates vary more than the models, due in part to the methodology by which the nutrient-based climatology is constructed. In the North Pacific, the modeled values of new production are all much lower than the observational estimates, probably as a result of the failure to form intermediate water with the right properties. The results demonstrate the potential usefulness of new production for evaluating circulation models.

26. Sarmiento, Jorge, J. P. Dunne, A. Gnanadesikan, R. M. Key, K. Matsumoto, and R. D. Slater, 2002: A new estimate of the CaCO₃ to organic carbon export ratio. Global Biogeochemical Cycles, 16(4), doi:10.1029/2002GB001919
    [ Abstract ]

    We use an ocean biogeochemical-transport box model of the top 100 m of the water column to estimate the CaCO₃ to organic carbon export ratio from observations of the vertical gradients of potential alkalinity and nitrate. We find a global average molar export ratio of 0.06 ± 0.03. This is substantially smaller than earlier estimates of 0.25 on which a majority of ocean biogeochemical models had based their parameterization of CaCO₃ production. Contrary to the pattern of coccolithophore blooms determined from satellite observations, which show high latitude predominance, we find maximum export ratios in the equatorial region and generally smaller ratios in the subtropical and subpolar gyres. Our results suggest a dominant contribution to global calcification by low-latitude nonbloom forming coccolithophores or other organisms such as foraminifera and pteropods.

27. Bianchi, D., Jorge Sarmiento, A. Gnanadesikan, R. M. Key, P. Schlosser, and R. Newton, in press: Simulations of oceanic 3HE distribution suggest low rates of mantle degassing. Nature Geosciences. 0/00.

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