Bibliography - K. B. Rodgers

1. 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.

2. 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.

3. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. E. Mikaloff-Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2010: Trends and regional distributions of land and ocean carbon sinks. Biogeosciences, www.biogeosciences.net/7/2351/2010/, 7, doi:10.5194/bg-7-2351-2010 2351-2367
   [ Abstract ]

   We show here an updated estimate of the net land carbon sink (NLS) as a function of time from 1960 to 2007 calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. Except for interannual variability, the net land carbon sink appears to have been relatively constant at a mean value of −0.27 PgC yr⁻¹ between 1960 and 1988, at which time it increased abruptly by −0.88 (−0.77 to −1.04) PgC yr⁻¹ to a new relatively constant mean of −1.15 PgC yr⁻¹ between 1989 and 2003/7 (the sign convention is negative out of the atmosphere). This result is detectable at the 99% level using a t-test. The land use source (LU) is relatively constant over this entire time interval. While the LU estimate is highly uncertain, this does imply that most of the change in the net land carbon sink must be due to an abrupt increase in the land sink, LS = NLS – LU, in response to some as yet unknown combination of biogeochemical and climate forcing. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the Northern Hemisphere land is a major sink of atmospheric CO₂, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

4. 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).

5. Sarmiento, Jorge, M. N. Gloor, N. Gruber, C. Beaulieu, A. R. Jacobson, S. M. Fletcher, Stephen W. Pacala, and K. B. Rodgers, 2009: Trends and Regional Distributions of Land and Ocean Carbon Sink. Biogeosciences, http://www.biogeosciences-discuss.net/6/10583/2009/bgd-6-10583-2009.html, (6), 10583-10624
   [ Abstract ]

   We show here a new estimate of the variability and long-term trends in the net land carbon sink from 1960 onwards calculated from the difference between fossil fuel emissions, the observed atmospheric growth rate, and the ocean uptake obtained by 5 recent ocean model simulations forced with reanalysis wind stress and heat and water fluxes. The net land carbon sink appears to have increased by −0.88 (−0.77 to −1.04) PgCyr−1 after 1988/1989 from a relatively constant mean of −0.27 PgCyr−1 before then to −1.15 PgCyr−1 thereafter (the sign convention is negative out of the atmosphere). This result is significant at the 1% critical level. The increase in net land 10 uptake is partially compensated by a reduction in the expected oceanic uptake, which we estimate from model simulations as about 0.35 (0.26 to 0.49) PgCyr−1. This implies that the atmospheric growth rate must have decreased by about −0.53 (−0.51 to −0.55) PgCyr−1 (equivalent to −0.25 ppm yr−1) below what would have been projected if the ocean uptake had continued to grow at the rate expected from a constant 15 climate model and if the net land uptake had continued at its pre-1988/1989 level. A regional synthesis and assessment of the land carbon sources and sinks over the post 1988/1989 period reveals broad agreement that the northern hemisphere land is a major sink of atmospheric CO2, but there remain major discrepancies with regard to the sign and magnitude of the net flux to and from tropical land.

6. Rodgers, K. B., Jorge Sarmiento, O. Aumont, C. Crevoisier, C.D.B. Montégut, and N. Metzl, 2008: A wintertime uptake window for anthropogenic CO₂ in the North Pacific. Global Biogeochemical Cycles, 22(GB2020), doi:10.1029/2006GB002920
   [ Abstract ]

   An ocean model has been forced with NCEP reanalysis fluxes over 1948–2003 to evaluate the pathways and timescales associated with the uptake of anthropogenic CO₂ over the North Pacific. The model reveals that there are two principal regions of uptake, the first in the region bounded by 35–45°N and 140–180°E, and the second along a band between 10–20°N and between 120°W and 180°E. For both of these regions, the dominant timescale of variability in uptake is seasonal, with maximum uptake occurring during winter and uptake being close to zero or slightly negative during summer when integrated over the basin. A decadal trend toward increased uptake of anthropogenic CO₂ consists largely of modulations of the uptake maximum in winter. For detection of anthropogenic changes, this implies that in situ measurements will need to resolve the seasonal cycle in order to capture decadal trends in ΔpCO₂. As uptake of anthropogenic CO₂ occurs preferentially during winter, observationally based estimates which do not resolve the full seasonal cycle may result in underestimates of the rate of uptake of anthropogenic CO₂. There is also a sizable circulation-driven decadal trend in the seasonal cycle of sea surface ΔpCO₂ for the North Pacific, with maximum changes found near the boundary separating the subtropical and subpolar gyres in western and central regions of the basin. These changes are due to a trend in the large-scale circulation of the gyres, which itself is driven by a trend in the wind stress over the basin scale. This trend in the three-dimensional circulation is more important than the local trend in mixed layer depth (MLD) in contributing to the decadal trend in ΔpCO₂.

7. Rodgers, K. B., O. Aumont, C. Menkes, and T. Gorgues, 2008: Decadal variations in equatorial Pacific ecosystems and ferrocline/pycnocline decoupling. Global Biogeochemical Cycles, 22(GB2019), doi:10.1029/2006GB002919
   [ Abstract ]

   The equatorial Pacific Ocean is known for its large interannual to decadal variability in circulation. In particular, the changes that occurred in 1976/1977 have received considerable attention in the climate dynamics literature, and recently there has been much attention focused on changes that may have occurred there in 1997/1998. Unfortunately, because of data sparsity, the impact of these changes or shifts on ocean biogeochemistry and ecosystems remains largely unknown. Here a three-dimensional ocean circulation model (the ORCA2 configuration of OPA) which has a food web/ biogeochemistry model (PISCES) embedded in it, and which has been forced with both NCEP-1 and ERA-40 reanalysis fluxes over multiple decades, is used as a tool to investigate decadal changes and their associated mechanisms. Our main finding with the model is that a decrease in the amplitude of the surface zonal wind stress in the tropical Pacific in the mid-to-late-1970s leads to a decrease in Fe and Chl concentrations in the upwelling regions of the eastern equatorial Pacific after 1976/ 1977. These changes find expression predominantly during the upwelling season (the seasonal maximum for Fe and Chl concentrations), when surface Fe and Chl concentrations tend to be significantly higher pre-1976/1977 than post-1976/1977. The changes in Chl concentrations need to be understood as modulations of the amplitude of the seasonal cycle, rather than as a ‘‘biological regime shift’’ (an abrupt transition from one mean state to another). In contrast to what is found for Fe and Chl, for NO₃ the decadal changes in surface concentrations in the upwelling region about 1976/1977 can be described as a shift in the mean state. It is shown that the response in surface Fe and Chl in the upwelling region about 1976/1977 is proportionally larger than the decadal changes in surface wind stress forcing, and it is also larger than the previously reported change in the strength of the meridional overturning strength of the subtropical cells (STCs). Importantly, this amplified response reflects a decoupling of the ferrocline and pycnocline within the equatorial Pacific. In this way, the presence of a time-invariant sediment source for Fe can substantially amplify the ecosystem response to decadal variability in ocean circulation.

8. Gorgues, T., C. Menkes, O. Aumont, Y. Dandonneau, G. Madec, and K. B. Rodgers, 2007: Indonesian Throughflow control of the eastern equatorial Pacific biogeochemistry. Geophysical Research Letters, 34(L05609), doi:10.1029/2006GL028210
   [ Abstract ]

   Two model simulations were performed to address the influence of the Indonesian throughflow (ITF) on the biogeochemical state of the equatorial Pacific. A simulation where the ITF is open is compared with an experiment where it is closed, and it is first shown that the impacts on the physical circulation are consistent with what has been found in previous modelling studies. In terms of biochemistry, closing the ITF results in increased iron concentration at the origin of the Equatorial Undercurrent (EUC). But the 11Sv of water otherwise transferred to the Indian Ocean remain in the equatorial Pacific, which result in a 30 m deepening of the thermocline/ferricline in the eastern Pacific. This deepening decreases the iron concentration of the equatorial wind driven upwelled water and cancels the iron increase advected by the EUC. The iron decrease of the equatorial upwelled water leads to decrease primary production by 15% along the equator.

9. Iudicone, D., K. B. Rodgers, R. Schopp, and G. Madec, 2007: An Exchange window for the injection of Antarctic Intermediate Water into the South Pacific. Journal of Physical Oceanography, 37, doi:10.1175/JPO2985.1 31-49
   [ Abstract ]

   Antarctic Intermediate Water (AAIW) occupies the intermediate horizon of most of the world oceans. Formed in the Southern Ocean, it is characterized by a relative salinity minimum. With a new, denser in situ National Oceanographic Data Center dataset, the authors have reanalyzed the export characteristics of AAIW from the Southern Ocean into the South Pacific Ocean. These new data show that part of the AAIW is exported from the subpolar frontal region by the large-scale circulation through an exchange window of 10° width situated east of 90°W in the southeast corner of the Pacific basin. This suggests the origin of this water to be in the Antarctic Circumpolar Current. A set of numerical modeling experiments has been used to reproduce these observed features and to demonstrate that the dynamics of the exchange window is controlled by the basin-scale meridional pressure gradient. The exchange of AAIW between the Southern and Pacific Oceans must therefore be understood in the context of the large basin-scale dynamical balance rather than simply local effects.

10. Laurian, A., A. Lazar, G. Reverdin, K. B. Rodgers, and P. Terray, 2006: Poleward propagation of spiciness anomalies in the North Atlantic Ocean. Geophysical Research Letters, 33(L13603), doi:10.1029/2006GL026155
    [ Abstract ]

    Recent modelling results suggest that subsurface salinity anomalies propagating from the tropics can reach and precondition the deep-water formation regions, thus modulating the THC variability. The forcing and propagative aspects of this mechanism are presented in the North Atlantic Ocean over 1948 -2002 using an OGCM. Density compensated salinity anomalies of 0.1 up to 0.35 psu along σ = 26 kg.m ^-3 are generated in the salinity maximum region at interannual to decadal frequency. The relation between subsurface conditions and late winter sea surface salinity variability supports the subduction mechanism. They circulate over isopycnals ranging from 25.6 σ to 26.2 σ at current speed between 150 m and 250 m depth toward Cape Hatteras via the Gulf of Mexico on a typical 6-year transit. Although mixing along the pathway reduces the amplitude of salinity anomalies by about 66%, they largely determine the subsurface spiciness of the Gulf Stream up to 30°N, upstream of the outcrop region.

11. Naegler, T., P. Ciais, K. B. Rodgers, and I. Levin, 2006: Excess radiocarbon constraints on air-sea gas exchange and the uptake of CO₂ by the oceans. Geophysical Research Letters, 33(L11802), doi:10.1029/2005GL025408
    [ Abstract ]

    We re-assess the constraints that estimates of the global ocean excess radiocarbon inventory (I^E) place on air-sea gas exchange. We find that the gas exchange scaling parameter aq cannot be constrained by I^E alone. Non-negligible biases in different global wind speed data sets require a careful adaptation of a_q to the wind field chosen. Furthermore, aq depends on the spatial and temporal resolution of the wind fields. We develop a new wind speed-and inventory-normalized gas exchange parameter a_q ^N which takes into account these biases and which is easily adaptable to any new estimate of I^E. Our study yields an average estimate of a_q of 0.32 ± 0.05 for monthly mean winds, lower than the previous estimate (0.39) from Wanninkhof (1992). We calculate a global annual average piston velocity for CO₂ of 16.7 ± 2.9 cm/hr and a gross CO₂ flux between atmosphere and ocean of 73 ± 10 PgC/yr, significantly lower than results from previous studies.

12. Dutay, J. C., P. Jean-Baptiste, J.-M. Campin, A. Ishida, E. Maier-Reimer, R. J. Matear, A. Mouchet, I. J. Totterdell, Y. Yamanaka, K. B. Rodgers, G. Madec, and J. C. Orr, 2004: Evaluation of OCMIP-2 Ocean Models’ Deep Circulation with Mantle Helium-3. Journal of Marine Systems, 48(1-4), doi:10.1016/j.jmarsys.2003.05.010 15-36
    [ Abstract ]

    We compare simulations of the injection of mantle helium-3 into the deep ocean from six global coarse resolution models which participated in the Ocean Carbon Model Intercomparison Project (OCMIP). We also discuss the results of a study carried out with one of the models, which examines the effect of the subgrid-scale mixing parameterization. These sensitivity tests provide useful information to interpret the differences among the OCMIP models and between model simulations and the data. We find that the OCMIP models, which parameterize subgrid-scale mixing using an eddy-induced velocity, tend to underestimate the ventilation of the deep ocean, based on diagnostics with δ³He. In these models, this parameterization is implemented with a constant thickness diffusivity coefficient. In future simulations, we recommend using such a parameterization with spatially and temporally varying coefficients in order to moderate its effect on stratification. The performance of the models with regard to the formation of AABW confirms the conclusion from a previous evaluation with CFC-11. Models coupled with a sea-ice model produce a substantial bottom water formation in the Southern Ocean that tends to overestimate AABW ventilation, while models that are not coupled with a sea-ice model systematically underestimate the formation of AABW. We also analyze specific features of the deep ³He distribution (³He plumes) that are particularly well depicted in the data and which put severe constraints on the deep circulation. We show that all the models fail to reproduce a correct propagation of these plumes in the deep ocean. The resolution of the models may be too coarse to reproduce the strong and narrow currents in the deep ocean, and the models do not incorporate the geothermal heating that may also contribute to the generation of these currents. We also use the context of OCMIP-2 to explore the potential of mantle helium-3 as a tool to compare and evaluate modeled deep-ocean circulations. Although the source function of mantle helium is known with a rather large uncertainty, we find that the parameterization used for the injection of mantle helium-3 is sufficient to generate realistic results, even in the Atlantic Ocean where a previous pioneering study [J. Geophys. Res. 100 (1995) 3829] claimed this parameterization generates inadequate results. These results are supported by a multi-tracer evaluation performed by considering the simulated distributions of both helium-3 and natural ¹⁴C, and comparing the simulated tracer fields with available data.

13. Rodgers, K. B., O. Aumont, G. Madec, and C. Menkes, 2004: Radiocarbon as a thermocline proxy in th eastern equatorial Pacific. Geophysical Research Letters, 31(L14314), doi:10.1029/2004GL019764
    [ Abstract ]

    An ocean model is used to test the idea that sea surface Δ¹⁴C behaves as a thermocline proxy in the eastern equatorial Pacific. The ORCA2 model, which includes Δ¹⁴C as a passive tracer, has been forced with reanalysis fluxes over 1948 – 1999, and the output is compared with a previously reported Galapagos Δ¹⁴C record. The model reproduces the abrupt increase in the seasonally minimum Δ¹⁴C in 1976/77 found in the data. This increase is associated with neither a shift of thermocline depth over the NINO3 region, nor a change in the relative proportion of Northern/Southern source waters. Rather, it is due to a decrease in the Sub-Antarctic Mode Water (SAMW) component of the upwelling water, thereby representing a decrease in entrainment of water from below the base of the directly ventilated thermocline.

14. Rodgers, K. B., P. Friedrichs, and M. Latif, 2004: Tropical Pacific Decadal Variability and Its Relation to Decadal Modulations of ENSO. Journal of Climate, 17(19), doi:10.1175/1520-0442(2004)017<3761:TPDVAI>2.0.CO;2 3761-3774
    [ Abstract ]

    A 1000-yr integration of a coupled ocean–atmosphere model (ECHO-G) has been analyzed to describe decadal to multidecadal variability in equatorial Pacific sea surface temperature (SST) and thermocline depth (Z20), and their relationship to decadal modulations of El Nin˜o–Southern Oscillation (ENSO) behavior. Although the coupled model is characterized by an unrealistically regular 2-yr ENSO period, it exhibits significant modulations of ENSO amplitude on decadal to multidecadal time scales. The authors’ main finding is that the structures in SST and Z20 characteristic of tropical Pacific decadal variability (TPDV) in the model are due to an asymmetry between the anomaly patterns associated with the model’s El Nin˜o and La Nin˜a states, with this asymmetry reflecting a nonlinearity in ENSO variability. As a result, the residual (i.e., the sum) of the composite El Nin˜o and La Nin˜a patterns exhibits a nonzero dipole structure across the equatorial Pacific, with positive perturbation values in the east and negative values in the west for SST and Z20. During periods when ENSO variability is strong, this difference manifests itself as a rectified change in the mean state. For comparison, a similar analysis was applied to a gridded SST dataset spanning the period 1871–1999. The data confirms that the asymmetry between the SST anomaly patterns associated with El Nin˜o and La Nin˜a for the model is realistic. However, ENSO in the observations is weaker and not as regular as in the model, and thus the changes due to ENSO asymmetries for the observations can only be detected in the Nin˜o-12 region.

15. Rodgers, K. B., S. Charbit, M. Kageyama, G. Philippon, G. Ramstein, C. Ritz, J. H. Yin, G. Lohmann, S. J. Lorenz, and M. Khodri, 2004: Sensitivity of Northern Hemispheric continental ice sheets to tropical SST during deglaciation. Geophysical Research Letters, 31(L02206), doi:10.1029/2003GL018375
    [ Abstract ]

    A thermomechanical ice sheet model (ISM) is used to investigate the sensitivity of the Laurentide and Fennoscandian ice sheets to tropical sea surface temperature (SST) perturbations during deglaciation. The ISM is driven by surface temperature and precipitation fields from three different atmospheric general circulation models (AGCMs). For each AGCM, the responses in temperature and precipitation over the ice sheets nearly compensate, such that ice sheet mass balance is not strongly sensitive to tropical SST boundary conditions. It was also found that there is significant variation in the response of the ISM to the different AGCM output fields.

16. Rodgers, K. B., and D. Bianchi, et al., in press: A Southern Ocean mechanism for atmospheric and oceanic Δ14C variations on centennial timescales. Paleoceanography. 0/00.

Direct link to page: http://cmi.princeton.edu/bibliography/results.php?author=3623