Mikaloff Fletcher
Sara E.
Mikaloff Fletcher
Sara E.
No Thumbnail Available
6 results
Search Results
Now showing
1 - 6 of 6
-
ArticleOceanic sources, sinks, and transport of atmospheric CO2(American Geophysical Union, 2009-02-18) Gruber, Nicolas ; Gloor, Emanuel ; Mikaloff Fletcher, Sara E. ; Doney, Scott C. ; Dutkiewicz, Stephanie ; Follows, Michael J. ; Gerber, Markus ; Jacobson, Andrew R. ; Joos, Fortunat ; Lindsay, Keith ; Menemenlis, Dimitris ; Mouchet, Anne ; Muller, Simon A. ; Sarmiento, Jorge L. ; Takahashi, TaroWe synthesize estimates of the contemporary net air-sea CO2 flux on the basis of an inversion of interior ocean carbon observations using a suite of 10 ocean general circulation models (Mikaloff Fletcher et al., 2006, 2007) and compare them to estimates based on a new climatology of the air-sea difference of the partial pressure of CO2 (pCO2) (Takahashi et al., 2008). These two independent flux estimates reveal a consistent description of the regional distribution of annual mean sources and sinks of atmospheric CO2 for the decade of the 1990s and the early 2000s with differences at the regional level of generally less than 0.1 Pg C a−1. This distribution is characterized by outgassing in the tropics, uptake in midlatitudes, and comparatively small fluxes in thehigh latitudes. Both estimates point toward a small (∼ −0.3 Pg C a−1) contemporary CO2 sink in the Southern Ocean (south of 44°S), a result of the near cancellation between a substantial outgassing of natural CO2 and a strong uptake of anthropogenic CO2. A notable exception in the generally good agreement between the two estimates exists within the Southern Ocean: the ocean inversion suggests a relatively uniform uptake, while the pCO2-based estimate suggests strong uptake in the region between 58°S and 44°S, and a source in the region south of 58°S. Globally and for a nominal period between 1995 and 2000, the contemporary net air-sea flux of CO2 is estimated to be −1.7 ± 0.4 Pg C a−1 (inversion) and −1.4 ± 0.7 Pg C a−1 (pCO2-climatology), respectively, consisting of an outgassing flux of river-derived carbon of ∼+0.5 Pg C a−1, and an uptake flux of anthropogenic carbon of −2.2 ± 0.3 Pg C a−1 (inversion) and −1.9 ± 0.7 Pg C a−1 (pCO2-climatology). The two flux estimates also imply a consistent description of the contemporary meridional transport of carbon with southward ocean transport throughout most of the Atlantic basin, and strong equatorward convergence in the Indo-Pacific basins. Both transport estimates suggest a small hemispheric asymmetry with a southward transport of between −0.2 and −0.3 Pg C a−1 across the equator. While the convergence of these two independent estimates is encouraging and suggests that it is now possible to provide relatively tight constraints for the net air-sea CO2 fluxes at the regional basis, both studies are limited by their lack of consideration of long-term changes in the ocean carbon cycle, such as the recent possible stalling in the expected growth of the Southern Ocean carbon sink.
-
ArticleAir-sea CO2 flux in the Pacific Ocean for the period 1990–2009(Copernicus Publications on behalf of the European Geosciences Union, 2014-02-06) Ishii, Masao ; Feely, Richard A. ; Rodgers, Keith B. ; Park, Geun-Ha ; Wanninkhof, Rik ; Sasano, D. ; Sugimoto, H. ; Cosca, Catherine E. ; Nakaoka, Shin-ichiro ; Telszewski, Maciej ; Nojiri, Yukihiro ; Mikaloff Fletcher, Sara E. ; Niwa, Y. ; Patra, Prabir K. ; Valsala, V. ; Nakano, Hideyuki ; Lima, Ivan D. ; Doney, Scott C. ; Buitenhuis, Erik T. ; Aumont, Olivier ; Dunne, John P. ; Lenton, Andrew ; Takahashi, TaroAir–sea CO2 fluxes over the Pacific Ocean are known to be characterized by coherent large-scale structures that reflect not only ocean subduction and upwelling patterns, but also the combined effects of wind-driven gas exchange and biology. On the largest scales, a large net CO2 influx into the extratropics is associated with a robust seasonal cycle, and a large net CO2 efflux from the tropics is associated with substantial interannual variability. In this work, we have synthesized estimates of the net air–sea CO2 flux from a variety of products, drawing upon a variety of approaches in three sub-basins of the Pacific Ocean, i.e., the North Pacific extratropics (18–66° N), the tropical Pacific (18° S–18° N), and the South Pacific extratropics (44.5–18° S). These approaches include those based on the measurements of CO2 partial pressure in surface seawater (pCO2sw), inversions of ocean-interior CO2 data, forward ocean biogeochemistry models embedded in the ocean general circulation models (OBGCMs), a model with assimilation of pCO2sw data, and inversions of atmospheric CO2 measurements. Long-term means, interannual variations and mean seasonal variations of the regionally integrated fluxes were compared in each of the sub-basins over the last two decades, spanning the period from 1990 through 2009. A simple average of the long-term mean fluxes obtained with surface water pCO2 diagnostics and those obtained with ocean-interior CO2 inversions are −0.47 ± 0.13 Pg C yr−1 in the North Pacific extratropics, +0.44 ± 0.14 Pg C yr−1 in the tropical Pacific, and −0.37 ± 0.08 Pg C yr−1 in the South Pacific extratropics, where positive fluxes are into the atmosphere. This suggests that approximately half of the CO2 taken up over the North and South Pacific extratropics is released back to the atmosphere from the tropical Pacific. These estimates of the regional fluxes are also supported by the estimates from OBGCMs after adding the riverine CO2 flux, i.e., −0.49 ± 0.02 Pg C yr−1 in the North Pacific extratropics, +0.41 ± 0.05 Pg C yr−1 in the tropical Pacific, and −0.39 ± 0.11 Pg C yr−1 in the South Pacific extratropics. The estimates from the atmospheric CO2 inversions show large variations amongst different inversion systems, but their median fluxes are consistent with the estimates from climatological pCO2sw data and pCO2sw diagnostics. In the South Pacific extratropics, where CO2 variations in the surface and ocean interior are severely undersampled, the difference in the air–sea CO2 flux estimates between the diagnostic models and ocean-interior CO2 inversions is larger (0.18 Pg C yr−1). The range of estimates from forward OBGCMs is also large (−0.19 to −0.72 Pg C yr−1). Regarding interannual variability of air–sea CO2 fluxes, positive and negative anomalies are evident in the tropical Pacific during the cold and warm events of the El Niño–Southern Oscillation in the estimates from pCO2sw diagnostic models and from OBGCMs. They are consistent in phase with the Southern Oscillation Index, but the peak-to-peak amplitudes tend to be higher in OBGCMs (0.40 ± 0.09 Pg C yr−1) than in the diagnostic models (0.27 ± 0.07 Pg C yr−1).
-
ArticleInverse estimates of anthropogenic CO2 uptake, transport, and storage by the ocean(American Geophysical Union, 2006-04-05) Mikaloff Fletcher, Sara E. ; Gruber, Nicolas ; Jacobson, Andrew R. ; Doney, Scott C. ; Dutkiewicz, Stephanie ; Gerber, Markus ; Follows, Michael J. ; Joos, Fortunat ; Lindsay, Keith ; Menemenlis, Dimitris ; Mouchet, Anne ; Muller, Simon A. ; Sarmiento, Jorge L.Regional air-sea fluxes of anthropogenic CO2 are estimated using a Green's function inversion method that combines data-based estimates of anthropogenic CO2 in the ocean with information about ocean transport and mixing from a suite of Ocean General Circulation Models (OGCMs). In order to quantify the uncertainty associated with the estimated fluxes owing to modeled transport and errors in the data, we employ 10 OGCMs and three scenarios representing biases in the data-based anthropogenic CO2 estimates. On the basis of the prescribed anthropogenic CO2 storage, we find a global uptake of 2.2 ± 0.25 Pg C yr−1, scaled to 1995. This error estimate represents the standard deviation of the models weighted by a CFC-based model skill score, which reduces the error range and emphasizes those models that have been shown to reproduce observed tracer concentrations most accurately. The greatest anthropogenic CO2 uptake occurs in the Southern Ocean and in the tropics. The flux estimates imply vigorous northward transport in the Southern Hemisphere, northward cross-equatorial transport, and equatorward transport at high northern latitudes. Compared with forward simulations, we find substantially more uptake in the Southern Ocean, less uptake in the Pacific Ocean, and less global uptake. The large-scale spatial pattern of the estimated flux is generally insensitive to possible biases in the data and the models employed. However, the global uptake scales approximately linearly with changes in the global anthropogenic CO2 inventory. Considerable uncertainties remain in some regions, particularly the Southern Ocean.
-
ArticleSea–air CO2 fluxes in the Southern Ocean for the period 1990–2009(Copernicus Publications on behalf of the European Geosciences Union, 2013-06-19) Lenton, Andrew ; Tilbrook, Bronte ; Law, R. M. ; Bakker, Dorothee C. E. ; Doney, Scott C. ; Gruber, Nicolas ; Ishii, Masao ; Hoppema, Mario ; Lovenduski, Nicole S. ; Matear, Richard J. ; McNeil, B. I. ; Metzl, Nicolas ; Mikaloff Fletcher, Sara E. ; Monteiro, Pedro M. S. ; Rodenbeck, C. ; Sweeney, Colm ; Takahashi, TaroThe Southern Ocean (44–75° S) plays a critical role in the global carbon cycle, yet remains one of the most poorly sampled ocean regions. Different approaches have been used to estimate sea–air CO2 fluxes in this region: synthesis of surface ocean observations, ocean biogeochemical models, and atmospheric and ocean inversions. As part of the RECCAP (REgional Carbon Cycle Assessment and Processes) project, we combine these different approaches to quantify and assess the magnitude and variability in Southern Ocean sea–air CO2 fluxes between 1990–2009. Using all models and inversions (26), the integrated median annual sea–air CO2 flux of −0.42 ± 0.07 Pg C yr−1 for the 44–75° S region, is consistent with the −0.27 ± 0.13 Pg C yr−1 calculated using surface observations. The circumpolar region south of 58° S has a small net annual flux (model and inversion median: −0.04 ± 0.07 Pg C yr−1 and observations: +0.04 ± 0.02 Pg C yr−1), with most of the net annual flux located in the 44 to 58° S circumpolar band (model and inversion median: −0.36 ± 0.09 Pg C yr−1 and observations: −0.35 ± 0.09 Pg C yr−1). Seasonally, in the 44–58° S region, the median of 5 ocean biogeochemical models captures the observed sea–air CO2 flux seasonal cycle, while the median of 11 atmospheric inversions shows little seasonal change in the net flux. South of 58° S, neither atmospheric inversions nor ocean biogeochemical models reproduce the phase and amplitude of the observed seasonal sea–air CO2 flux, particularly in the Austral Winter. Importantly, no individual atmospheric inversion or ocean biogeochemical model is capable of reproducing both the observed annual mean uptake and the observed seasonal cycle. This raises concerns about projecting future changes in Southern Ocean CO2 fluxes. The median interannual variability from atmospheric inversions and ocean biogeochemical models is substantial in the Southern Ocean; up to 25% of the annual mean flux, with 25% of this interannual variability attributed to the region south of 58° S. Resolving long-term trends is difficult due to the large interannual variability and short time frame (1990–2009) of this study; this is particularly evident from the large spread in trends from inversions and ocean biogeochemical models. Nevertheless, in the period 1990–2009 ocean biogeochemical models do show increasing oceanic uptake consistent with the expected increase of −0.05 Pg C yr−1 decade−1. In contrast, atmospheric inversions suggest little change in the strength of the CO2 sink broadly consistent with the results of Le Quéré et al. (2007).
-
ArticleInverse estimates of the oceanic sources and sinks of natural CO2 and the implied oceanic carbon transport(American Geophysical Union, 2007-02-10) Mikaloff Fletcher, Sara E. ; Gruber, Nicolas ; Jacobson, Andrew R. ; Gloor, Emanuel ; Doney, Scott C. ; Dutkiewicz, Stephanie ; Gerber, Markus ; Follows, Michael J. ; Joos, Fortunat ; Lindsay, Keith ; Menemenlis, Dimitris ; Mouchet, Anne ; Muller, Simon A. ; Sarmiento, Jorge L.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.
-
ArticleAn atmospheric constraint on the seasonal Air-Sea exchange of oxygen and heat in the extratropics(American Geophysical Union, 2021-07-22) Morgan, Eric J. ; Manizza, Manfredi ; Keeling, Ralph F. ; Resplandy, Laure ; Mikaloff Fletcher, Sara E. ; Nevison, Cynthia D. ; Jin, Yuming ; Bent, Jonathan D. ; Aumont, Olivier ; Doney, Scott C. ; Dunne, John P. ; John, Jasmin G. ; Lima, Ivan D. ; Long, Matthew C. ; Rodgers, Keith B.The air-sea exchange of oxygen (O2) is driven by changes in solubility, biological activity, and circulation. The total air-sea exchange of O2 has been shown to be closely related to the air-sea exchange of heat on seasonal timescales, with the ratio of the seasonal flux of O2 to heat varying with latitude, being higher in the extratropics and lower in the subtropics. This O2/heat ratio is both a fundamental biogeochemical property of air-sea exchange and a convenient metric for testing earth system models. Current estimates of the O2/heat flux ratio rely on sparse observations of dissolved O2, leaving it fairly unconstrained. From a model ensemble we show that the ratio of the seasonal amplitude of two atmospheric tracers, atmospheric potential oxygen (APO) and the argon-to-nitrogen ratio (Ar/O2), exhibits a close relationship to the O2/heat ratio of the extratropics (40–70°). The amplitude ratio, A APO/A ArN2, is relatively constant within the extratropics of each hemisphere due to the zonal mixing of the atmosphere. A APO/A ArN2 is not sensitive to atmospheric transport, as most of the observed spatial variability in the seasonal amplitude of δAPO is compensated by similar variations in δ(Ar/N2). From the relationship between O2/heat and A APO/A ArN2 in the model ensemble, we determine that the atmospheric observations suggest hemispherically distinct O2/heat flux ratios of 3.3 ± 0.3 and 4.7 ± 0.8 nmol J-1 between 40 and 70° in the Northern and Southern Hemispheres respectively, providing a useful constraint for O2 and heat air-sea fluxes in earth system models and observation-based data products.