Lovenduski
Nicole S.
Lovenduski
Nicole S.
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ArticleToward a mechanistic understanding of the decadal trends in the Southern Ocean carbon sink(American Geophysical Union, 2008-08-16) Lovenduski, Nicole S. ; Gruber, Nicolas ; Doney, Scott C.We investigate the multidecadal and decadal trends in the flux of CO2 between the atmosphere and the Southern Ocean using output from hindcast simulations of an ocean circulation model with embedded biogeochemistry. The simulations are run with NCEP-1 forcing under both preindustrial and historical atmospheric CO2 concentrations so that we can separately analyze trends in the natural and anthropogenic CO2 fluxes. We find that the Southern Ocean (<35°S) CO2 sink has weakened by 0.1 Pg C a−1 from 1979–2004, relative to the expected sink from rising atmospheric CO2 and fixed physical climate. Although the magnitude of this trend is in agreement with prior studies (Le Quéré et al., 2007), its size may not be entirely robust because of uncertainties associated with the trend in the NCEP-1 atmospheric forcing. We attribute the weakening sink to an outgassing trend of natural CO2, driven by enhanced upwelling and equatorward transport of carbon-rich water, which are caused by a trend toward stronger and southward shifted winds over the Southern Ocean (associated with the positive trend in the Southern Annular Mode (SAM)). In contrast, the trend in the anthropogenic CO2 uptake is largely unaffected by the trend in the wind and ocean circulation. We regard this attribution of the trend as robust, and show that surface and interior ocean observations may help to solidify our findings. As coupled climate models consistently show a positive trend in the SAM in the coming century [e.g., Meehl et al., 2007], these mechanistic results are useful for projecting the future behavior of the Southern Ocean carbon sink.
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ArticleObservational needs supporting marine ecosystems modeling and forecasting: from the global ocean to regional and coastal systems(Frontiers Media, 2019-10-15) Capotondi, Antonietta ; Jacox, Michael ; Bowler, Chris ; Kavanaugh, Maria T. ; Lehodey, Patrick ; Barrie, Daniel ; Brodie, Stephanie ; Chaffron, Samuel ; Cheng, Wei ; Dias, Daniela F. ; Eveillard, Damien ; Guidi, Lionel ; Iudicone, Daniele ; Lovenduski, Nicole S. ; Nye, Janet A. ; Ortiz, Ivonne ; Pirhalla, Douglas ; Pozo Buil, Mercedes ; Saba, Vincent S. ; Sheridan, Scott ; Siedlecki, Samantha A. ; Subramanian, Aneesh C. ; de Vargas, Colomban ; Di Lorenzo, Emanuele ; Doney, Scott C. ; Hermann, Albert J. ; Joyce, Terrence M. ; Merrifield, Mark ; Miller, Arthur J. ; Not, Fabrice ; Pesant, StephaneMany coastal areas host rich marine ecosystems and are also centers of economic activities, including fishing, shipping and recreation. Due to the socioeconomic and ecological importance of these areas, predicting relevant indicators of the ecosystem state on sub-seasonal to interannual timescales is gaining increasing attention. Depending on the application, forecasts may be sought for variables and indicators spanning physics (e.g., sea level, temperature, currents), chemistry (e.g., nutrients, oxygen, pH), and biology (from viruses to top predators). Many components of the marine ecosystem are known to be influenced by leading modes of climate variability, which provide a physical basis for predictability. However, prediction capabilities remain limited by the lack of a clear understanding of the physical and biological processes involved, as well as by insufficient observations for forecast initialization and verification. The situation is further complicated by the influence of climate change on ocean conditions along coastal areas, including sea level rise, increased stratification, and shoaling of oxygen minimum zones. Observations are thus vital to all aspects of marine forecasting: statistical and/or dynamical model development, forecast initialization, and forecast validation, each of which has different observational requirements, which may be also specific to the study region. Here, we use examples from United States (U.S.) coastal applications to identify and describe the key requirements for an observational network that is needed to facilitate improved process understanding, as well as for sustaining operational ecosystem forecasting. We also describe new holistic observational approaches, e.g., approaches based on acoustics, inspired by Tara Oceans or by landscape ecology, which have the potential to support and expand ecosystem modeling and forecasting activities by bridging global and local observations.
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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).
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ArticleEnhanced CO2 outgassing in the Southern Ocean from a positive phase of the Southern Annular Mode(American Geophysical Union, 2007-06-20) Lovenduski, Nicole S. ; Gruber, Nicolas ; Doney, Scott C. ; Lima, Ivan D.We investigate the interannual variability in the flux of CO2 between the atmosphere and the Southern Ocean on the basis of hindcast simulations with a coupled physical-biogeochemical-ecological model with particular emphasis on the role of the Southern Annular Mode (SAM). The simulations are run under either pre-industrial or historical CO2 concentrations, permitting us to separately investigate natural, anthropogenic, and contemporary CO2 flux variability. We find large interannual variability (±0.19 PgC yr−1) in the contemporary air-sea CO2 flux from the Southern Ocean (<35°S). Forty-three percent of the contemporary air-sea CO2 flux variance is coherent with SAM, mostly driven by variations in the flux of natural CO2, for which SAM explains 48%. Positive phases of the SAM are associated with anomalous outgassing of natural CO2 at a rate of 0.1 PgC yr−1 per standard deviation of the SAM. In contrast, we find an anomalous uptake of anthropogenic CO2 at a rate of 0.01 PgC yr−1 during positive phases of the SAM. This uptake of anthropogenic CO2 only slightly mitigates the outgassing of natural CO2, so that a positive SAM is associated with anomalous outgassing in contemporaneous times. The primary cause of the natural CO2 outgassing is anomalously high oceanic partial pressures of CO2 caused by elevated dissolved inorganic carbon (DIC) concentrations. These anomalies in DIC are primarily a result of the circulation changes associated with the southward shift and strengthening of the zonal winds during positive phases of the SAM. The secular, positive trend in the SAM has led to a reduction in the rate of increase of the uptake of CO2 by the Southern Ocean over the past 50 years.
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ArticleThe influence of ocean topography on the upwelling of carbon in the Southern Ocean(American Geophysical Union, 2021-09-27) Brady, Riley X. ; Maltrud, Mathew E. ; Wolfram, Phillip J. ; Drake, Henri F. ; Lovenduski, Nicole S.The physical circulation of the Southern Ocean sets the surface concentration and thus air-sea exchange of CO2. However, we have a limited understanding of the three-dimensional circulation that brings deep carbon-rich waters to the surface. Here, we introduce and analyze a novel high-resolution ocean model simulation with active biogeochemistry and online Lagrangian particle tracking. We focus our attention on a subset of particles with high dissolved inorganic carbon (DIC) that originate below 1,000 m and eventually upwell into the near-surface layer (upper 200 m). We find that 71% of the DIC-enriched water upwelling across 1,000 m is concentrated near topographic features, which occupy just 33% of the Antarctic Circumpolar Current. Once particles upwell to the near-surface layer, they exhibit relatively uniform pCO2 levels and DIC decorrelation timescales, regardless of their origin. Our results show that Southern Ocean bathymetry plays a key role in delivering carbon-rich waters to the surface.