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Assessment of River Margin Air-Sea CO 2 Fluxes

Assessment of River Margin Air-Sea CO 2 Fluxes. Steven E. Lohrenz, Wei-Jun Cai, Xiaogang Chen, Merritt Tuel, and Feizhou Chen. Introduction.

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Assessment of River Margin Air-Sea CO 2 Fluxes

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  1. Assessment of River Margin Air-Sea CO2 Fluxes Steven E. Lohrenz, Wei-Jun Cai, Xiaogang Chen, Merritt Tuel, and Feizhou Chen 2006 OCRT Meeting, Providence

  2. Introduction • A major objective of the U.S. Global Change Research Program Climate Change Science Program Strategic Plan and the North American Carbon Program is the application of satellite ocean color to characterize the spatial variability of air-sea CO2 flux in the oceans adjacent to the North American continent

  3. Introduction • Despite their relatively small surface area, ocean margins may have a significant impact on global biogeochemical cycles, and potentially, in the global air-sea fluxes of CO2

  4. Introduction • Margin ecosystems receive inputs of terrestrial organic and mineral matter and exhibit intense geochemical and biological processing of carbon and other elements • Nowhere is this more evident that in regions influenced by large rivers

  5. Introduction • Recent studies (Lohrenz and Cai, 2006) in the northern Gulf of Mexico and elsewhere (Tsunogai et al., 1999; Ternon et al., 2000; Cai, 2003) demonstrate that enhanced biological production in large river plumes may lower surface pCO2 levels such that these regions may exhibit a net surface influx of atmospheric CO2 • But characterizations have not been conducted over representative seasonal and discharge conditions

  6. Introduction • Systems are likely to differ depending on the both biological, chemical, and meteorological properties • Quantification of the contributions of river-influenced margins to regional CO2 fluxes is difficult due to the high degree of spatial and temporal variability in these regions

  7. Introduction • Satellite-based regional approaches (e.g., Lefevre et al., 2002; Ono et al., 2003; Olsen et al., 2004; Lohrenz and Cai, 2006) can be used to extend the spatial and temporal coverage for broad scale assessments of pCO2 distributions and air-sea fluxes of CO2 • The primary objective of our research is to apply these approaches to the characterization of pCO2 and air-sea fluxes of CO2 in the river-influenced margin of the northern Gulf of Mexico

  8. Outline • Brief description of approach as applied to the Mississippi River outflow region • Summary of initial findings • June 2003 • August 2004 • October 2005 • Challenges and future directions

  9. Winds Air-Sea CO2 Flux Approach

  10. Approach • In situ measurements of surface pCO2 and environmental variables (T, S, chlorophyll) conducted

  11. Approach • Algorithm based on empirical relationships of in situ measurements of surface pCO2 to environmental variables (T, S, chlorophyll) • June 2003

  12. T,S,Chl PCA pCO2 Map Empircal Algorithm MODIS Products + In situ pCO2 Approach • Apply algorithm to products retrieved from MODIS-Aqua imagery

  13. June 2003 T,S,Chl PCA pCO2 Map Empircal Algorithm MODIS Products + In situ pCO2 Approach • Produce pCO2 map

  14. Approach • Additional cruises in August 2004 and October 2005 • August 2004 October 2005

  15. Approach • PCA algorithms • August 2004 (r2=0.858) October 2005 (r2=0.974)

  16. Results • Aug 2004 August 2004

  17. Results

  18. Results • Aug 2004 October 2005

  19. Results

  20. Results • Air-sea flux calculations: • CO2 fluxes across the air-sea interface were calculated by the one-dimensional thin-film model: • CO2 Flux = k[pCO2(w) – pCO2(a)] • where k is the gas transfer velocity and has the largest uncertainty in the flux calculation;  is the solubility of CO2 at given temperature and salinity

  21. Results • Air-sea flux calculations: • Various sets of the gas transfer velocity, k, vs. wind speed relationships were used to provide a range of values to bracket the gas flux including: • Liss and Merlivat, 1986; Wanninkhof and McGillis, 1999; Nightingale et al., 2000; McGillis et al., 2001

  22. Results • There was net uptake of CO2 in June, but a net release in August and October • October 2005 followed two major storm events accompanied by coastal flooding; strong winds

  23. Challenges and Future Directions • Challenges • Variability in conditions in coastal waters limits the generality of algorithms across temporal and spatial ranges • Remedy is to provide ample in situ data, including ship-based surveys and moored time-series • Couple ship-based operations with radiometric measurements to provide complementary data set

  24. Challenges and Future Directions • Future Directions • Continued characterization of Mississippi River system to capture range of seasonal and discharge conditions • Comparison of other river systems and shelf environments • Relate patterns to biogeochemical rates (primary production, net ecosystem metabolism) • Couple observations with ecosystem models to provide more informed understanding of carbon cycle controlling mechanisms

  25. Acknowledgements • Funding for this work was provided by NASA (NNG05GD22G and NNS04AB84H)

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