Our project involves a synthesis of satellite-based ocean color and temperature, in situ ocean data, and atmospheric O 2 /N 2 and N 2 O measurements to provide improved estimates of export flux and subsurface ventilation in the Southern Ocean. The satellite data will be incorporated into improved algorithms to generate time series of ocean phytoplankton photosynthetic pigments, net primary production (NPP), and export flux (f) ratios. The improved f ratios will be guided by analysis of a large new database of Th isotope and sediment trap measurements in the Southern Ocean. The export fluxes, calculated as the product of NPP∙f, will be combined with a simple mixed-layer balance to estimate air-sea O 2 fluxes, which will be used to force an atmospheric tracer transport model. The model results will be compared to atmospheric O 2 /N 2 observations at southern hemisphere monitoring stations that have been corrected for thermal and terrestrial signals using standard methods as well as for biological O 2 ventilation signals based on a new method involving atmospheric N 2 O. Simulations initially have focused on evaluating the ability of export flux algorithms to reproduce mean seasonal cycles in atmospheric O 2 /N 2 and later will be extended to examine interannual variability. Southern Ocean Export Flux and Air-Sea O 2 Exchange: A Synthesis of Direct Observations, Satellite Data and Atmospheric O 2 /N 2 Measurements Overview Cynthia Nevison 1, Matt Charette 2, Mati Kahru 3, Ralph Keeling 3, Kanchan Maiti 2, and Greg Mitchell 3 1 University of Colorado, Boulder, CO, 2 Woods Hole Oceanographic Institution, Woods Hole, MA, 3 Scripps Institution of Oceanography, La Jolla, CA Figure 2. EF (POC export) vs Net Primary Production observations from the Southern Ocean reveal different values for f at similar temperature and NPP values, suggesting that additional variables may need to be incorporated into the commonly used Laws et al. [2004] model. The abundance of autotrophic to heterotrophic micro-organisms or ballasting material (biogenic silica, calcite) may contribute to the variability in f. We will use the data compilation in Figure 1 to explore whether these trophic abundances can be quantified optically as a function of the backscattering: absorption ratio (bb:a) or the bb spectral slope from OCTS, SeaWiFS and MODIS-Aqua data. Atmospheric O 2 /N 2 Figure 5. Climatological atmospheric O 2 /N 2 seasonal cycles observed at 6 atmospheric monitoring stations (black curves). The red, blue and green curves are the atmospheric tracers produced by forcing the MATCH atmospheric transport model with the export fluxes (EF) from Figure 4. While the observed O 2 /N 2 seasonal cycles are driven in part by sea-to-air exchange of oxygen left behind in the mixed layer by carbon export, the observed cycles are also influenced by additional factors, as explained below, and thus cannot be used directly to evaluate the export flux tracers. This limitation has hindered the use of O 2 /N 2 data to evaluate satellite EF algorithms in past studies. Direct Observations Satellite Data Figure 1. In the last two decades, extensive studies on the production and fate of biomass in Antarctic ecosystems have been carried out; however, these data have not been properly synthesized. Here we present a compilation of the available POC flux data collected by three different techniques: moored sediment traps, surface-tethered traps and 234 Th-based measurements. Figure 3. Composite image of export flux of organic carbon (mgC m -2 day -1 ) for February computed from SeaWiFS chl-a and PAR, and surface temperature from AVHRR or MODIS. The locations of Th, sediment trap, NPP and optical data are indicated as black symbols. The GasEx study region is shown in the South Atlantic sector. Monitoring stations for atmospheric O 2 /N 2 are indicated by stars. Figure 4. Current estimates of export flux (EF) differ up to a factor of 3 using different satellite methods. This is true for the NPP component of EF alone. Uncertainties in the parameterization of f introduce additional uncertainties. This figure shows time-series of satellite-based EF using three different estimates of NPP as input to the Laws [2004] model for a domain bounded by the polygon in the GasEx process study region (see Figure 3). Since EF is the fundamental parameter most relevant to the plankton role in carbon export and associated O 2 release to the atmosphere, it is important to evaluate EF estimates based on different, independent methods. Atmospheric O 2 /N 2 data provide a logical independent validation tool. (O 2 /N 2 ) obs = (O 2 /N 2 ) therm + (O 2 /N 2 ) land + (O 2 /N 2 ) vent + (O 2 /N 2 ) prod (Equation 1) Figure 7. Compares satellite-based calculations of 3 atmospheric export flux (EF) tracers (red, blue and green curves) at Cape Grim, Tasmania with observations (magenta curve). The observations are based on O 2 /N 2 measurements from Cape Grim, corrected for thermal, land and deep ocean ventilation signals as described in Figure 6. The calculations use export fluxes computed using the Laws et al. [2004] model of f ratio and NPP estimated from 3 different satellite algorithms. To convert EF into a surface oxygen flux, a stoichiometric ratio of 1.4 mol O 2 /mol C is assumed. The O 2 fluxes are used to drive the MATCH atmospheric transport model to compute the resulting atmospheric O 2 /N 2 variations. Acknowledgements: We acknowledge the support of NASA grant NNX08AB48G and thank Natalie Mahowald and the National Center for Atmospheric Research for help with the MATCH transport model. A GasEx B Figure 6. Seasonal cycle in atmospheric O 2 /N 2 at Cape Grim, Tasmania decomposed into its 4 component signals as described by Equation 1. The estimation of the land and ocean ventilation, surface production, and thermal terms is described in detail in Nevison et al. [2005]. (O 2 /N 2 ) therm is estimated based on Ar/N 2 data, (O 2 /N 2 ) land is estimated based on CO 2 data, and (O 2 /N 2 ) vent is estimated based on N 2 O data. (O 2 /N 2 ) prod, the signal due to marine production, is calculated as a residual of observed O 2 /N 2 minus the other 3 terms. (O 2 /N 2 ) prod can be compared to transport simulations forced by export fluxes, as shown below in Figure 7. References Laws, E. A Export flux and stability as regulators of community composition in pelagic marine biological communities: Implications for regime shifts. Progress in Oceanography, 60(2-4): Nevison, C. D., Keeling, R. F., Weiss, R. F., Popp, B. N., Jin, X., Fraser, P. J., Porter, L. W., & Hess, P. G Southern Ocean ventilation inferred from seasonal cycles of atmospheric N 2 O and O 2 /N 2 at Cape Grim, Tasmania. Tellus Series B-Chemical and Physical Meteorology, 57(3):