Oxygen (O(2)) is a critical constraint on marine ecosystems. As oceanic O(2) falls to hypoxic concentrations, habitability for aerobic organisms decreases rapidly. We show that the spatial extent of hypoxia is highly sensitive to small changes in the ocean's O(2) content, with maximum responses at suboxic concentrations where anaerobic metabolisms predominate. In model-based reconstructions of historical oxygen changes, the world's largest suboxic zone, in the Pacific Ocean, varies in size by a factor of 2. This is attributable to climate-driven changes in the depth of the tropical and subtropical thermocline that have multiplicative effects on respiration rates in low-O(2) water. The same mechanism yields even larger fluctuations in the rate of nitrogen removal by denitrification, creating a link between decadal climate oscillations and the nutrient limitation of marine photosynthesis.

[1] Long-term trends and average seasonal variability in the upper ocean carbon cycle are investigated at Station ALOHA, the site of the U.S. JGOFS Hawaii Ocean Time series program (HOT), on the basis of a 14-year time series (1988)(1989)(1990)(1991)(1992)(1993)(1994)(1995)(1996)(1997)(1998)(1999)(2000)(2001)(2002) . More than half of the rates of change in sDIC and oceanic pCO 2 , and most of the change in 13 C/ 12 C, are attributed to the uptake of isotopically light anthropogenic CO 2 from the atmosphere. The residual trends appear to be caused mainly by a regional change in the net freshwater budget, perhaps associated with a regime change of the North Pacific climate system near 1997. Computed oceanic pCO 2 is below atmospheric pCO 2 for nearly the entire year, leading to an annual mean surface ocean pCO 2 undersaturation of about 18 matm, and to an annual uptake of CO 2 from the atmosphere, which we compute to be 1.0 ± 0.1 mol m À2 yr À1 . We estimate that about 30% of this flux relates to the uptake of anthropogenic CO 2 , and the remainder to biologically mediated export of organic carbon. Using a modified version of the diagnostic model of Gruber et al. [1998], constrained by d 13 C oc , we infer net community production of organic carbon (NCP) to be the dominant process generating the observed seasonal variability in sDIC. The annual integral of NCP, 2.3 ± 0.8 mol m À2 yr À1 , is comparable to previous estimates of biological production in the subtropical North Pacific. Annually integrated fluxes of air-sea gas exchange and NCP at Station ALOHA are each about two thirds of those computed for the upper ocean near Bermuda using similar methods of estimation [Gruber et al., 1998. However, the seasonal amplitudes of sDIC and d 13 C oc near Hawaii are only half as large as near Bermuda, because air-sea gas exchange and NCP tend to oppose each other near Hawaii, but reinforce each other near Bermuda.

A B S T R A C TUsing an Observing System Simulation Experiment (OSSE), we investigate the impact of JAXA Greenhouse gases Observing SATellite 'IBUKI' (GOSAT) sampling on the estimation of terrestrial biospheric flux with the NASA Carbon Monitoring System Flux (CMS-Flux) estimation and attribution strategy. The simulated observations in the OSSE use the actual column carbon dioxide (X CO 2 ) b2.9 retrieval sensitivity and quality control for the year 2010 processed through the Atmospheric CO 2 Observations from Space algorithm. CMSFlux is a variational inversion system that uses the GEOS-Chem forward and adjoint model forced by a suite of observationally constrained fluxes from ocean, land and anthropogenic models. We investigate the impact of GOSAT sampling on flux estimation in two aspects: 1) random error uncertainty reduction and 2) the global and regional bias in posterior flux resulted from the spatiotemporally biased GOSAT sampling. Based on Monte Carlo calculations, we find that global average flux uncertainty reduction ranges from 25% in September to 60% in July. When aggregated to the 11 land regions designated by the phase 3 of the Atmospheric Tracer Transport Model Intercomparison Project, the annual mean uncertainty reduction ranges from 10% over North American boreal to 38% over South American temperate, which is driven by observational coverage and the magnitude of prior flux uncertainty. The uncertainty reduction over the South American tropical region is 30%, even with sparse observation coverage. We show that this reduction results from the large prior flux uncertainty and the impact of non-local observations. Given the assumed prior error statistics, the degree of freedom for signal is Â1132 for 1-yr of the 74 055 GOSAT X CO 2 observations, which indicates that GOSAT provides Â1132 independent pieces of information about surface fluxes. We quantify the impact of GOSAT's spatiotemporally sampling on the posterior flux, and find that a 0.7 gigatons of carbon bias in the global annual posterior flux resulted from the seasonally and diurnally biased sampling when using a diagonal prior flux error covariance.

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