[1] Results are presented of export production, dissolved organic matter (DOM) and dissolved oxygen simulated by 12 global ocean models participating in the second phase of the Ocean Carbon-cycle Model Intercomparison Project. A common, simple biogeochemical model is utilized in different coarse-resolution ocean circulation models. The model mean (±1s) downward flux of organic matter across 75 m depth is 17 ± 6 Pg C yr À1 . Model means of globally averaged particle export, the fraction of total export in dissolved form, surface semilabile dissolved organic carbon (DOC), and seasonal net outgassing (SNO) of oxygen are in good agreement with observation-based estimates, but particle export and surface DOC are too high in the tropics. There is a high sensitivity of the results to circulation, as evidenced by (1) the correlation of surface DOC and export with circulation metrics, including chlorofluorocarbon inventory and deep-ocean radiocarbon, (2) very large intermodel differences in Southern Ocean export, and (3) greater export production, fraction of export as DOM, and SNO in models with explicit mixed layer physics. However, deep-ocean oxygen, which varies widely among the models, is poorly correlated with other model indices. Cross-model means of several biogeochemical metrics show better agreement with observation-based estimates when restricted to those models that best simulate deep-ocean radiocarbon. Overall, the results emphasize the importance of physical processes in marine biogeochemical modeling and suggest that the development of circulation models can be accelerated by evaluating them with marine biogeochemical metrics.

[1] We diagnose the contribution of four main phytoplankton functional groups to the production and export of particulate organic carbon (POC), CaCO 3 , and opal by combining in a restoring approach global oceanic observations of nitrate, silicic acid, and alkalinity with a simple size-dependent ecological/biogeochemical model. In order to determine the robustness of our results, we employ three different variants of the ocean general circulation model (OGCM) required to transport and mix the nutrients and alkalinity into the upper ocean. In our standard model, the global export of CaCO 3 is diagnosed as 1.1 PgC yr À1 (range of sensitivity cases 0.8 to 1.2 PgC yr À1 ) and that of opal as 180 Tmol Si yr À1 (range 160 to 180 Tmol Si yr À1 ). CaCO 3 export is found to have three maxima at approximately 40°S, the equator, and around 40°N. In contrast, the opal export is dominated by the Southern Ocean with a single maximum at around 60°S. The molar export ratio of inorganic to organic carbon is diagnosed in our standard model to be about 0.09 (range 0.07 to 0.10) and found to be remarkably uniform spatially. The molar export ratio of opal to organic nitrogen varies substantially from values around 2 to 3 in the Southern Ocean south of 45°S to values below 0.5 throughout most of the rest of the ocean, except for the North Pacific. Irrespective of which OGCM is used, large phytoplankton dominate the export of POC, with diatoms alone accounting for 40% of this export, while the contribution of coccolithophorids is only about 10%. Small phytoplankton dominate net primary production (NPP) with a fraction of $70%. Diatoms and coccolithophorids account for about 15% and less than 2% of NPP, respectively. These diagnosed contributions of the main phytoplankton functional groups to NPP are also robust across all OGCMs investigated. Correlation and regression analyses reveal that the variations in the relative contributions of diatoms and coccolithophorids to NPP can be predicted reasonably well on the basis of a few key parameters.

[1] Ocean iron fertilization is being considered as a strategy for mitigating the buildup of anthropogenic CO 2 in the atmosphere. Assessment of this strategy requires consideration of its unintended consequences, such as an enhancement of ocean N 2 O emissions. This feedback could offset the radiative benefit from the atmospheric CO 2 reduction significantly, because N 2 O is a much more powerful greenhouse gas than CO 2 itself. Our model results show that the magnitude of this offsetting effect is substantial, but is highly dependent on the location and duration of fertilization. We find the largest offsets (of the order of 100%) when fertilization is undertaken in the tropics, particularly when it is of limited duration and size. Smaller, but still substantial effects are found when fertilization is undertaken elsewhere and over longer periods. These results suggest that any assessment of ocean fertilization as a mitigating option is incomplete without consideration of the N 2 O feedback.

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[1] An ecosystem model embedded in a global ocean general circulation model is used to quantify roles of biological and physical processes on seasonal oxygen variations. We find that the thermally induced seasonal net outgassing (SNO) of oxygen is overestimated by about 30% if gas phase equilibrium is assumed, and we find that seasonal variations in thermocline oxygen due to biology are approximated well using the oxygen anomaly. Outside the tropics and the north Indian Ocean, biological SNO is, on average, 56% of net community production (defined as net oxygen production above 76 m) during the outgassing period and 35% of annual net community production. In the same region the seasonal drawdown of the oxygen anomaly within the upper thermocline (76-500 m) is 76% of the remineralization during the drawdown and 48% of annual remineralization. Applying model-derived relationships to observed O 2 climatologies and using independent estimates for tropical and monsoonal systems, we estimate global net community production to be 14.9 ± 2.5 Pg C yr À1 .

Observations, primarily from satellites, have shown a statistical relationship between the surface wind stress and underlying sea surface temperature (SST) on intermediate space and time scales, in many regions inclusive of eastern boundary upwelling current systems. In this paper, this empirical SST-wind stress relationship is utilized to provide a simple representation of mesoscale air-sea coupling for an oceanic model forced by surface winds, namely, the Regional Oceanic Modeling System (ROMS). This model formulation is applied to an idealized upwelling problem with prevailing equatorward winds to determine the coupling consequences on flow, SST, stratification, and wind evolutions. The initially uniform wind field adjusts through coupling to a cross-shore profile with weaker nearshore winds, similar to realistic ones. The modified wind stress weakens the nearshore upwelling circulation and increases SST in the coastal zone. The SST-induced wind stress curl strengthens offshore upwelling through Ekman suction. The total curl-driven upwelling exceeds the coastal upwelling. The SST-induced changes in the nearshore wind stress field also strengthen and broaden the poleward undercurrent. The coupling also shows significant impact on the developing mesoscale eddies by damaging cyclonic eddies more than anticyclonic eddies, which leads to dominance by the latter. Dynamically, this is a consequence of cyclones with stronger SST gradients that induce stronger wind perturbations in this particular upwelling problem and that are therefore generally more susceptible to disruption than anticyclones at finite Rossby number. The net effect is a weakening of eddy kinetic energy.

Abstract. Using numerical simulations, we quantify the impact of changes in the ocean's biological pump on the air-sea balance of CO 2 by fertilizing a small surface patch in the high-nutrient, low-chlorophyll region of the eastern tropical Pacific with iron. Decade-long fertilization experiments are conducted in a basin-scale, eddy-permitting coupled physical/biogeochemical/ecological model. In contrast to previous studies, we find that most of the dissolved inorganic carbon (DIC) removed from the euphotic zone by the enhanced biological export is replaced by uptake of CO 2 from the atmosphere. Atmospheric uptake efficiencies, the ratio of the perturbation in air-sea CO 2 flux to the perturbation in export flux across 100 m, integrated over 10 years, are 0.75 to 0.93 in our patch size-scale experiments. The atmospheric uptake efficiency is insensitive to the duration of the experiment. The primary factor controlling the atmospheric uptake efficiency is the vertical distribution of the enhanced biological production and export. Iron fertilization at the surface tends to induce production anomalies primarily near the surface, leading to high efficiencies. In contrast, mechanisms that induce deep production anomalies (e.g. altered light availability) tend to have a low uptake efficiency, since most of the removed DIC is replaced by lateral and vertical transport and mixing. Despite high atmospheric uptake efficiencies, patchscale iron fertilization of the ocean's biological pump tends to remove little CO 2 from the atmosphere over the decadal timescale considered here.