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  1. Abstract

    This study develops a new regional model of the Southern Ocean including an improved representation of the iron biogeochemistry and ecosystem component, nesting within a biogeochemical ocean state estimate, and benchmarking with a suite of observations. The regional domain focuses on the Udintsev Fracture Zone (UFZ) in the central Pacific sector of the Southern Ocean. The UFZ is characterized by the deep gap between the Pacific‐Antarctic Ridge and the East Pacific Rise, which is one of the key “choke points” of the Antarctic Circumpolar Current where major Southern Ocean fronts are constrained within close proximity to this topographic feature. It is also a region of elevated mesoscale eddy activity, especially downstream of the UFZ. The model reproduces observed partial pressure of carbon dioxide in the surface water (pCO2) remarkably well from seasonal to interannual timescales relative to prior studies (r = 0.89). The seasonality of pCO2is difficult to simulate correctly because it is a small residual between the opposing influences of temperature and carbon. This model represents an intermittent double peak pattern of pCO2; one driven by the summertime high temperature and another from the wintertime high of dissolved inorganic carbon. The model also captures the spatial and temporal structure of the regional net primary production with respect to the satellite ocean color products (r = 0.57). The model is further validated by comparing it with biogeochemical float observations from the Southern Ocean Carbon and Climate Observations and Modeling project, revealing the model performance and challenges to accurately represent physical and biogeochemical properties in frontal regions.

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  2. Abstract

    Observations of dissolved iron (dFe) in the subtropical North Atlantic revealed remarkable features: While the near‐surface dFe concentration is low despite receiving high dust deposition, the subsurface dFe concentration is high. We test several hypotheses that might explain this feature in an ocean biogeochemistry model with a refined Fe cycling scheme. These hypotheses invoke a stronger lithogenic scavenging rate, rapid biological uptake, and a weaker binding between Fe and a ubiquitous, refractory ligand. While the standard model overestimates the surface dFe concentration, a 10‐time stronger biological uptake run causes a slight reduction in the model surface dFe. A tenfold decrease in the binding strength of the refractory ligand, suggested by recent observations, starts reproducing the observed dFe pattern, with a potential impact for the global nutrient distribution. An extreme value for the lithogenic scavenging rate can also match the model dFe with observations, but this process is still poorly constrained.

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  3. The Southern Ocean is an important region of ocean carbon uptake, and observations indicate its air‐sea carbon flux fluctuates from seasonal to decadal timescales. Carbon fluxes at regional scales remain highly uncertain due to sparse observation and intrinsic complexity of the biogeochemical processes. The objective of this study is to better understand the mechanisms influencing variability of carbon uptake in the Drake Passage. A regional circulation and biogeochemistry model is configured at the lateral resolution of 10 km, which resolves larger mesoscale eddies where the typical Rossby deformation radius is(50 km). We use this model to examine the interplay between mean and eddy advection, convective mixing, and biological carbon export that determines the surface dissolved inorganic carbon and partial pressure of carbon dioxide variability. Results are validated against in situ observations, demonstrating that the model captures general features of observed seasonal to interannual variability. The model reproduces the two major fronts: Polar Front (PF) and Subantarctic Front (SAF), with locally elevated level of eddy kinetic energy and lateral eddy carbon flux, which play prominent roles in setting the spatial pattern, mean state and variability of the regional carbon budget. The uptake of atmospheric CO2, vertical entrainment during cool seasons, and mean advection are the major carbon sources in the upper 200 m of the region. These sources are balanced by the biological carbon export during warm seasons and mesoscale eddy transfer. Comparing the induced advective carbon fluxes, mean flow dominates in magnitude, however, the amplitude of variability is controlled by the eddy flux.

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