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The inventory and variability of oceanic dissolved inorganic carbon (DIC) is driven by the interplay of physical, chemical, and biological processes. Quantifying the spatiotemporal variability of these drivers is crucial for a mechanistic understanding of the ocean carbon sink and its future trajectory. Here, we use the Estimating the Circulation and Climate of the Ocean-Darwin ocean biogeochemistry state estimate to generate a global-ocean, data-constrained DIC budget and investigate how spatial and seasonal-to-interannual variability in three-dimensional circulation, air-sea CO2 flux, and biological processes have modulated the ocean sink for 1995-2018. Our results demonstrate substantial compensation between budget terms, resulting in distinct upper-ocean carbon regimes. For example, boundary current regions have strong contributions from vertical diffusion while equatorial regions exhibit compensation between upwelling and biological processes. When integrated across the full ocean depth, the 24-year DIC mass increase of 64 Pg C (2.7 Pg C year-1) primarily tracks the anthropogenic CO2 growth rate, with biological processes providing a small contribution of 2% (1.4 Pg C). In the upper 100 m, which stores roughly 13% (8.1 Pg C) of the global increase, we find that circulation provides the largest DIC gain (6.3 Pg C year-1) and biological processes are the largest loss (8.6 Pg C year-1). Interannual variability is dominated by vertical advection in equatorial regions, with the 1997-1998 El Niño-Southern Oscillation causing the largest year-to-year change in upper-ocean DIC (2.1 Pg C). Our results provide a novel, data-constrained framework for an improved mechanistic understanding of natural and anthropogenic perturbations to the ocean sink.

Edited by James McWilliams, University of California, Los Angeles, CA; received April 19, 2022; accepted September 3, 2022 The expansive gyres of the subtropical ocean account for a significant fraction of global organic carbon export fromthe upper ocean. In the gyre interior, verticalmixing and the heaving of nutrient-rich waters into the euphotic layer sustain local productivity, in turn depleting the layers below. However, the nutrient pathways by which these subeuphotic layers are themselves replenished remain unclear. Using a global, eddy-permitting simulation of ocean physics and biogeochemistry, we quantify nutrient resupplymechanisms along and across density surfaces, including the contribution of eddy-scale motions that are challenging to observe. We find that mesoscale eddies (10 to 100 km) flux nutrients from the shallow flanks of the gyre into the recirculating interior, through time-varying motions along density surfaces. The subeuphotic layers are ultimately replenished in approximately equal contributions by this mesoscale eddy transport and the remineralization of sinking particles.Themesoscale eddy resupply ismost important in the lower thermocline for the whole subtropical region but is dominant at all depths within the gyre interior. Subtropical gyre productivity may therefore be sustained by a nutrient relay, where the lateral transport resupplies nutrients to the thermocline and allows vertical exchanges to maintain surface biological production and carbon export. [ABSTRACT FROM AUTHOR]

Anaerobic microbial activity in the ocean causes losses of bioavailable nitrogen and emission of nitrous oxide to the atmosphere, but its predictability at global scales remains limited. Resource ratio theory suggests that anaerobic activity becomes sustainable when the ratio of oxygen to organic matter supply is below the ratio required by aerobic metabolisms. Here, we demonstrate the relevance of this framework at the global scale using three‐dimensional ocean datasets, providing a new interpretation of existing observations. Evaluations of the location and extent of anoxic zones and a diagnostic rate of pelagic nitrogen loss are consistent with previous estimates. However, we demonstrate that a threshold based on substrate‐supply fluxes is qualitatively different from a threshold based solely on the ambient oxygen concentration. This implies that use of the flux‐based threshold in global biogeochemical models can result in different predictions of anaerobic activity and nitrogen loss. Plain Language Summary: Where oxygen concentrations are sufficiently low in the ocean, nutrients are lost to the atmosphere because of specialized activity by microorganisms. Typically, biogeochemical models prescribe a critical oxygen concentration as a threshold to predict this activity. Here, we explain and demonstrate that an alternative threshold is more appropriate. The new threshold takes into account the supply flux of oxygen, rather than the concentration, as well as the supply flux of organic matter that is consumed by the microorganisms. We use global datasets to demonstrate that this threshold is relevant at the global scale. Since it is distinct from and more mechanistic than the typical oxygen concentration threshold, it may be used in global biogeochemical climate models to improve their ability to predict the loss of nutrients to the atmosphere and the impact on the climate system. Key Points: We show that a threshold for anaerobic activity based on oxygen and organic matter fluxes is consistent with global ocean observationsOur diagnostic calculation uses global datasets to define the three major pelagic anoxic zones in a new wayThe flux‐based threshold is qualitatively different than a threshold based on the oxygen concentration [ABSTRACT FROM AUTHOR]

A mechanistic framework for quantitatively mapping the regional drivers of air-sea CO2 fluxes at a global scale is developed. The framework evaluates the interplay between (1) surface heat and freshwater fluxes that influence the potential saturated carbon concentration, which depends on changes in sea surface temperature, salinity and alkalinity, (2) a residual, disequilibrium flux influenced by upwelling and entrainment of remineralized carbon- and nutrient-rich waters from the ocean interior, as well as rapid subduction of surface waters, (3) carbon uptake and export by biological activity as both soft tissue and carbonate, and (4) the effect on surface carbon concentrations due to freshwater precipitation or evaporation. In a steady state simulation of a coarse-resolution ocean circulation and biogeochemistry model, the sum of the individually determined components is close to the known total flux of the simulation. The leading order balance, identified in different dynamical regimes, is between the CO2 fluxes driven by surface heat fluxes and a combination of biologically driven carbon uptake and disequilibrium-driven carbon outgassing. The framework is still able to reconstruct simulated fluxes when evaluated using monthly averaged data and takes a form that can be applied consistently in models of different complexity and observations of the ocean. In this way, the framework may reveal differences in the balance of drivers acting across an ensemble of climate model simulations or be applied to an analysis and interpretation of the observed, real-world air-sea flux of CO2. [ABSTRACT FROM AUTHOR]

Background: Malaria presents public health challenge despite extensive intervention campaigns. A 30-year hindcast of the climatic suitability for malaria transmission in India is presented, using meteorological variables from a state of the art seasonal forecast model to drive a process-based, dynamic disease model. Methods: The spatial distribution and seasonal cycles of temperature and precipitation from the forecast model are compared to three observationally-based meteorological datasets. These time series are then used to drive the disease model, producing a simulated forecast of malaria and three synthetic malaria time series that are qualitatively compared to contemporary and pre-intervention malaria estimates. The area under the Relative Operator Characteristic (ROC) curve is calculated as a quantitative metric of forecast skill, comparing the forecast to the meteorologically-driven synthetic malaria time series. Results and discussion: The forecast shows probabilistic skill in predicting the spatial distribution of Plasmodium falciparum incidence when compared to the simulated meteorologically-driven malaria time series, particularly where modelled incidence shows high seasonal and interannual variability such as in Orissa, West Bengal, and Jharkhand (North-east India), and Gujarat, Rajastan, Madhya Pradesh and Maharashtra (North-west India). Focusing on these two regions, the malaria forecast is able to distinguish between years of “high”, “above average” and “low” malaria incidence in the peak malaria transmission seasons, with more than 70% sensitivity and a statistically significant area under the ROC curve. These results are encouraging given that the three month forecast lead time used is well in excess of the target for early warning systems adopted by the World Health Organization. This approach could form the basis of an operational system to identify the probability of regional malaria epidemics, allowing advanced and targeted allocation of resources for combatting malaria in India. [ABSTRACT FROM AUTHOR]

Phytoplankton growth in many high-latitude and equatorial regions of the world ocean is limited by low iron (Fe), while regions of lower nutrient upwelling and higher dust supply such as the subtropical gyres are typically not considered Fe-limited. Recent work which showed that adding Saharan dust leachate promotes blooms of Vibrio bacteria in the North Atlantic Ocean challenges this simple paradigm, because Fe is one of the most prevalent micronutrients in the leachate. We also find that adding high quantities of dust leachate to the North Atlantic near Bermuda stimulates Vibrio growth. However, inorganic Fe alone did not stimulate the growth of Vibrio in any of eleven field experiments in the North Atlantic, central North Pacific, or the coastal Pacific, suggesting that some other growth factor besides Fe is partially or completely responsible for the Vibrio growth response with added dust leachate. We consider the possibility that organic carbon, phosphorous, or other trace-metals in dust might have stimulated Vibrio growth either directly or indirectly. Laboratory culture experiments demonstrate that dust leachate can stimulate the growth of Vibrio even when sufficient Fe is present in the medium. Global modeling suggests that dust fluxes high enough to stimulate Vibrio growth are rare. • The addition of Fe does not stimulate Vibrio growth in three different marine locations. • Dust leachate does stimulate Vibrio growth, though the mechanism is unclear. • Global modeling suggests natural dust fluxes are rarely large enough to stimulate Vibrio blooms. [ABSTRACT FROM AUTHOR]