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Ocean deoxygenation is an important consequence of climate change that poses an imminent threat to marine life and global food security. However, our understanding of the complex interactions between changes in circulation, solubility, and respiration that drive global‐scale deoxygenation is incomplete. Here, we consider idealized biogeochemical steady states in equilibrium with perpetually slower and warmer oceans constructed from climate‐model simulations of the 2090s that we hold constant in time. In contrast to simulations of the end‐of‐century transient state, our idealized states are intensely deoxygenated in the abyss, consistent with perpetually reduced ventilation and throttled Antarctic Bottom Water formation. We disentangle the effects of the deoxygenation drivers on preformed oxygen and true oxygen utilization (TOU) using the novel concept of upstream exposure time, which precisely connects TOU to oxygen utilization rates and preformed oxygen to ventilation. For our idealized steady states, deoxygenation below 2,000 m depth is due to increased TOU, driven dominantly by slower circulations that allow respiration to act roughly 2–3 times longer thereby overwhelming the effects of reduced respiration rates. Above 500 m depth, decreased respiration and slower circulation closely compensate, resulting in little expansion of upper‐ocean hypoxia. The bulk of preformed oxygen loss is driven by ventilation shifting equatorward to where warmer surface waters hold less oxygen. Warming‐driven declines in solubility account for less than 10% of the total oxygen loss. Although idealized, our analysis suggests that long‐term changes in the marine oxygen cycle could be driven dominantly by changes in circulation rather than by thermodynamics or biology. Plain Language Summary: Climate change is driving oxygen out of the ocean, threatening marine life and global food security. However, the precise contributions of the chemical, physical, and biological processes that control oxygen levels are not well known because of their complex interactions. To better understand these interactions, we consider idealized simulations of a global oxygen cycle that is fully equilibrated with a perpetually warmer and slower ocean constructed from climate‐model simulations of the 2090s but held constant in time for our analyses. Compared to typical predictions, these idealized states exhibit intense deep‐ocean deoxygenation, for which we precisely quantify the contributions from changes in solubility, respiration, and ocean circulation. We find that deep‐ocean deoxygenation is driven by the slower circulation allowing respiration to act for 2–3 times longer thereby overcoming lower respiration rates. The surface origin of oxygen shifts away from cold high‐latitude waters toward warmer waters, in which atmospheric oxygen is less soluble, further reducing oxygen levels. Warming‐driven decreases in solubility alone only account for a mere 10% of the total oxygen loss. The upper ocean remains well oxygenated because changes in respiration and circulation compensate almost perfectly. Our results highlight the central importance of circulation in controlling oxygen in the ocean. Key Points: Key drivers of deoxygenation are quantified for oxygen cycles idealized by being in equilibrium with perpetually slower oceansWidespread intense abyssal ocean deoxygenation is driven predominantly by slower circulations allowing respiration to act over longer timesMost of the reduction in preformed oxygen is driven by changes in ventilation patterns and not by warming‐driven reduced solubility [ABSTRACT FROM AUTHOR]

The equatorial Pacific is the largest oceanic source of carbon dioxide to the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal climate variability. New production, the amount of phytoplankton net primary production driven by upwelled nitrate, plays a significant role in modulating air‐sea CO2 fluxes as the biological carbon pump removes carbon from the surface ocean. We aim to understand how the physical drivers of sea surface temperature and wind speed influence interannual and decadal variability of the equatorial Pacific carbon cycle. In the equatorial Pacific, there are three biogeochemical regimes: the upwelling cold tongue east of 140°W and south of the equator (3°N–15°S); the eastern Pacific warm pool north of the equator (3°–15°N); and the 28.5°C western Pacific warm pool, west of 140°W. We find that between 2000 and 2020, air‐sea CO2 flux and ΔpCO2 increased in the cold tongue (45 mmolC m−2 yr−2, 1.5 μatm yr−1, respectively) but decreased elsewhere, while new production decreased everywhere. The western Pacific occasionally became a weak carbon sink, depending on ENSO and this sink was strongest at 165°E during central Pacific "Modoki" El Niño events. We find that changes in wind speed, temperature and ENSO frequency have altered the surface carbon budget. The mean basin‐wide (150°E−90°W and 15°N–15°S) new production for 2000–2020 was 1.2 ± 0.1 PgC yr−1 and air‐sea CO2 flux was 0.5 ± 0.1 PgC yr−1. New production decreased at −7.7 ± 1.6 TgC yr−2, compared to the CO2 flux trend of −1.7 ± 1.4 TgC yr−2. Plain Language Summary: The equatorial Pacific is the largest source of carbon dioxide that outgasses from the ocean into the atmosphere. This outgassing varies depending on the El Niño‐Southern Oscillation (ENSO) and decadal changes in the climate. Winds in the eastern Pacific drive upwelling which supplies nutrients and carbon to the surface. Some of this carbon is outgassed to the atmosphere, some is consumed by phytoplankton and can either move through the food web or sink into the ocean interior. We are interested in how much carbon is removed from the surface equatorial Pacific through outgassing of carbon and biological consumption. We find that changes in wind speeds, surface temperatures, freshening surface water and changing patterns of ENSO have influenced the equatorial Pacific carbon budget. For example, between 2000 and 2020, CO2 release to the atmosphere increased in the upwelling zone but decreased elsewhere, while biological consumption decreased everywhere. The western Pacific occasionally absorbs carbon from the atmosphere during central Pacific "Modoki" El Niño events, a particular type of El Niño that is becoming more frequent. Key Points: Decadal shifts in upwelling have increased CO2 flux in the cold tongue but not biological production, which is likely nutrient limitedIncreasing frequency of central Pacific El Niño events may amplify decadal CO2 flux trends and drive sink conditions in the westIncreasing surface temperatures contribute to stratification and decreasing new production despite increasing dissolved inorganic carbon [ABSTRACT FROM AUTHOR]

We diagnose changes in the decadal‐mean ocean transport over the 21st century as simulated by the ACCESS‐1.3 model under RCP8.5 forcing. A matrix formulation of advection‐diffusion is used to quantify transport into the ocean interior and from the interior back to the surface. We compute the ocean volumes last ventilated at the surface, as well as the volumes next reexposed to the atmosphere, per unit area, and the distributions of water in transit between key ventilation regions. We find that for the 2090s circulation, Antarctic Bottom Water formation and Antarctic ventilation have collapsed but are compensated by increased mode water formation and ventilation at the poleward edge of the southern subtropical gyres. Reexposure of deep water back to the atmosphere in the Antarctic Southern Ocean is also reduced but compensated by enhanced reexposure in the subpolar Southern Ocean. Labrador Sea ventilation is suppressed, while there is increased ventilation in the eastern subpolar Atlantic. The 2090s surface‐to‐surface interbasin transport is much more sluggish than that of the 1990s, with 30–40% reduced volume flow rates. For transport from last ventilation in the subpolar North Atlantic to next ventilation in the tropical Eastern Pacific, the 2090s residence‐time distribution has a ∼40% longer mode with much more slowly decaying tails and correspondingly lagged path densities. The 2090s mean times since last ventilation, as well as the mean times to reexposure are older in the Antarctic Southern Ocean and elsewhere progressively with depth, by up to ∼1,000 years in the abyss.Plain Language Summary: The ocean's uptake and distribution of greenhouse gases, oxygen, and heat are determined by the time scales and pathways with which water is transported from the surface into the ocean interior, and from the interior back to the surface. Here we use highly efficient numerical methods to analyze climate model simulations of the global ocean circulation over the 21st century for a scenario with strong greenhouse gas emissions. We find that for the 2090s circulation, Antarctic deep‐water formation has nearly collapsed and the ocean volume that had last surface contact in the Southern Ocean has been roughly halved. To compensate, the volume that forms in subantarctic regions and provides nutrients to much of the world ocean nearly doubles. Less water is reexposed to the atmosphere in Antarctic seas while more is reexposed in adjacent waters to the north. Compared to the 1990s, the 2090s circulation is much more sluggish. Although the pathways from North Atlantic deep‐water formation to reexposure in the tropical eastern Pacific have similar geographic patterns, their most likely transport times are about 40% slower. For the ocean's deepest waters, the mean surface‐to‐surface transit times along the paths of the 2090s circulation are up to 2,000 years longer.Key Points: Transport time scales into and out of the abyssal ocean slow by nearly 1,000 years for the 2090s circulationThrottled Antarctic Bottom Water formation and high‐latitude reexposure are compensated by enhanced subpolar ventilation and reexposureThe residence time partitioned pathways of the 2090s have geographic patterns similar to the 1990s but are strongly lagged in residence time [ABSTRACT FROM AUTHOR]

Ocean temperature extreme events such as marine heatwaves are expected to intensify in coming decades due to anthropogenic global warming. Reported ecological and economic impacts of marine heatwaves include coral bleaching, local extinction of mangrove and kelp forests and elevated mortalities of invertebrates, fishes, seabirds and marine mammals. In contrast, little is known about the impacts of marine heatwaves on microbes that regulate biogeochemical processes in the ocean. Here we analyse the daily output of a near‐global ocean physical–biogeochemical model simulation to characterize the impacts of marine heatwaves on phytoplankton blooms in 23 tropical and temperate oceanographic regions from 1992 to 2014. The results reveal regionally coherent anomalies of shallower surface mixing layers and lower surface nitrate concentrations during marine heatwaves. These anomalies exert counteracting effects on phytoplankton growth through light and nutrient limitation. Consequently, the responses of phytoplankton blooms are mixed, but can be related to the background nutrient conditions of the study regions. The blooms are weaker during marine heatwaves in nutrient‐poor waters, whereas in nutrient‐rich waters, the heatwave blooms are stronger. The corresponding analyses of sea‐surface temperature, chlorophyll a and nitrate based on satellite observations and in situ climatology support this relationship between phytoplankton bloom anomalies and background nitrate concentration. Given that nutrient‐poor waters are projected to expand globally in the 21st century, this study suggests increased occurrence of weaker blooms during marine heatwaves in coming decades, with implications for higher trophic levels and biogeochemical cycling of key elements. [ABSTRACT FROM AUTHOR]

The tropical Pacific Ocean is a globally significant region of climate‐driven biogeochemical variability. Satellite ocean color algorithms have been used for over 20 years, providing a substantial historical record of global ocean chlorophyll‐a variability. Current chlorophyll algorithms perform better in the tropical Pacific than for the globe. Nevertheless, improvements can be made to produce a robust historical record of chlorophyll variability, which is essential to accurately identify ocean‐atmosphere carbon fluxes and long‐term trends in ocean productivity. We use a large in situ chlorophyll database to tune empirical ocean color algorithms to reduce bias in the equatorial Pacific. Traditional band ratio chlorophyll algorithms (OCx) perform adequately but exhibit errors at low chlorophyll concentrations. A new algorithm, the Ocean Color Index (OCI; Hu et al., 2012, https://doi.org/10.1029/2011JC007395), is more effective at calculating low chlorophyll concentrations in the mesotrophic tropical Pacific Ocean. Existing ocean color algorithms underestimate tropical Pacific chlorophyll by 5.8%, 14%, and 2% for three satellite ocean color sensors: SeaWiFS, MODIS‐Aqua, and MERIS. In this paper, we develop regionally tuned sensor‐specific coefficients and blending windows between the OCI to OCx algorithms to reduce systematic biases in the tropical Pacific. We assess cross‐sensor consistency to produce robust 21‐year time series trends. These updated estimates increase chlorophyll concentrations in open water and decrease around island and warm‐pool regions, with implications for our understanding of El Niño‐Southern Oscillation driven carbon fluxes and net primary productivity. Plain Language Summary: The tropical Pacific Ocean is a globally significant region where climate drives the variability of phytoplankton, nutrient availability, and primary productivity. Satellite‐based chlorophyll observations provide the most comprehensive large‐scale estimate of phytoplankton abundance in the upper ocean. With over two decades of observations available from satellites, it is essential that these satellite measurements are as accurate as possible in order for us to document chlorophyll variability and trends accurately. In this paper, we evaluate the performance of satellite chlorophyll observations in the tropical Pacific Ocean and suggest algorithm improvements. Reduced errors in chlorophyll estimates will provide essential insights into critical processes like primary productivity and biologically driven CO2 transport. No significant long‐term trends are found for the tropical Pacific Ocean so far. However, decreased error in our estimates will allow future climate studies to more accurately describe phytoplankton variability in this region. Key Points: Satellite ocean color measurements underestimate chlorophyll concentrations in the tropical Pacific Ocean, and we removed the systematic biasModifying algorithm coefficients and blending window size between algorithms can improve the overall ocean color estimatesImproved chlorophyll estimates have implications for historical and future Pacific carbon cycle and net primary productivity estimates [ABSTRACT FROM AUTHOR]

Abstract: The Great Australian Bight (GAB), a coastal sea bordered by the Pacific, Southern, and Indian Oceans, sustains one of the largest fisheries in Australia but the geographical origin of nutrients that maintain its productivity is not fully known. We use 12 years of modeled data from a coupled hydrodynamic and biogeochemical model and an Eulerian‐Lagrangian approach to quantify nitrate supply to the GAB and the region between the GAB and the Subantarctic Australian Front (GAB‐SAFn), identify phytoplankton growth within the GAB, and ascertain the source of nitrate that fuels it. We find that nitrate concentrations have a decorrelation timescale of ∼60 days; since most of the water from surrounding oceans takes longer than 60 days to reach the GAB, 23% and 75% of nitrate used by phytoplankton to grow are sourced within the GAB and from the GAB‐SAFn, respectively. Thus, most of the nitrate is recycled locally. Although nitrate concentrations and fluxes into the GAB are greater below 100 m than above, 79% of the nitrate fueling phytoplankton growth is sourced from above 100 m. Our findings suggest that topographical uplift and stratification erosion are key mechanisms delivering nutrients from below the nutricline into the euphotic zone and triggering large phytoplankton growth. We find annual and semiannual periodicities in phytoplankton growth, peaking in the austral spring and autumn when the mixed layer deepens leading to a subsurface maximum of phytoplankton growth. This study highlights the importance of examining phytoplankton growth at depth and the utility of Lagrangian approaches. [ABSTRACT FROM AUTHOR]

The implications of a mesoscale eddy for relevant properties of the Southern Ocean carbon cycle are examined with in situ observations. We explored carbon properties inside a large (~190 km diameter) cyclonic eddy that detached from the Subantarctic Front (SAF) south of Tasmania in March 2016. Based on remote sensing, the eddy was present for ~2 months in the Subantarctic Zone (SAZ), an important region of oceanic carbon dioxide (CO2) uptake throughout the annual cycle and carbon subduction (i.e., where mode and intermediate waters form), before it was reabsorbed into the SAF. The eddy was sampled during the middle of its life, 1 month after it spawned. Comparatively, the eddy was ~3°C colder, 0.5 practical salinity unit fresher, and less biologically productive than surrounding SAZ waters. The eddy was also richer in dissolved inorganic carbon (DIC) and had lower saturation states of aragonite and calcite than the surrounding SAZ waters. As a consequence, it was a strong source of CO2 to the atmosphere (with fluxes up to +25 mmol C m−2 d−1). Compared to the SAF waters, from which it originated, DIC concentration in the eddy was ~20 μmol kg−1 lower, indicating lateral mixing, small-scale recirculation, or eddy stirring with lower-DIC SAZ waters by the time the eddy was observed. As they are commonly spawned from the Antarctic Circumpolar Current, and as 50% of them decay in the SAZ (the rest being reabsorbed by the SAF-N), these types of eddies may represent a significant south-north transport pathway for carbon across the ACC and may alter the carbon properties of SAZ waters. [ABSTRACT FROM AUTHOR]

Surface ocean productivity mediates the transfer of carbon to the deep ocean and in the process regulates atmospheric CO2 levels. A common axiom in oceanography is that large phytoplankton contribute disproportionally to the transfer of carbon to the deep ocean because of their greater ability to escape grazing pressure, build biomass, and sink. In the present study, we assessed the relationship of net community production to phytoplankton assemblages and plankton size distribution in the Sub-Antarctic Zone and northern reaches of the Polar Frontal Zone in the Australian sector of the Southern Ocean. We reanalyzed and synthesized previously published estimates of O2/Ar net community oxygen production (NCP) and triple-O2 isotopes gross primary oxygen production (GPP) along with microscopic and pigment analyses of the microbial community. Overall, we found that the axiom that large phytoplankton drive carbon export was not supported in this region. Mixed-layer-depth-integrated NCP was correlated to particulate organic carbon (POC) concentration in the mixed layer. While lower NCP/GPP and NCP/POC values were generally associated with communities dominated by smaller plankton size (as would be expected), these communities did not preclude high values for both properties. Vigorous NCP in some regions occurred in the virtual absence of large phytoplankton (and specifically diatoms) and in communities dominated by nanoplankton and picoplankton. We also observed a positive correlation between NCP and the proportion of the phytoplankton community grazed by microheterotrophs, supporting the mediating role of grazers in carbon export. The novel combination of techniques allowed us to determine how NCP relates to upper ocean ecosystem characteristics and may lead to improved models of carbon export. [ABSTRACT FROM AUTHOR]

We use a database of more than 4.4 million observations of ocean pCO2 to investigate oceanic pCO2 growth rates. We use pCO2 measurements, with corresponding sea surface temperature and salinity measurements, to reconstruct alkalinity and dissolved inorganic carbon to understand what is driving these growth rates in different ocean regions. If the oceanic pCO2 growth rate is faster (slower) than the atmospheric CO2 growth rate, the region can be interpreted as having a decreasing (increasing) atmospheric CO2 uptake. Only the Western subpolar and subtropical North Pacific, and the Southern Ocean are found to have sufficient spatial and temporal observations to calculate the growth rates of oceanic pCO2 in different seasons. Based on these regions, we find the strength of the ocean carbon sink has declined over the last two decades due to a combination of regional drivers (physical and biological). In the subpolar North Pacific reduced atmospheric CO2 uptake in the summer is associated with changes in the biological production, while in the subtropical North Pacific enhanced uptake in winter is associated with enhanced biological production. In the Indian and Pacific sectors of the Southern Ocean a reduced winter atmospheric CO2 uptake is associated with a positive SAM response. Conversely in the more stratified Atlantic Ocean sector enhanced summer uptake is associated with increased biological production and reduced vertical supply. We are not able to separate climate variability and change as the calculated growth rates are at the limit of detection and are associated with large uncertainties. Ongoing sustained observations of global oceanic pCO2 and its drivers, including dissolved inorganic carbon and alkalinity, are key to detecting and understanding how the ocean carbon sink will evolve in future and what processes are driving this change. [ABSTRACT FROM AUTHOR]

Despite improvements in our understanding of the Southern Ocean air-sea flux of CO2, discrepancies still exist between a variety of differing ocean/atmosphere methodologies. Here we employ an independent method to estimate the Southern Ocean air-sea flux of CO2 that exploits all available surface ocean measurements for dissolved inorganic carbon (DIC) and total alkalinity (ALK) beyond 1986. The DIC concentrations were normalized to the year 1995 using coinciding CFC measurements in order to account for the anthropogenic CO2 signal. We show that independent of season, surface-normalized DIC and ALK can be empirically predicted to within ∼ μmol/kg using standard hydrographic properties. The predictive equations were used in conjunction with World Ocean Atlas (2001) climatologies to give a first estimate of the annual cycle of DIC and ALK in the surface Southern Ocean. These seasonal distributions will be very useful in both validating biogeochemistry in general circulation models and for use in situ biological studies within the Southern Ocean. Using optimal CO2 dissociation constants, we then estimate an annual cycle of pCO2 and associated net air-sea CO2 flux. Including the effects of sea ice, we estimate a Southern Ocean (>50°S) CO2 sink of 0.4 ± 0.25 Pg C/yr. Our analysis also indicates a substantial CO2 sink of 1.1 ± 0.6 Pg C/yr within the sub-Antarctic zone (40°S-50°S), associated with strong cooling and high winds. Our results imply the Southern Ocean CO2 flux south of 50°S to be very similar to those found by Takahashi et al. (2002), but on the higher end of a range of atmospheric/oceanic CO2 inversion methodologies. This paper estimates for the first time basic seasonal carbon cycle parameters within the circumpolar Southern Ocean, which have up to now been extremely difficult to measure and sparse. The application of such an empirical technique using more widely available hydrographic parameters in the Southern Ocean provides an important independent estimate to not only CO2 uptake, but also for other future biogeochemical studies. Refining and testing these empirical methods with new carbon measurements will be important to further reduce uncertainties and extend our understanding of Southern Ocean CO2 dynamics. [ABSTRACT FROM AUTHOR]

A biogeochemical ocean general circulation model, driven with NCEP-RI and observed atmospheric CO2 history, is used to investigate and quantify the role that the Southern Annular Mode (SAM), identified as the leading mode of climate variability, has in driving interannual variability in Southern Ocean air-sea CO2 fluxes between 1980 and 2000. Our simulations show the Southern Ocean to be a region of decreased CO2 uptake during the positive SAM phase. The SAM induces changes in Southern Ocean CO2 uptake with a 2-month time lag explaining 42% of the variance in the total interannual variability in air-sea CO2 fluxes. Our analysis shows that the response of the Southern Ocean to the SAM is primarily governed by changes in ΔpCO2 (67%), and that this response is driven by changes in ocean physics that control the supply of nutrients to the upper ocean, primarily Dissolved Inorganic Carbon (DIC). The SAM is predicted to become stronger and more positive in response to climate change and our results suggest this will decrease the Southern Ocean CO2 uptake by 0.1PgC/yr per unit change in the SAM. [ABSTRACT FROM AUTHOR]

A sampling strategy to estimate the annual mean CO2 uptake by the Southern Ocean was developed by applying two-dimensional Fourier transforms and signal-to-noise ratios to the simulated air-sea CO2 fluxes and ΔpCO2 from an ocean biogeochemical model driven with NCEP-R1. Observations of pCO2 were used to validate the statistical properties of the model and to estimate the mesoscale variability not captured by the model resolution. Sampling regularly every 3 months, at every 30° in longitude and 3° in latitude is sufficient to determine the net Southern Ocean CO2 uptake. We applied this sampling strategy to the simulated air-sea fluxes to estimate a net annual mean CO2 uptake of 0.6 ± 0.1 PgC/yr (1990-1999). This uncertainty in the estimate was dominated by the simulated interannual variability, and not by errors in the sampling or unresolved mesoscale variability. Therefore sampling at higher resolutions in space and time would not reduce the uncertainty in the Southern Ocean annual mean uptake any further. These results show that a doubling of the current Southern Ocean sampling (in longitude) would be required to constrain the net annual mean air-sea CO2 fluxes to within the natural variability of the system. [ABSTRACT FROM AUTHOR]