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Global climate change has impacted ocean biogeochemistry and physical dynamics, causing increases in acidity and temperature, among other phenomena. These changes can lead to deleterious effects on marine ecosystems and communities that rely on these ecosystems for their livelihoods. To better quantify these changes, an array of floats fitted with biogeochemical sensors (BGC‐Argo) is being deployed throughout the ocean. This paper presents an algorithm for deriving a deployment strategy that maximizes the information captured by each float. The process involves using a model solution as a proxy for the true ocean state and carrying out an iterative process to identify optimal float deployment locations for constraining the model variance. As an example, we use the algorithm to optimize the array for observing ocean surface dissolved carbon dioxide concentrations (pCO2) in a region of strong air–sea gas exchange currently being targeted for BGC‐Argo float deployment. We conclude that 54% of the pCO2 variability in the analysis region could be sampled by an array of 50 Argo floats deployed in specified locations. This implies a relatively coarse average spacing, though we find the optimal spacing is nonuniform, with a denser sampling being required in the eastern equatorial Pacific. We also show that this method could be applied to determine the optimal float deployment along ship tracks, matching the logistics of real float deployment. We envision this software package to be a helpful resource in ocean observational design anywhere in the global oceans. [ABSTRACT FROM AUTHOR]

In the tropical Pacific, weak ventilation and intense microbial respiration at depth give rise to a low dissolved oxygen (O2) environment that is thought to be ventilated primarily by the equatorial current system (ECS). The role of mesoscale eddies and vertical mixing as potential pathways of O2 supply in this region, however, remains poorly known due to sparse observations and coarse model resolution. Using an eddy resolving simulation of ocean circulation and biogeochemistry, we assess the contribution of these processes to the O2 budget balance and find that vertical mixing of O2, which is modulated by the surface wind speed and the vertical shear of the eddying currents, contributes substantially to the replenishment of O2 in the upper equatorial Pacific thermocline, complementing the advective supply of O2 by the ECS and meridional circulation at depth. These transport processes vary seasonally in conjunction with the wind: mixing of O2 into the upper thermocline is strongest during boreal summer and fall when the vertical shear and eddy kinetic energy are intensified. The relationship between eddy activity and the downward mixing of O2 arises from the modulation of equatorial turbulence by Tropical Instability Waves via their impacts on the vertical shear. This interaction of processes across scales sustains a local pathway of O2 delivery into the equatorial Pacific interior and highlights the need for adequate observations and models of turbulent mixing and mesoscale processes for understanding and predicting the fate of the tropical Pacific O2 content in a warmer and more stratified ocean. Plain Language Summary: The eastern tropical Pacific interior is an O2 deficient environment, due to intense O2 consumption by microbial communities that is not vigorously replenished by ocean circulation at depth. In this study, we use a high resolution simulation of ocean circulation and biogeochemistry to understand the role of finer scale processes such as turbulence and eddies in injecting O2 locally. We find that mixing due to turbulence along the equator supplies a key portion of O2 into the ocean by exchanging waters between the well‐aerated mixed layer near the surface and the ocean's interior where O2 falls precipitously with depth. We also find that this mixing varies considerably with the seasons. This annual cycle in mixing arises from the seasonal variability in wind stress and the passage of eddies, which amplify turbulence through their influence on the subsurface currents along the equator, and represents a previously unexplored but potentially important route of O2 delivery into the ocean's interior. As the upper ocean warms and becomes less dense, the ocean's O2 content is expected to decrease, and thus observing and accurately modeling these O2 pathways will be crucial to monitoring how marine ecosystem habitats will shift in a warmer climate. Key Points: Mesoscale‐resolving ocean biogeochemistry simulations show vertical mixing is a key source of oxygen to the equatorial Pacific thermoclineThe supply of oxygen by advection and vertical mixing is strongly seasonal and is driven by seasonal variability in the windThe vertical mixing of oxygen is modulated by the wind stress and mesoscale eddy impacts on equatorial shear‐driven turbulence [ABSTRACT FROM AUTHOR]

A state estimate of the tropical Pacific Ocean is used to analyze regional volume, temperature, and salinity budgets during 2014–2018. The simulated ocean state is constrained by both model dynamics and assimilated observations. Comparisons with independent mooring data show that the state estimate is consistent with the observed variability in temperature and velocity. Budgets are analyzed between 5°S and 5°N in the upper 300 m, inside a box defined to represent the central and eastern equatorial Pacific. Transports through the faces of this box are quantified to understand the processes responsible for variability in box‐mean properties. Vertical mixing across 300 m is negligible; temperature and salinity tendencies are balanced by surface fluxes and advective divergence, which is decomposed into geostrophic and ageostrophic components. The onset and recovery of the 2015/2016 El Niño event is found to be dominated by anomalous surface fluxes and horizontal advection. During the onset phase, weaker trade winds cause the shallow meridional overturning circulation to slow down, which reduces the poleward transport of heat and leads to upper ocean warming. Anomalous precipitation and advection of fresh water from the western Pacific drive the net freshening of the region. Relaxation from El Niño conditions is dominated by wind‐driven meridional advection at 5°N. As the meridional advection regains strength, Ekman advection efficiently exports the warm, fresh surface water out of the equatorial region. Quantifying the heat and salt transport changes in response to wind variability strengthens our understanding of global ocean heat transport. Plain Language Summary: We have combined observations and an ocean model to produce an estimate of the tropical Pacific Ocean properties. Of particular interest are changes to the upper‐ocean temperature and salinity over the period 2014–2018. The largest changes occur in the central and eastern equatorial Pacific, and to diagnose the causes we define an analysis box representative of that region. Transports through five faces of this box (the sixth face is a land boundary) are diagnosed to understand the processes responsible for variability in box‐average properties. We examine the cooling and freshening that occurs during the El Niño event of 2015/2016. The region is typically characterized by a near surface poleward flow away from the equator and a compensating flow toward the equator from below. This circulation is wind driven and slows down due to changing winds associated with the El Niño. When the winds return to their average state, currents are re‐invigorated, leading to the dissipation of El Niño conditions. This paper focuses on determining the size and timing of these processes. Key Points: A data‐assimilating model of tropical Pacific Ocean state is validated against independent mooring observationsTemperature, salinity, and volume budgets are quantified during the onset and recovery of the 2015/2016 El NiñoSurface fluxes and horizontal advection are the main drivers of regional property changes [ABSTRACT FROM AUTHOR]

Ocean acidification has potentially large impacts on calcifying organisms and ecosystems. Argo floats equipped with biogeochemical (BGC) sensors have been continuously measuring Southern Ocean pH since 2014. These BGC‐Argo floats were deployed as part of the Southern Ocean Carbon and Climate Observations and Modeling project. Here we present a SOCCOM‐era Objectively Mapped pH (SOM‐pH) 2014–2019 climatology and explain the method for constructing this product. We show example SOM‐pH fields demonstrating the spatial and temporal structure of Southern Ocean pH. Comparison with previous ship‐based measurements reveals decreases in pH of up to 0.02 per decade, with a structure decaying with depth. An assessment of the trend structure reveals a pattern indicative of the meridional overturning circulation. Upwelling waters that have not been in recent contact with the atmosphere show negligible or small trends, while surface and downwelling waters that have had more exposure to the atmosphere show the strongest trends. Thus comparison of this new BGC‐Argo mapped pH estimate to historic observations allows quantifying the structure of Southern Ocean acidification. Plain Language Summary: Robotic instruments that measure ocean acidity levels (pH) have been deployed in the Southern Ocean since 2014. Here we compile these observations into a map that is an estimate of the current Southern Ocean pH. This paper discusses how the map is made and gives a brief description of its features. We then compare it to pH measurements made from ships prior to 2014. Analyzing the structure of these differences in space and time reveals how the Southern Ocean is becoming more acidic. We find that there is significant structure to the changes. In general the changes are largest at the surface and become smaller at greater depth. However, there are horizontal differences. Ocean locations where winds pull up deep waters have smaller trends than regions where the winds are pushing waters down. Thus the ocean changes show a signature associated with the large‐scale circulation. Key Points: We present a novel 12‐month Southern Ocean pH mapped product, made possible by the Biogeochemical‐Argo array initiated in 2014Comparing to ship‐based measurements above 1,500 m reveals a decrease in pH of up to 0.02 per decadepH changes are widespread with varying magnitudes reflecting the pattern of the meridional overturning circulation [ABSTRACT FROM AUTHOR]

Measurements of pH and nitrate from the Southern Ocean Carbon and Climate Observations and Modeling array of profiling floats were used to assess the ratios of dissolved inorganic carbon (DIC) and nitrate (NO3) uptake during the spring to summer bloom period throughout the Southern Ocean. Two hundred and forty‐three bloom periods were observed by 115 floats from 30°S to 70°S. Similar calculations were made using the Takahashi surface DIC and nitrate climatology. To separate the effects of atmospheric CO2 exchange and mixing from phytoplankton uptake, the ratios of changes in DIC to nitrate of surface waters (ΔDIC/ΔNO3) were computed in the Biogeochemical Southern Ocean State Estimate (B‐SOSE) model. Phytoplankton uptake of DIC and nitrate are fixed in B‐SOSE at the Redfield Ratio (RR; 6.6 mol C/mol N). Deviations in the B‐SOSE ΔDIC/ΔNO3 must be due to non‐biological effects of CO2 gas exchange and mixing. ΔDIC/ΔNO3 values observed by floats and in the Takahashi climatology were corrected for the non‐biological effects using B‐SOSE. The corrected, in situ biological uptake ratio (C:N) occurs at values similar to the RR, with two major exceptions. North of 40°S biological DIC uptake is observed with little or no change in nitrate giving high C:N. In the latitude band at 55°S, the Takahashi data give a low C:N value, while floats are high. This may be due to a change in CO2 air‐sea exchange in this region from uptake during the Takahashi reference year of 2005 to outgassing of CO2 during the years sampled by floats. Plain Language Summary: Phytoplankton take up dissolved inorganic carbon (DIC) and nitrate as they grow. This results in a decrease in DIC and nitrate during the spring through summer bloom periods each year. The ratio of DIC to nitrate uptake is typically near 6.6 mol C/mol N, a value termed the Redfield Ratio (RR). Here, we used sensor data from an array of profiling floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling program in the Southern Ocean to examine the ratio of C:N uptake by phytoplankton during 243 bloom periods from October through February. We find uptake occurred at values near the RR throughout the Southern Ocean, with two exceptions. North of 40°S, C:N ratios exceed the Redfield value, most likely due to phytoplankton production of a gel‐like organic matter deficient in nitrogen. Near 55°S in the Antarctic Southern Zone, an apparent increase in the C:N ratio over the past decade may reflect a change from an air‐sea flux of CO2 into the ocean to a flux out of the ocean. Key Points: Carbon:nitrate uptake ratios by phytoplankton are near the Redfield Ratio in Southern Ocean waters south of 40°SCarbon:nitrate uptake ratios north of 40°S are much higher, likely due to production of dissolved organic matter with little nitrogenA change in air‐sea CO2 flux during October to February from a sink in 2005 to a source in recent years may have occurred near 55°S [ABSTRACT FROM AUTHOR]

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. [ABSTRACT FROM AUTHOR]

Ocean circulation connects geographically distinct ecosystems across a wide range of spatial and temporal scales via exchanges of physical and biogeochemical properties. Remote oceanographic processes can be especially important for ecosystems in the Southern Ocean, where the Antarctic Circumpolar Current transports properties across ocean basins through both advection and mixing. Recent tracking studies have indicated the existence of two large‐scale, open ocean habitats in the Southern Ocean used by grey petrels (Procellaria cinerea) from two populations (i.e., Kerguelen and Antipodes islands) during their nonbreeding season for extended periods during austral summer (i.e., October to February). In this work, we use a novel combination of large‐scale oceanographic observations, surface drifter data, satellite‐derived primary productivity, numerical adjoint sensitivity experiments, and output from a biogeochemical state estimate to examine local and remote influences on these grey petrel habitats. Our aim is to understand the oceanographic features that control these isolated foraging areas and to evaluate their ecological value as oligotrophic open ocean habitats. We estimate the minimum local primary productivity required to support these populations to be much <1% of the estimated local primary productivity. The region in the southeast Indian Ocean used by the birds from Kerguelen is connected by circulation to the productive Kerguelen shelf. In contrast, the region in the south‐central Pacific Ocean used by seabirds from the Antipodes is relatively isolated suggesting it is more influenced by local factors or the cumulative effects of many seasonal cycles. This work exemplifies the potential use of predator distributions and oceanographic data to highlight areas of the open ocean that may be more dynamic and productive than previously thought. Our results highlight the need to consider advective connections between ecosystems in the Southern Ocean and to re‐evaluate the ecological relevance of oligotrophic Southern Ocean regions from a conservation perspective. [ABSTRACT FROM AUTHOR]

The upper ocean dissolved inorganic carbon (DIC) concentration is regulated by advective and diffusive transport divergence, biological processes, freshwater, and air-sea CO2 fluxes. The relative importance of these mechanisms in the Southern Ocean is uncertain, as year-round observations in this area have been limited. We use a novel physical-biogeochemical state estimate of the Southern Ocean to construct a closed DIC budget of the top 650 m and investigate the spatial and temporal variability of the different components of the carbon system. The dominant mechanisms of variability in upper ocean DIC depend on location and time and space scales considered. Advective transport is the most influential mechanism and governs the local DIC budget across the 10 day-5 year timescales analyzed. Diffusive effects are nearly negligible. The large-scale transport structure is primarily set by upwelling and downwelling, though both the lateral ageostrophic and geostrophic transports are significant. In the Antarctic Circumpolar Current, the carbon budget components are also influenced by the presence of topography and biological hot spots. In the subtropics, evaporation and air-sea CO2 flux primarily balances the sink due to biological production and advective transport. Finally, in the subpolar region sea ice processes, which change the seawater volume and thus the DIC concentration, compensate the large impact of the advective transport and modulate the timing of biological activity and air-sea CO2 flux. [ABSTRACT FROM AUTHOR]