In situ measurements of atmospheric O2 and CO2 reveal an unexpected O2 signal over the tropical Atlantic Ocean

We present the first meridional transects of atmospheric O2 and CO2 over the Atlantic Ocean. We combine these measurements into the tracer atmospheric potential oxygen (APO), which is a measure of the oceanic contribution to atmospheric O2 variations. Our new in situ measurement system, deployed on board a commercial container ship during 2015, performs as well as or better than existing similar measurement systems. The data show small short-term variability (hours to days), a step-change corresponding to the position of the Intertropical Convergence Zone (ITCZ), and seasonal cycles that vary with latitude. In contrast to data from the Pacific Ocean and to previous modeling studies, our Atlantic Ocean APO data show no significant bulge in the tropics. This difference cannot be accounted for by interannual variability in the position of the ITCZ or the Atlantic Meridional Mode Index and appears to be a persistent feature of the Atlantic Ocean system. Modeled APO using the TM3 atmospheric transport model does exhibit a significant bulge over the Atlantic and overestimates the interhemispheric gradient in APO over the Atlantic Ocean. These results indicate that either there are inaccuracies in the oceanic flux data products in the equatorial Atlantic Ocean region, or that there are atmospheric transport inaccuracies in the model, or a combination of both. Our shipboard O2 and CO2 measurements are ongoing and will reveal the long-term nature of equatorial APO outgassing over the Atlantic as more data become available

Immediate and Long-Lasting Impacts of the Mt. Pinatubo Eruption on Ocean Oxygen and Carbon Inventories

Large volcanic eruptions drive significant climate perturbations through major anomalies in radiative fluxes and the resulting widespread cooling of the surface and upper ocean. Recent studies suggest that these eruptions also drive important variability in air-sea carbon and oxygen fluxes. By simulating the Earth system using two initial-condition large ensembles, with and without the aerosol forcing associated with the Mt. Pinatubo eruption in June 1991, we isolate the impact of this volcanic event on physical and biogeochemical properties of the ocean. The Mt. Pinatubo eruption forced significant anomalies in surface fluxes and the ocean interior inventories of heat, oxygen, and carbon. Pinatubo-driven changes persist for multiple years in the upper ocean and permanently modify the ocean's heat, oxygen, and carbon inventories. Positive anomalies in oxygen concentrations emerge immediately post-eruption and penetrate into the deep ocean. In contrast, carbon anomalies intensify in the upper ocean over several years post-eruption, and are largely confined to the upper 150 m. In the tropics and northern high latitudes, the change in oxygen is dominated by surface cooling and subsequent ventilation to mid-depths, while the carbon anomaly is associated with solubility changes and eruption-generated El Niño—Southern Oscillation variability. We do not find significant impact of Pinatubo on oxygen or carbon fluxes in the Southern Ocean; but this may be due to Southern Hemisphere aerosol forcing being underestimated in Community Earth System Model 1 simulations

Quantification of ocean heat uptake from changes in atmospheric O2 and CO2 composition

The ocean is the main source of thermal inertia in the climate system. Ocean heat uptake during recent decades has been quantified using ocean temperature measurements. However, these estimates all use the same imperfect ocean dataset and share additional uncertainty due to sparse coverage, especially before 2007. Here, we provide an independent estimate by using measurements of atmospheric oxygen (O2) and carbon dioxide (CO2) – levels of which increase as the ocean warms and releases gases – as a whole ocean thermometer. We show that the ocean gained 1.29 ± 0.79 × 1022 Joules of heat per year between 1991 and 2016, equivalent to a planetary energy imbalance of 0.80 ± 0.49 W watts per square metre of Earth’s surface. We also find that the ocean-warming effect that led to the outgassing of O2 and CO2 can be isolated from the direct effects of anthropogenic emissions and CO2 sinks. Our result – which relies on high-precision O2 atmospheric measurements dating back to 1991 – leverages an integrative Earth system approach and provides much needed independent confirmation of heat uptake estimated from ocean data

Simulations of ocean deoxygenation in the historical era: insights from forced and coupled models

Ocean deoxygenation due to anthropogenic warming represents a major threat to marine ecosystems and fisheries. Challenges remain in simulating the modern observed changes in the dissolved oxygen (O2). Here, we present an analysis of upper ocean (0-700m) deoxygenation in recent decades from a suite of the Coupled Model Intercomparison Project phase 6 (CMIP6) ocean biogeochemical simulations. The physics and biogeochemical simulations include both ocean-only (the Ocean Model Intercomparison Project Phase 1 and 2, OMIP1 and OMIP2) and coupled Earth system (CMIP6 Historical) configurations. We examine simulated changes in the O2 inventory and ocean heat content (OHC) over the past 5 decades across models. The models simulate spatially divergent evolution of O2 trends over the past 5 decades. The trend (multi-model mean and spread) for upper ocean global O2 inventory for each of the MIP simulations over the past 5 decades is 0.03 ± 0.39×1014 [mol/decade] for OMIP1, −0.37 ± 0.15×1014 [mol/decade] for OMIP2, and −1.06 ± 0.68×1014 [mol/decade] for CMIP6 Historical, respectively. The trend in the upper ocean global O2 inventory for the latest observations based on the World Ocean Database 2018 is −0.98×1014 [mol/decade], in line with the CMIP6 Historical multi-model mean, though this recent observations-based trend estimate is weaker than previously reported trends. A comparison across ocean-only simulations from OMIP1 and OMIP2 suggests that differences in atmospheric forcing such as surface wind explain the simulated divergence across configurations in O2 inventory changes. Additionally, a comparison of coupled model simulations from the CMIP6 Historical configuration indicates that differences in background mean states due to differences in spin-up duration and equilibrium states result in substantial differences in the climate change response of O2. Finally, we discuss gaps and uncertainties in both ocean biogeochemical simulations and observations and explore possible future coordinated ocean biogeochemistry simulations to fill in gaps and unravel the mechanisms controlling the O2 changes

Toward an integrated pantropical ocean observing system

Global climate is regulated by the ocean, which stores, releases, and transports large amounts of mass, heat, carbon, and oxygen. Understanding, monitoring, and predicting the exchanges of these quantities across the ocean’s surface, their interactions with the atmosphere, and their horizontal and vertical pathways through the global oceans, are key for advancing fundamental knowledge and improving forecasts and longer-term projections of climate, weather, and ocean ecosystems. The existing global observing system provides immense value for science and society in this regard by supplying the data essential for these advancements. The tropical ocean observing system in particular has been developed over decades, motivated in large part by the far-reaching and complex global impacts of tropical climate variability and change. However, changes in observing needs and priorities, new challenges associated with climate change, and advances in observing technologies demand periodic evaluations to ensure that stakeholders’ needs are met. Previous reviews and assessments of the tropical observing system have focused separately on individual basins and their associated observing needs. Here we provide a broader perspective covering the tropical observing system as a whole. Common gaps, needs, and recommendations are identified, and interbasin differences driven by socioeconomic disparities are discussed, building on the concept of an integrated pantropical observing system. Finally, recommendations for improved observations of tropical basin interactions, through oceanic and atmospheric pathways, are presented, emphasizing the benefits that can be achieved through closer interbasin coordination and international partnerships

Climate Modulations of Air-Sea Oxygen, Carbon, and Heat Exchange

The exchanges of oxygen (O2), carbon dioxide (CO2), and heat across the air-sea interface have broad and profound implications for climate and marine ecosystems. In this thesis, I use observations and models to improve our process understanding of how natural climate variability modulates these exchanges. In chapter 2, I investigate the impacts of El Niño Southern Oscillation (ENSO) on air-sea O2 exchange. I use atmospheric inversions of global, continuous timeseries of atmospheric O2 and CO2 and ocean models to evaluate links between ENSO and air-sea O2 exchange and explore driving mechanisms using ocean and atmospheric models. I find that El Niño events lead to anomalous outgassing of oceanic O2, a response that is driven primarily by changes in the source and intensity of upwelling in the equatorial Pacific. In Chapter 3, I examine the impacts of tropical volcanic eruptions on air-sea exchanges of O2, CO2 and heat using coupled model simulations and observations. Here, I find that volcanic events lead to substantial oceanic heat loss that is accompanied by large oceanic uptakes of oxygen and carbon. An El Niño-like pattern emerges following tropical eruptions and plays a major role in modulating the oceanic response to volcanic forcing. In Chapter 4, I explore the use of global continuous atmospheric measurements of O2 and CO2 to evaluate claims that enhanced ocean heat uptake caused the recent global surface warming hiatus, based on a potential negative relationship between air-sea heat and gas exchange. Here, I find that the relationship between air-sea oxygen, carbon and heat fluxes due to natural variability is complex; air-sea heat and O2 exchange are positively coupled in the tropical Pacific, but are negatively coupled at higher latitudes. This spatially distinct relationship complicates the attribution of observed decadal trends in atmospheric O2 and CO2 to changes in ocean heat uptake, but may present an opporunitity to develop regional constraints. Collectively, the results of this thesis contribute to a quantitative and mechanistic framework enabling interpretation of O2 and CO2 trends in the context of ongoing ocean warming and deoxygenation

Mesoscale Modulation of Dissolved Oxygen in the Tropical Pacific

&amp;lt;p&amp;gt;The distribution of dissolved oxygen in the tropical Pacific acts as a major control on marine ecosystems habitats and the foraging range of tuna fiheries in this region. A basic understanding of processes driving the mean structure and variability of the oxygen minimum zones (OMZs) in this region, however, remains challenged by sparse observations and coarse model resolution. In this study, we examine the influence of mesoscale processes on equatorial Pacific oxygen distribution and variability, with a particular focus on tropical instability vortices (TIVs). We employ an eddy-resolving configuration of the Community Earth System Model (CESM) and Lagrangian analysis to evaluate the impacts and governing mechanisms by which TIVs influence oxygen distribution and budgets in this region. The westward seasonal propagation of TIVs from summer through winter is found to drive a deepening of the oxygen minima along the equatorial Pacific band (10&amp;lt;sup&amp;gt;o&amp;lt;/sup&amp;gt;N-10&amp;lt;sup&amp;gt;o&amp;lt;/sup&amp;gt;S), and thus a seasonal expansion of the equatorial oxygenated tongue separating the north and south tropical Pacific OMZs. Strong hemispheric asymmetry is evident in TIV impacts on oxygen due to relatively weaker TIV activity and less pronounced oxygen gradients south of the equator. Mechanisms governing TIV oxygenation of the upper equatorial Pacific include a complex interplay of physical and biogeochemical processes. Isopycnal displacements act in concert with vortex trapping and lateral stirring to mix oxygenated waters from the upper layers into the equatorial boundaries of the north and south tropical Pacific OMZs. TIV-induced advection and upwelling, on the other hand, intensifies nutrient supply and productivity, organic carbon export, and oxygen respiration demand at depth, thus acting (though only slightly) to counteract the physical effects. The influence of these processes varies with TIV phase, from vortex generation in the eastern Pacific through vortex dissipation in the west. TIVs are found to have a profound influence on upper equatorial Pacifc oxygen distribution and budget, with major implications for understanding the coupling between oxygen and&amp;amp;#160;ocean circulation, predicting marine ecosystem dynamics, and designing observation networks in this region.&amp;lt;/p&amp;gt; </jats:p

Eddy-Mediated Mixing of Oxygen in the Equatorial Pacific

Code for Eddy-Mediated Mixing of Oxygen in the Equatorial Pacifi

Climate Modulations of Air-Sea Oxygen, Carbon, and Heat Exchange

The exchanges of oxygen (O2), carbon dioxide (CO2), and heat across the air-sea interface have broad and profound implications for climate and marine ecosystems. In this thesis, I use observations and models to improve our process understanding of how natural climate variability modulates these exchanges. In chapter 2, I investigate the impacts of El Niño Southern Oscillation (ENSO) on air-sea O2 exchange. I use atmospheric inversions of global, continuous timeseries of atmospheric O2 and CO2 and ocean models to evaluate links between ENSO and air-sea O2 exchange and explore driving mechanisms using ocean and atmospheric models. I find that El Niño events lead to anomalous outgassing of oceanic O2, a response that is driven primarily by changes in the source and intensity of upwelling in the equatorial Pacific. In Chapter 3, I examine the impacts of tropical volcanic eruptions on air-sea exchanges of O2, CO2 and heat using coupled model simulations and observations. Here, I find that volcanic events lead to substantial oceanic heat loss that is accompanied by large oceanic uptakes of oxygen and carbon. An El Niño-like pattern emerges following tropical eruptions and plays a major role in modulating the oceanic response to volcanic forcing. In Chapter 4, I explore the use of global continuous atmospheric measurements of O2 and CO2 to evaluate claims that enhanced ocean heat uptake caused the recent global surface warming hiatus, based on a potential negative relationship between air-sea heat and gas exchange. Here, I find that the relationship between air-sea oxygen, carbon and heat fluxes due to natural variability is complex; air-sea heat and O2 exchange are positively coupled in the tropical Pacific, but are negatively coupled at higher latitudes. This spatially distinct relationship complicates the attribution of observed decadal trends in atmospheric O2 and CO2 to changes in ocean heat uptake, but may present an opporunitity to develop regional constraints. Collectively, the results of this thesis contribute to a quantitative and mechanistic framework enabling interpretation of O2 and CO2 trends in the context of ongoing ocean warming and deoxygenation

Oxygen

peer reviewe