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

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

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

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

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

Impacts of ENSO on air-sea oxygen exchange: observations and mechanisms

Models and observations of Atmospheric Potential Oxygen (APO ≃ O2 + 1.1*CO2) are used to investigate the influence of El Niño Southern Oscillation (ENSO) on air-sea O2 exchange. An atmospheric transport inversion of APO data from the Scripps flask network shows significant interannual variability in tropical APO fluxes that is positively correlated with the Niño3.4 index, indicating anomalous ocean outgassing of APO during El Niño. Hindcast simulations of the Community Earth System Model (CESM) and the Institut Pierre-Simon Laplace (IPSL) model show similar APO sensitivity to ENSO, differing from the Geophysical Fluid Dynamic Laboratory (GFDL) model, which shows an opposite APO response. In all models, O2 accounts for most APO flux variations. Detailed analysis in CESM shows the O2 response is driven primarily by ENSO-modulation of the source and rate of equatorial upwelling, which moderate the intensity of O2 uptake due to vertical transport of low-O2 waters. These upwelling changes dominate over counteracting effects of biological productivity and thermally-driven O2 exchange. During El Niño, shallower and weaker upwelling leads to anomalous O2 outgassing, whereas deeper and intensified upwelling during La Niña drives enhanced O2 uptake. This response is strongly localized along the central and eastern equatorial Pacific, leading to an equatorial zonal dipole in atmospheric anomalies of APO. This dipole is further intensified by ENSO-related changes in winds, reconciling apparently conflicting APO observations in the tropical Pacific. These findings suggest a substantial and complex response of the oceanic O2 cycle to climate variability that is significantly (>50%) underestimated in magnitude by ocean models

External Forcing Explains Recent Decadal Variability of the Ocean Carbon Sink

The ocean has absorbed the equivalent of 39% of industrial‐age fossil carbon emissions, significantly modulating the growth rate of atmospheric CO2 and its associated impacts on climate. Despite the importance of the ocean carbon sink to climate, our understanding of the causes of its interannual‐to‐decadal variability remains limited. This hinders our ability to attribute its past behavior and project its future. A key period of interest is the 1990s, when the ocean carbon sink did not grow as expected. Previous explanations of this behavior have focused on variability internal to the ocean or associated with coupled atmosphere/ocean modes. Here, we use an idealized upper ocean box model to illustrate that two external forcings are sufficient to explain the pattern and magnitude of sink variability since the mid‐1980s. First, the global‐scale reduction in the decadal‐average ocean carbon sink in the 1990s is attributable to the slowed growth rate of atmospheric pCO2. The acceleration of atmospheric pCO2 growth after 2001 drove recovery of the sink. Second, the global sea surface temperature response to the 1991 eruption of Mt Pinatubo explains the timing of the global sink within the 1990s. These results are consistent with previous experiments using ocean hindcast models with variable atmospheric pCO2 and with and without climate variability. The fact that variability in the growth rate of atmospheric pCO2 directly imprints on the ocean sink implies that there will be an immediate reduction in ocean carbon uptake as atmospheric pCO2 responds to cuts in anthropogenic emissions. Plain Language Summary Humans have added 440 Pg of fossil fuel carbon to the atmosphere since 1750, driving up the atmospheric CO2 concentration. But not all of this carbon remains in the atmosphere. The ocean has absorbed 39%, substantially mitigating anthropogenic climate change. Though this “ocean carbon sink” is a critical climate process, our understanding of its mechanisms remains limited. Of great interest is the unexplained slow‐down of the ocean carbon sink in the 1990s and a subsequent recovery. In this work, we use a simple globally‐averaged model to show that two processes external to the ocean are sufficient to explain the slowing of the ocean carbon sink in the 1990s. First, a reduced rate of accumulation of carbon in the atmosphere after 1989 reduced the atmosphere–ocean gradient that drives the ocean sink. Second, the eruption of Mt Pinatubo led to changes in ocean temperature that modified the timing of the sink from 1991 to 2001. We illustrate that the most important control on the decade‐averaged magnitude of the ocean sink is variability in the growth rate of atmospheric CO2. This implies that as future fossil fuel emission cuts drive reduced growth of atmospheric CO2, the ocean sink will immediately slow down