Suki Cheuk-Kiu Wong

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The ocean carbon sink has absorbed approximately 29 % of anthropogenic carbon dioxide (CO₂) emissions in the last decade, mitigating climate change. Over time, the ocean carbon sink has grown nearly proportionally to atmospheric CO₂ concentrations. Natural variability also modulates the ocean carbon sink, but the decadal-timescale mechanisms of this variability are not well-understood. Using LDEO-Hybrid Physics Data, a unique observation-based product that merges observations with hindcast models, we assess the decadal variability of global air-sea CO₂ fluxes for 1959–2024, with a focus on the dominant regional contributions. The dominant mode of decadal air-sea CO₂ flux variability exhibits strong synchronous signals between the tropical Pacific and the Southern Ocean. This synchronicity is modulated by Pacific Decadal Variability (PDV) and the interannually varying El Niño-Southern Oscillation (ENSO) as measured by the Multivariate ENSO Index. When PDV and MEI indices are positive, the Southern Ocean experiences stronger westerly winds and deeper mixed layers. However, fully explaining the weakening of the Southern Ocean carbon sink in recent decades, which had predominantly negative PDV and MEI, will require additional constraints, particularly for biological processes.

The El Niño‐Southern Oscillation (ENSO) in the equatorial Pacific is the dominant mode of global air‐sea carbon dioxide (CO₂) flux interannual variability (IAV). Air‐sea CO₂ fluxes are driven by the difference between atmospheric and surface ocean pCO₂, with variability of the latter driving flux variability. Previous studies found that models in Coupled Model Intercomparison Project Phase 5 (CMIP5) failed to reproduce the observed ENSO‐related pattern of CO₂ fluxes and had weak pCO₂ IAV, which were explained by both weak upwelling IAV and weak mean vertical dissolved inorganic carbon (DIC) gradients. We assess whether the latest generation of CMIP6 models can reproduce equatorial Pacific pCO₂ IAV by validating models against observations‐based data products. We decompose pCO₂ IAV into thermally and non‐thermally driven anomalies to examine the balance between these competing anomalies, which explain the total pCO₂ IAV. The majority of CMIP6 models underestimate pCO₂ IAV, while they overestimate sea surface temperature IAV. Insufficient compensation of non‐thermal pCO₂ to thermal pCO₂ IAV in models results in weak total pCO₂ IAV. We compare the relative strengths of the vertical transport of temperature and DIC and evaluate their contributions to thermal and non‐thermal pCO₂ anomalies. Model‐to‐observations‐based product comparisons reveal that modeled mean vertical DIC gradients are biased weak relative to their mean vertical temperature gradients, but upwelling acting on these gradients is insufficient to explain the relative magnitudes of thermal and non‐thermal pCO₂ anomalies.

The El Niño-Southern Oscillation (ENSO) in the equatorial Pacific is the dominant mode of global air-sea CO ₂ flux interannual variability (IAV). Air-sea CO ₂ fluxes are driven by the difference between atmospheric and surface ocean pCO ₂ , with variability of the latter driving flux variability. Previous studies found that models in Coupled Model Intercomparison Project Phase 5 (CMIP5) failed to reproduce the observed ENSO-related pattern of CO ₂ fluxes and had weak pCO ₂ IAV, which were explained by both weak upwelling IAV and weak mean vertical DIC gradients. We assess whether the latest generation of CMIP6 models can reproduce equatorial Pacific pCO ₂ IAV by validating models against observations-based data products. We decompose pCO ₂ IAV into thermally and non-thermally driven anomalies to examine the balance between these competing anomalies, which explain the total pCO ₂ IAV. The majority of CMIP6 models underestimate pCO ₂ IAV, while they overestimate SST IAV. Thermal and non-thermal pCO ₂ anomalies are not appropriately balanced in models, such that the resulting pCO ₂ IAV is too weak. We compare the relative strengths of the vertical transport of temperature and DIC and evaluate their contributions to thermal and non-thermal pCO ₂ anomalies. Model-to-observations-based product comparisons reveal that modeled mean vertical DIC gradients are biased weak relative to their mean vertical temperature gradients, but upwelling acting on these gradients is insufficient to explain the relative magnitudes of thermal and non-thermal pCO ₂ anomalies.