Search for: All records

Abstract The Southern Ocean is an important region for both heat and carbon uptake, due in large part to wind-driven circulation. This region also continually experiences strong winds associated with the passage of synoptic storms, which influence the upper ocean through strong fluxes of momentum, heat, freshwater, and gases. While studies have found that storms can induce strong carbon outgassing, their role in the combined heat and carbon uptake remains unknown. In this work, we explore the climatological impact of storms on the Southern Ocean combined heat and carbon uptake through two preindustrial coupled climate model runs with contrasting seasonal carbon fluxes. We use a feature tracking system to identify storms and create composites for storm-following and post-storm anomalous fluxes of heat and carbon. Storms induce a net anomalous release of heat and carbon from the ocean throughout the year, with clear seasonality in the magnitude of the fluxes that coincide with the background seasonal cycles. We find a strong model dependency for the storm-driven anomalous carbon fluxes, both in terms of the seasonal range and timing of maximum outgassing. Storm-induced anomalous fluxes are dampened on the order of days after the storm passes, with a small continued release of heat that is most persistent in the winter. Our study underlines the high uncertainty about the seasonal nature of storm impacts on the ocean and suggests that evolving atmospheric and oceanic conditions could impose opposing shifts in the future seasonality of storm impacts. 

The notion of climate sensitivity has become synonymous with equilibrium climate sensitivity (ECS), or the equilibrium response of the Earth system to a doubling of CO₂. But there is a hierarchy of measures of climate sensitivity, which can be arranged in order of increasing complexity and societal relevance and which mirror the historical development of climate modeling. Elements of this hierarchy include the well-known ECS and transient climate response and the lesser-known transient climate response to cumulative emissions and zero emissions commitment. This article describes this hierarchy of climate sensitivities and associated modeling approaches. Key concepts reviewed along the way include climate forcing and feedback, ocean heat uptake, and the airborne fraction of cumulative emissions. We employ simplified theoretical models throughout to encapsulate well-understood aspects of these quantities and to highlight gaps in our understanding and areas for future progress.▪There is a hierarchy of measures of climate sensitivity, which exhibit a range of complexity and societal relevance.▪Equilibrium climate sensitivity is only one of these measures, and our understanding of it may have reached a plateau.▪The more complex measures introduce new quantities, such as ocean heat uptake coefficient and airborne fraction, which deserve increased attention. 

Abstract Density-driven steric seawater changes are a leading-order contributor to global mean sea level rise. However, intermodel differences in the magnitude and spatial patterns of steric sea level rise exist at regional scales and often emerge during the spinup and preindustrial control integrations of climate models. Steric sea level results from an eddy-permitting climate model, GFDL CM4, are compared with a lower-resolution counterpart, GFDL-ESM4. The results from both models are examined through basin-scale heat budgets and watermass analysis, and we compare the patterns of ocean heat uptake, redistribution, and sea level differ in ocean-only [i.e., Ocean Model Intercomparison Project (OMIP)] and coupled climate configurations. After correcting for model drift, both GFDL CM4 and GFDL-ESM4 simulate nearly equivalent ocean heat content change and global sea level rise during the historical period. However, the GFDL CM4 model exhibits as much as a 40% increase in surface ocean heat uptake in the Southern Ocean and subsequent increases in horizontal export to other ocean basins after bias correction. The results suggest regional differences in the processes governing Southern Ocean heat export, such as the formation of Antarctic Intermediate Water (AAIW), Subpolar Mode Water (SPMW), and gyre transport between the two models, and that sea level changes in these models cannot be fully bias-corrected. Since the process-level differences between the two models are evident in the preindustrial control simulations of both models, these results suggest that the control simulations are important for identifying and correcting sea level–related model biases. 

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 O₂inventory and ocean heat content (OHC) over the past 5 decades across models. The models simulate spatially divergent evolution of O₂trends over the past 5 decades. The trend (multi-model mean and spread) for upper ocean global O₂inventory 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 O₂inventory 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 O₂inventory 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 O₂changes.