Heat and carbon uptake in the Southern Ocean: the state of the art and future priorities

Although the circulation of the Southern Ocean has traditionally been viewed through a zonally-averaged framework, research in recent years has emphasized the importance of standing meanders and associated zonal asymmetries in the Antarctic Circumpolar Current.  These meanders, which result from topography-mean flow interactions, create regions with vigorous variability at meso- to submeso-scales, indicated by elevated eddy kinetic energy.  These hotspots of eddy activity are known to influence the circulation and frontal structure of the Southern Ocean, yet their influence on the movement of carbon through this key region remains poorly understood.  In this talk, Dr Gray will present analysis of seven years of in situ observations collected by Biogeochemical-Argo floats deployed by the Southern Ocean Carbon and Climate Observations and Modeling project.  These data, which have unprecedented spatial and temporal coverage, suggest that regions of high eddy kinetic energy are associated with enhanced ventilation between the surface mixed layer and the ocean interior.  The impact of this increased exchange on the spatial variability of the Southern Ocean carbon cycle and air-sea exchange will be discussed.

Southern Ocean net primary production consumes upwelled nutrients and maintains interior pools of respired carbon that contribute to the regional carbon cycle. Climate projections confidently predict that Southern Ocean net primary production will increase in the future in response to changing availability of the micronutrient iron. However, manganese, which is bio-essential is also an important regulator of upper ocean productivity and the biological carbon pump, but is neglected by earth system models. This suggests that we have an incomplete picture of the drivers of Southern Ocean primary production, which questions the confidence across model projections and the response of the region to a changing climate. Ultimately, Southern Ocean net primary production is controlled by a unique set of controls that are dissimilar to those at lower latitudes and have shaped the evolution of Antarctic phytoplankton since the opening of Drake Passage. Addressing these knowledge gaps across the links between Southern Ocean physics and biogeochemistry is crucial for providing realistic assessments of future change. 

Arguably the most important scientific endeavour to date is the ability to understand and predict future ecosystems states, system-scale feedbacks and the emergence of abrupt change and tipping points to the carbon-climate system for the implementation of effective policy-driven mitigations. The Southern Ocean (SO) is disproportionately more important when it comes to buffering the impacts of climate change, taking up 50% of the oceanic uptake of CO₂ (Gruber et al, 2019; Gregor et al, 2019) and 75% of the excess heat generated by anthropogenic CO₂ (Frolicher et al, 2015). SO phytoplankton primary production (PP) plays an important role in carbon uptake and biogeochemical cycling by exporting organic material into the ocean's interior regulating nutrient supply to thermocline waters, which in turn drives low latitude PP and carbon export (Sarmiento et al, 2004). Quantifying the strength and efficiency of the biological carbon pump (BCP) and its sensitivity to predicted changes in the Earth’s climate is fundamental to our ability to predict long term changes in the global carbon cycle and by extension, the impact of continued anthropogenic perturbation of atmospheric CO₂ levels (Henson et al, 2011; Laufkotter et al, 2017). There is little agreement however on Earth System Models projections of the climate sensitivity of the SO BCP with a lack of consensus in even the direction of predicted change (eg Hauck et al, 2015), highlighting a gap in our understanding and ability to accurately represent a major planetary carbon flux. The Southern Ocean Carbon-Climate Observatory (SOCCO) based at the CSIR in Cape Town, South Africa aims to address some of these knowledge gaps in order to shed light on how future conditions may impact phytoplankton ecophysiology and the ocean’s biogeochemical cycling of key nutrients. This presentation will share their approach to addressing this challenge together with some key recent findings that address the sensitivity of the SO BCP to climate change. 

The Southern Ocean chapter of RECCAP2, which is the Global Carbon Project’s second systematic study on Regional Carbon Cycle Assessment and Processes, assesses the Southern Ocean carbon sink 1985-2018 from ocean biogeochemical models, surface pCO₂-based data products, and data-assimilated models. This analysis quantifies annual and seasonal air-sea CO₂ fluxes on the level of large-scale ocean biomes. In contrast to previous comparisons, the researchers find surprisingly good agreement between the ensemble mean of models and data-products and one data-assimilated regional model on a contemporary CO₂ uptake of about 0.75 PgC/yr between 1985 and 2018. They mainly attribute this change to the use of an updated data set for the river flux adjustments to account for the different components of the CO₂ flux measured by different methods, although differences in contributing models and data-products as well as in the defined boundary of the Southern Ocean may contribute as well. Admittedly, the models have a large spread with a mean uptake between 0.25 and 1.1 PgC/yr. The model spread is to first order explained by a diverse representation of the natural CO₂ fluxes. The modelled anthropogenic CO₂ uptake is more congruent, although comparison with observed interior ocean anthropogenic carbon accumulation points to an underestimation by most models. The researchers' assessment further confirms the discrepancies among data streams on the magnitude of winter outgassing of CO₂. However, across latitude, model spread in multi-year averaged summer CO₂ uptake is larger than in winter pointing to difficulties capturing the delicate balance between physical and biological processes.

Over the past decade, but especially in the past 5 years, the ocean surface CO₂ flux and pCO₂ observational community has made important advances in constraints for the interannual variability of the seasonal cycle of pCO₂ and FCO₂ in the Southern Ocean.  These seasonal cycle constraints for pCO2 have improved confidence in model derived variability and trends of FCO₂ as well as the mean annual contribution of the Southern Ocean to the global carbon budget.  New autonomous platforms (Gliders and Floats) have enabled further advances towards both addressing the spatial and temporal observational gaps in ocean pCO₂ as well as opened the door to investigate intra-seasonal variability and the contribution they make to a new understanding of the seasonal variability, its mechanisms, and the impact on the mean annual flux.  Here the researchers present some of the key results from the SOSCEx series of integrated glider experiments 2013-2019 conducted by the South African SOCCO programme in both the Sub-Antarctic and the Polar Upwelling Zones of the SE Atlantic Ocean.  They use a metric – the Seasonal Cycle Reproducibility (SCR) to show how the Southern Ocean comprises a mosaic of seasonal to synoptic modes of variability.  Their observational results point towards an unexpected importance of intraseasonal modes of variability of pCO₂ and they also propose the mechanistic basis for this variability and contrast how forced models of the Southern Ocean reflect these synoptic modes.  They also analyse the implications of seasonally biased ship observations as well as intraseasonal modes on the reconstruction of pCO₂ variability in the Southern Ocean.

The balance of known emissions and sinks of CO₂ reveal a gap in the assessment of global carbon flows. The “carbon budget imbalance” represents the frontier in carbon cycle research. This presentation will examine the potential role of the Southern Ocean to account for some of the carbon budget imbalance, building largely on the work of the SONATA (Southern OceaN optimal Approach To Assess the carbon state, variability and climatic drivers) project. It will examine the coherence of estimates based on atmospheric and oceanic observations and those based on process models and draw the emerging patterns of variability. Overall, the available evidence suggests that climate variability was inducing a decrease in the ocean CO₂ sink during the period 1990 to approximately 2010, a trend that is corroborated here with the identification of a climate fingerprint for DIC in the ocean interior and would slightly reduce the carbon budget imbalance in this period. However, in the past decade, a strong increase in the Southern Ocean CO₂ sink inferred by ocean data-products is difficult to reconcile with atmospheric observations, process models, and the carbon budget imbalance. The SONATA project shows that multiple constraints are a powerful tool to keep track of trends and variability in the Southern Ocean CO₂ sink. 

The role of the Southern Ocean is discussed in terms of its contribution to the global climate response to carbon emissions based upon Earth system model projections following a 1% rise in atmospheric CO₂. The Southern Ocean plays a key role in sequestering heat and carbon from the atmosphere; this response  is not due to enhanced local storage of heat and carbon, but rather due to the northward transport into the other ocean basins. The local thermal response over the Southern Ocean is strongly affected by physical climate feedbacks, particularly involving clouds and surface albedo, where a reduction in clouds and sea ice acts to enhance surface warming. This combination of heat and carbon uptake in the Southern Ocean, and the physical feedbacks acting over this region, all contribute to the global climate response.  Surface warming projections are understood in terms of two key global climate metrics: the slope of surface warming versus cumulative carbon emissions (called the Transient Climate Response to Emissions) and whether surface warming continues when emissions ceases (called the Zero Emissions Commitment). Both of these climate metrics are controlled by partially-competing thermal and carbon mechanisms, involving how heat and carbon are sequestered from the atmosphere, and the extent that radiative forcing leads to surface warming or an increase in ocean heat storage. The researchers' analysis of Earth system models reveals a wide inter-model spread in these climate metrics, which are particularly driven by differences in the evolution in physical climate feedbacks and the ocean uptake of heat, together by differences in the terrestrial carbon cycle and a lesser extent by the ocean uptake of carbon. The Southern Ocean is playing an important role in determining these global climate metrics through both the temporal change in regional physical feedbacks and its enhanced sequestration of heat and carbon.

Strong winds in Southern Ocean storms drive significant, two-way air-sea exchanges of carbon and heat. These spatially and temporally varying fluxes are vital to Earth’s climate system; this is especially true during the ongoing transient response to anthropogenic forcing where the ocean moderates the pace of carbon and heat increases in the atmosphere. Evaluating the ability of fully coupled climate models to accurately simulate the circulation and biogeochemistry relative to observations is crucial for closing the global carbon budget and for building confidence in climate model projections and advancing model fidelity. The lack of historical observations of key metrics associated with the circulation and biogeochemistry has led to large disagreement among Earth system model simulations of the Southern Ocean, but the recent revolutions in autonomous ocean float technology and sensor design allow us to estimate ocean carbon inventory changes in near real time. This shift from decadal surveys to a continual global ocean biogeochemistry data stream, in conjunction with satellite wind measurements and the latest Earth system models has moved ocean carbon into our operational oceanographic toolkit which is critical for enabling global carbon budget constraints and carbon accounting. Unfortunately, the current satellite constellation that remotely senses winds temporally under-samples the growing storms and the higher winds within them, leading to potentially large biases in Southern Ocean wind re-analyses and the carbon and heat fluxes derived from them. Evidence from a range of sources indicates that the wind speeds that drive Southern Ocean fluxes are increasing. Using the Biogeochemical Southern Ocean State Estimate to explore the effects of a 20% increase in wind speed on the air-sea carbon fluxes over the Southern Ocean, the researchers explore the new near-real-time observing system requirements for prediction of our changing ocean and climate system.