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.

Professor Alison Gray, University of Washington, USA

Professor Alison Gray, University of Washington, USA

Alison Gray is an oceanographer working to advance scientific understanding of the large-scale physics and chemistry of the ocean and the impacts on Earth’s climate and ecosystems. Her research primarily focuses on observing the global ocean using autonomous instruments that measure physical and biogeochemical ocean properties and analysing those observations using state-of-the-art methods from statistics and data science. An Assistant Professor in the School of Oceanography at the University of Washington, she enjoys teaching and mentoring undergraduate and graduate students in oceanography and data analysis. Previously, Dr Gray was at Princeton University, where she studied the circulation and carbon cycle of the Southern Ocean as a National Oceanographic and Atmospheric Administration Climate and Global Change Postdoctoral Fellow. She completed her graduate work at the University of Washington, where she received an MS and a PhD in physical oceanography, as well as an MS in applied mathematics.

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. 

Professor Alessandro Tagliabue, University of Liverpool, UK

Professor Alessandro Tagliabue, University of Liverpool, UK

Professor Tagliabue is a Professor at the University of Liverpool and an ocean biogeochemist, interested in how the cycling of resources in the sea affects biological activity and vice-versa. He is particularly interested in trace micronutrients and how they interact together to shape primary production, ecosystem structure and their response to global change.  His science links numerical models, at both global and idealised scales, with both fieldwork and synthesis of datasets.

Professor Tagliabue is heavily involved in the international GEOTRACES programme, was a lead author on the IPCC Special Report on Oceans and Cryosphere in a Changing Climate and is a member of the governing council of the UK Challenger Society for Marine Science. He is also UK Chair for SCOR and sit on the Royal Society Global Environment Research Committee.

 

 

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. 

Dr Sandy Thomalla, Southern Ocean Carbon and Climate Observatory (SOCCO), CSIR, South Africa

Dr Sandy Thomalla, Southern Ocean Carbon and Climate Observatory (SOCCO), CSIR, South Africa

Sandy Thomalla is a Principal Scientist and Research Group Leader of the Southern Ocean Carbon - Climate Observatory (SOCCO) at the CSIR, in Cape Town, South Africa. She obtained her PhD from the University of Cape Town in association with the National Oceanography Centre, Southampton, UK. Her research focuses primarily on understanding the biological carbon pump through a range of observational approaches that include measurements of primary production, photophysiology, carbon export, satellite ocean colour and in situ bio-optics. A key objective is to examine the modes of variability in ecosystem physical drivers towards a better process understanding of biogeochemical responses, particularly the Biological Carbon Pump and its sensitivity to climate.  

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.

Dr Judith Hauck, Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Germany

Dr Judith Hauck, Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research, Germany

Judith studied marine environmental sciences in Oldenburg in Germany, before moving to the Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research for her PhD. For her thesis entitled 'Processes in the Southern Ocean carbon cycle: Dissolution of carbonate sediments and inter-annual variability of carbon fluxes' she was awarded the Annette-Barthelt Award for the best PhD thesis in the field of marine sciences in Germany. She is now head of the Helmholtz Young Investigator Group for Marine Carbon and Ecosystem Feedbacks in the Earth System (MarESys), as well as the Deputy Head of Marine Biogeosciences at the Alfred Wegener Institute. She is a member of several committees looking to assess and report on the global carbon budget and ocean carbon research.  

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.

Dr Pedro Monteiro, CSIR-CHPC, South Africa

Dr Pedro Monteiro, CSIR-CHPC, South Africa

Dr Pedro M S Monteiro is a Chief Oceanographer at CSIR in South Africa in the Southern Ocean Carbon-Climate Observatory (SOCCO).  He has a special interest in understanding the processes and dynamics that link ocean physics to CO₂ variability on synoptic to interannual scales and using this to contribute towards reducing the uncertainties and biases in oceanCO₂ reconstructions. The use of high-resolution numerical modelling and ocean robotics as experimental platforms, to examine the scale sensitivities of Southern Ocean coupled physics – CO₂ to climate variability and change, is a key focus. He was a Coordinating Lead Author for the Carbon chapter 5 in AR6-WG1 and a member of the Joint Steering Committee of the WCRP.

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. 

Professor Corinne Le Quéré CBE FRS, University of East Anglia, UK

Professor Corinne Le Quéré CBE FRS, University of East Anglia, UK

Professor Corinne Le Quéré FRS CBE, is Royal Society Research Professor at the University of East Anglia, where she conducts research on the interactions between the carbon cycle and climate change, particularly those mediated by the world’s oceans. She leads two projects in the NERC’s programme on the Role of the Southern Ocean in the Earth System (RoSES), one examining Southern Ocean CO2 variability in recent decades, one exploring its evolution this century. More broadly, she spearheaded the development of marine carbon-cycle models with new ways to represent plankton diversity and ecology. She also led for over a decade the annual update of the ‘global carbon budget’, an international effort to keep track of global carbon emissions and their fate in the environment, and authored three assessment reports of the Intergovernmental Panel on Climate Change (IPCC).

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.

Professor Ric Williams, University of Liverpool, UK

Professor Ric Williams, University of Liverpool, UK

Ric Williams is a Professor in Ocean Sciences at University of Liverpool. His climate research focuses on how surface warming increases linearly with carbon emissions and how there may be delayed warming even after carbon emissions cease. His ocean research focuses on how climate projections for heat and carbon storage differ from each other, and the coupling between surface temperature, overturning and cloud feedbacks. Ric Williams obtained a Physics degree at Bristol University, a master’s degree in Atmospheric Physics at Imperial College and a PhD in Physical Oceanography at University of East Anglia. He has worked as researcher at Imperial College and won a 2-year NOAA fellowship at Massachusetts Institute of Technology, USA, and then moved to University of Liverpool. He has over 95 peer-reviewed publications and a textbook on Ocean Dynamics and the Carbon Cycle. He is currently the co-Director of the University of Liverpool research theme: Climate Futures.

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.

Professor Joellen Russell, University of Arizona, USA

Professor Joellen Russell, University of Arizona, USA

Dr Joellen Russell is an oceanographer, climate scientist and University Distinguished Professor at the University of Arizona. Professor Russell uses robot floats, supercomputers and satellites to observe and predict the ocean’s role in climate and the carbon cycle. Professor Russell is the lead for the modeling theme of the Southern Ocean Carbon and Climate Observations and Modeling project (SOCCOM) including its Southern Ocean Model Intercomparison Project (SOMIP). She currently serves as Co-Chair of the NOAA Science Advisory Board’s Climate Working Group and on the National Center for Atmospheric Research’s Community Earth System Model Advisory Board. Professor Russell is one of the 14 scientists behind an amicus curiae brief supporting the plaintiff in the historic 2007 U.S. Supreme Court decision on carbon dioxide emissions and climate change, Commonwealth of Massachusetts, et al v U.S. Environmental Protection Agency. Before joining UA, Dr Russell was a Research Scientist at Princeton University and the National Ocean and Atmospheric Administration’s Geophysical Fluid Dynamics Laboratory (NOAA/GFDL). She received her AB in Environmental Geoscience from Harvard and her PhD in Oceanography from Scripps Institution of Oceanography, University of California, San Diego.