A Carbon and transient tracer measurement programme in the Atlantic and Southern Ocean under Oceans 2025

Lead Research Organisation: University of East Anglia
Department Name: Environmental Sciences

Abstract

The oceans act as a heat reservoir, exchanger and transporter, and on time scales longer than a few days it is necessary to include their influence in order to predict weather or climate. Over times of years to centuries, they are also a sink for much of the carbon we are currently releasing into the atmosphere: the heat and carbon budgets are in fact intimately related. The oceans have absorbed almost half the accumulated fossil fuel emissions since the industrial revolution, without which the atmospheric content would be about 60ppm higher than it is today. Ocean uptake of carbon has therefore slowed the pace of human-induced climate change substantially. Theme 1 of the Oceans 2025 proposal includes a programme aimed at studying the role of the large scale ocean circulation in climate, with a focus on the overturning circulation of the Atlantic and the Southern Oceans. A major aim, both in as a scientific product in itself and as a stepping-stone to better understanding of the underlying processes, is to estimate new property budgets and transports of heat, mass, freshwater, and carbon. The new observations which form the basis for this are ocean sections in the Atlantic and Southern Oceans, to be undertaken in the years 2008-2010 by NOC and collaborators. This programme was designed to include carbon and chemical tracer measurements, since these are essential to deliver the carbon inventories. NOC does not have the expertise specialised measurements however, and this proposal is to fund UEA to make them. We already have evidence that the carbon budget of both the Atlantic and Southern Oceans is changing quite rapidly. We do not know to what extent this is a response to human-induced climate change and how much is variation that would occur naturally. The research proposed will enable essential observations to help us document this change, understand its causes, and predict the future of the oceanic sink for the CO2 that we are emitting into the oceans.

Publications

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Description Transient Tracers and Inorganic Carbon in the Oceans 2025 Programme The programme was developed at the University of East Anglia (UEA) in collaboration with the National Oceanography Centre (NOC) as part of the Oceans 2025 programme. It had these objectives: (a) Estimates of the inventory and transport of natural and anthropogenic carbon in the Atlantic and the Atlantic sector of the Southern Ocean, (b) an assessment of how these may be changing on decadal time scales, (c) an overall budgeting of carbon by region that links the changes in inventories to the fluxes by water transport, biological production and air-sea fluxes,(d) improved estimates of overturning transport of other properties (mass, freshwater, heat) as a result of the constraints that the chlorofluorocarbon (CFC) observations place on these quantities. The project included carbon and tracer measurements on high-resolution, full-depth hydrographic sections in the Atlantic Ocean on 4 cruises between 2009 and 2010, namely JC31 (Drake Passage sections A21 and SR1B), JC32 (24°S), DI332 (Arctic Gateway), and DI346 (24.5°N). UEA acquired high quality data of dissolved inorganic carbon, alkalinity and a series of transient tracers (namely CFCs CFC-11, CFC-12, CFC-113, CCl4 and sulphur hexafluoride, SF6). All the data have been submitted to the British Oceanographic Data Centre (BODC). The carbon data have also been submitted to the Global Data Analysis Project (version 2), an international synthesis project that seeks to create an internally consistent global ocean carbon data set for the 2000s. Transient tracer and carbon measurements from the 2010 occupation of 24.5°N enabled the calculation of 3 independent estimates of anthropogenic and natural carbon. We also calculated these for historical sections in 1992, 1998 and 2004; • The trends in anthropogenic carbon (Cant) in water masses at 24.5°N were analysed in collaboration with the marine institute in Vigo, Spain. The study shows an increasing Cant content in the upper water masses (UNACW, LNACW, AAIW). Generally, back-calculation methods (_CT0, TROCA) and a tracer-based method (TTD) agree well, but the strength of this agreement varies between water masses (E. F. Guallart et al., in preparation). • Combination of carbon data with geostrophy-derived velocity fields allowed an investigation into the temporal variability of the carbon transports and associated air-sea fluxes. Current net southward carbon transports at 24.5°N are weaker than in the pre-industrial era due to the surface-intensified northward anthropogenic carbon transport. Whilst water column carbon storage is increasing over time, the variability in contemporary carbon transport and air-sea fluxes is largely driven by the natural carbon cycle. Organic carbon transport appears crucial in closing the North Atlantic carbon budget (Brown, et al., in preparation). Anthropogenic carbon was investigated for 1990 and 2009 repeats of the Drake Passage A21 section (G. Evans, NOC PhD thesis, in preparation). Substantial concentrations of Cant are entering mode and intermediate waters to the northern end of the section, whilst mid-depth water shows limited changes. A slight signal in the deepest waters hugging the Antarctic continental slope may relate to recently ventilated Antarctic Bottom Water originating in the Weddell Sea. The data highlight variability of the natural carbon system. The tracer measurements inform on the Atlantic Meridional Overturning Circulation (AMOC): • The 24°S section sampled an SF6 plume released in the deep Brazil Basin 13 years earlier. The 1-dimensional diffusion equation was used to model the vertical spread of the tracer, yielding a mean diapycnal diffusivity estimate of about 3 x 10-4 m2/s at 4 km depth. This estimate is similar to that found by surveys of the tracer plume made between 1996 and 2000, and provides evidence for the long-term stability of that result (Rye et al., 2012). • The Atlantic 24.5°N section sampled several excess SF6 plumes originating from the 1996 release in the Greenland Sea during the Greenland Sea Tracer Experiment. This shows that tagged Greenland Sea Arctic intermediate water has contributed to NADW as far as 24.5°N within a transfer time of 13 years. Two signals were identified: one in the Florida Strait within the Deep Western Boundary Current and the main one on the eastern flank of the Mid-Atlantic Ridge around 1500 m depth suggesting a new path of the lower limb of the AMOC (Messias et al., in preparation). The findings illustrate the value of the high quality carbon and tracer data acquired by UEA during the project for study of the global carbon cycle, ocean ventilation and for validation and constraining of global circulation models and climatologies.
Exploitation Route NA The high quality carbon and tracer data acquired by UEA during the project are used for study of the global carbon cycle, ocean ventilation and for validation and constraining of global circulation models and climatologies.
Sectors Environment

 
Description Collaboration with NOC and University of Southampton 
Organisation National Oceanography Centre
Country United Kingdom 
Sector Academic/University 
PI Contribution Tracer release experiment observation and analysis teams
Collaborator Contribution Provision of the James Cook for UK cruises on DIMES. Provision of National Marine Facilities staff and equipment. Physical oceanography input.
Impact Most of the papers have joint authorship
Start Year 2008