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The role of Southern Ocean Carbon cycle under CLImate change

Periodic Report Summary 2 - SOCCLI (The role of Southern Ocean Carbon cycle under CLImate change)

The project “The role of the Southern Ocean carbon cycle under climate change (SOCCLI)” aimed at realizing a high-level staff exchange and networking activity in the field of climate research, in particular ocean carbon cycle research, between key research groups of South Africa (CSIR, NTC-UCT) and Europe (Norway: UiB, NERSC; France: CNRS-IPSL). The topic to be jointly investigated was the Southern Ocean’s role in global carbon cycling as the Southern Ocean has been shown to play a key role in controlling the atmospheric carbon dioxide concentration both in the past and the high CO2 state of the Earth system.

The main objective of SOCCLI was to better determine the Southern Ocean’s role in global carbon cycling through establishing and reinforcing long-term research cooperation on marine biogeochemical research between South Africa and European partners. Specific objectives were to: (i) integrate the South African observational data of Southern Ocean surface ocean pCO2 and air-sea CO2 fluxes into worldwide data sets, (ii) understand and quantify the integrated role of short-term variability (sub-seasonal, seasonal, interannual) and mesoscale variability (oceanic eddies) for modulating long-term trends in Southern Ocean CO2 fluxes and associated feedbacks to climate change, (iii) study and assess the long-term sensitivities of the magnitude and phasing of the seasonal cycle of CO2 fluxes in the Southern Ocean to climate forcing, and (iv) disseminate the results in bi-lateral outreach events to policy makers and to large international research programmes (EU projects, South African projects, IGBP core projects, SOLAS and IMBER).

Work performed and main results
Within the second reporting period of SOCCLI (months 25-48), 12 secondments were performed, thereof 5 being outgoing and 7 being incoming ones. The secondments followed up on and extended the work started in reporting period 1 (ref. periodic report submitted on 24 September 2014). Main results achieved within the past 24 months include:
1. Southern Ocean phytoplankton dynamics have been investigated with a novel model approach under an ensemble of more than 1200 idealised physical scenarios used as input to a complex biogeochemical model (Llort et al., 2015). Crucial states of phytoplankton blooms in the Southern Ocean were defined (onset, climax, and apex dates). All onsets consistently happened in the (north-) winter. As most important parameter for the bloom onset, the upper layer mixing was identified. For the bloom climax, variable iron supply was an important modulation factor.
2. For a proper quantification of the variable Southern Ocean carbon sink (for anthropogenic CO2), adequate sampling strategies have been assessed. As it turns out, resolving the seasonal cycle with respective measurements is insufficient to reduce the error in the regional CO2 air-sea flux estimate (Monteiro et al., 2015). Short-term variability, in particular storm events (with high gas-transfer rates) need to be taken into account in order to provide reliable interannual CO2 flux quantifications. This has to be taken into account for future observing system design.
3. A suite of CMIP5 (Coupled Model Intercomparison Project phase 5) results from fully-fledged Earth system models were analysed for the strength of the Southern Ocean sink for anthropogenic carbon (Kessler and Tjiputra, 2016). For the strong RCP8.5 business as usual scenario, the Southern Ocean turned out to be the only region in the World Ocean which continues to be a growing sink of human-made CO2 from the atmosphere. Inconsistencies on the simulation of the seasonal cycle in the models make an assessment of the process attribution for the Southern Ocean difficult. Improvements in Sothern Ocean carbon observing systems are needed to clarify this important issue (see also Monteiro et al., 2015).

Expected final results and potential impact
Key results of the SOCCLI project are: (i) The joint South African-European work provided new process-based insights into the functioning of the coupled physical-biogeochemical system of the Southern Ocean within the Southern Ocean. (ii) The combination of field data and a hierarchy of models helped to establish relationships between physical mixing, mixed layer depth, and mesoscale variability for controlling biological production and coupled oxygen-carbon fluxes in the Southern Ocean. (iii) The Southern Ocean is considered as the emerging dominating sink for anthropogenic carbon within the world ocean. An improved understanding of the processes underlying this sink, as provided by SOCCLI, is mandatory for adequate future projections using Earth system models. The correct quantification of the Southern Ocean carbon sink is important especially for estimating greenhouse gas emission reductions appropriately in order to meet given climate mitigation targets. (iv) Research supported by SOCCLI contributed significantly to the development of operational oceanography in South Africa, benefitting marine environmental monitoring, safety and resources (including fisheries), extreme weather prediction as well as climate and seasonal forecasting activities. The research generated new knowledge of relevance for policy making and management. During the course of the project, outreach events with policy makers both in Europe and South Africa provided the respective communities with essential material for informed decisions on greenhouse gas emission reductions and mitigation of climate change.

Through SOCCLI, already existing cooperation between the partner institutions was fostered and new links were established by involving also early stage researchers. The staff exchange provided a very efficient opportunity to promote joint publications and to optimally exploit the complementary expertise. The project was embedded in and linked with important European collaborative projects such as EU FP7 project CARBOCHANGE as well as other large international research projects.