Overturning circulation and its implications for global carbon cycle in coupled models

Present ongoing climate change is expected to alter the ocean circulation. Project ORGANIC (Overturning Circulation and its Implications for Global Carbon Cycle in Coupled Models) goal is to advance our understanding of the structure and variability of the large scale overturning ocean circulation and its impact on the climate and ocean biogeochemistry across various time scales. Applying state-of-the-art Norwegian Earth System Model (NorESM), we uncover the temporal stability of the overturning strength and its spatial extent in response to climate change. Under previously assumed stable preindustrial climate, we find considerable variability in the Atlantic Meridional Overturning Circulation (AMOC) strength, operating at interannual (2-3 years) to centennial (60-200 years) time scales. The centennial variability coincides with the long-term climate time scale and is found to be associated with the variations in the strength of northward heat transport to the subpolar North Atlantic and the Nordic Seas. This signal also propagates into the ocean interior as time progresses. Beyond the physical footprint, changes in AMOC strength also affect the distribution of interior ocean biogeochemical tracers, which are important for the marine ecosystem. Our model simulations show that the ocean overturning is the main mechanism for transporting anthropogenic carbon and oxygen taken up from the atmosphere into the ocean interior. Consequently, changes in the AMOC strength will alter the rate of ocean acidification and deoxygenation. For example, future slowing down of the overturning, as expected from climate warming, will enhance deoxygenation while acidification continues. Further, our results reveals that profound impact on the ocean interior will persist over millennial timescales, even if we manage to mitigate the ongoing climate change. Monitoring these changes in the coming future should be an area of climate research priority. Development of long-term climate mitigation policies should carefully takes the broad circulation changes into consideration.

Project outcomes: - Improved dynamical understanding of the ocean physics-biogeochemistry interactions. - Advanced understanding of delayed climate change impact on ocean ecosystems. - New methodologies for accelerated spin-up and more efficient model development. - Trained three early-career scientists in new and growing interdisciplinary fields. - Improved leadership, management, and supervision skills of the project leader. - High number of publications in internationally leading journals. Project impacts: - Increased national and international profiles of project scientists through new publications and dissemination. - Invitations to international workshops to present project findings. - Established new interdisciplinary research activity at the Bjerknes Centre, linking biogeochemistry and paleoceanography. - New networks between the project team and international paleoclimate scientists. - Advanced framework for model development, reduces computational cost.

ORGANIC applies for funding to study the physical and biogeochemical interactions in the climate system using state-of-the-art model system. The project will enhance our knowledge in climate variability simulated by the NorESM model and identify uncertainty that comes with its future projections. The focus is to elucidate the linkage between large scale overturning circulation with the biogeochemical cycling in the ocean. This link is necessary since hydrography tracers such as temperature and salinity do not give us a comprehensive overview on the overturning circulation. On the other hand, biogeochemical tracers such as nutrient and CFCs are closely tied to the ocean circulation and can be used as indicators for patterns and ventilation rates of the ocean. Due to the non linear interactions between climate and ocean carbon cycle, it is vital for an Earth system model to accurately simulate the relevant former and latter processes individually as well as interactively in order for it to produce a sound future climate projections. The outcome of ORGANIC will be of highly relevant for both global and regional climate studies, particularly in regions where the ocean ventilation will be perturbed by anthropogenic forcing. The work is divided in three work packages: WP1: Sensitivity of thermohaline circulation patterns and rates WP2: Equilibrium states of ocean biogeochemistry WP3: Marine sediment dynamics The proposed interdisciplinary work will involve scientists from natural, mathematical and computational scientists. The study utilizes the nationally developed Norwegian Earth system model and observational sets from contemporary and paleo periods. The methods that will be developed throughout the project, the Matrix Free Newton Krylov, will provide novel and efficient approach to increase our understanding in the sophisticated interactions between the physical and biogeochemical processes in the climate system.