Arctic Ocean mixing processes and vertical fluxes of energy and matter

AROMA is motivated by the rapid changes that are now occurring in the Arctic climate system, manifested by thinning sea ice, warming ocean and atmosphere temperatures, with consequences for the uptake of atmospheric carbon, net primary productivity, strong climate feedbacks, and dramatic implications for society. The overall objective of AROMA is to advance our understanding of the role of vertical mixing for the heat and carbon budgets of the Arctic Ocean, the role of ocean heat in changing sea ice thickness and area, and the associated feedbacks. To address this objective, AROMA participated in the international MOSAiC program to collect measurements in the water column under sea ice, as the research icebreaker Polarstern drifted across the Arctic Ocean for one year, between September 2019 and October 2020. Measurements were made by scientists participating in the drift, as well as from autonomous instruments and platforms deployed in the ice camp. The observations are analyzed together with related studies conducted under the Nansen LEGACY project, in the Barents Sea and north of Svalbard, expanding the scope and the regional coverage. In the first phase of AROMA, researchers participated in the MOSAiC drift. Additionally, we participated in the Nansen LEGACY physical process cruises to the Barents Sea, in October 2020 and in winter 2021, to study the dynamics of the polar front between the warm Atlantic-origin waters and cold polar waters. In collaboration with scientists from the University of Alaska Fairbanks, we analysed data collected by ocean moorings in the Eurasian Basin of the Arctic Ocean from 2003 to 2018. The new results show that heat associated with Atlantic water entering the Arctic Ocean is spreading and increasing, and that a new feedback mechanism is contributing to accelerating sea ice loss. In the second phase of AROMA, we analyzed the data collected during MOSAiC and the Nansen LEGACY. From MOSAiC, we documented the evolution of ocean turbulence under ice for a full year. We used a novel kind of ascending microstructure profiler to measure the structure of turbulence in the upper ocean up to the ice surface, providing a detailed description of ocean-ice interactions. In addition to profiling measurements, we used a tracer-based method in winter to compare and obtain reliable estimates of the strength of upper ocean mixing, an important parameter for the calculation of vertical transport. Combining year-round microstructure observations from MOSAiC with the numerous cruises of the Nansen LEGACY project, we investigated how far existing methods to quantify turbulence from standard observations are applicable to the unique Arctic Ocean conditions. Our observations from north of Svalbard emphasize the role of tides as an important source of mixing in the Arctic Ocean. Understanding the pathway for the energy from tides to turbulence, the magnitude and distribution of the ocean mixing rates, and the role of feedbacks between mixing rates, stratification, sea ice, and the tide are key to predicting the fate of the Atlantic water in the Arctic and the evolution of the Arctic Ocean in a warming world.

We have generated high-quality datasets and conducted thorough analyses to address the goals and objectives of the project. All of our datasets are openly accessible and will serve as a valuable resource for future research. Additionally, the project has facilitated the career development of two early career scientists. Through a series of publications, we provide new insights into oceanographic processes and dynamical mechanisms in a region where warm Atlantic water undergoes substantial transformations. We have developed instrumentation, documented its capabilities and limitations, and tested methods and parameterizations related to ocean mixing in the Arctic Ocean, laying a strong foundation for further research. Activities with similar focuses will provide opportunities for further discoveries, with the potential to address remaining uncertainties in the functioning of the Arctic Ocean and its future evolution. Our observations are being used for process evaluation and to constrain various ocean models. They are also being utilized for training artificial intelligence-based estimates of ocean mixing.

AROMA is a research proposal to support a comprehensive program to observe, understand, and quantify the fundamental processes of vertical mixing and vertical fluxes that shape the oceanographic and biogeochemical structure of the Arctic Ocean. AROMA is a collaboration with and participation in the international program MOSAiC, the Year of Polar Prediction, and the national multidisciplinary program The Nansen LEGACY. AROMA is motivated by the rapid changes that are now occurring in the Arctic climate system, manifested by thinning sea ice, warming ocean and atmosphere temperatures, with consequences for the uptake of atmospheric carbon, net primary productivity, strong climate feedbacks, and dramatic implications for society. The experimental approach is designed to advance understanding of, and provide observational constraints on the coupled physical, biological, and chemical processes in the Arctic Ocean. AROMA assembles the best expertise in experimental ocean mixing and carbon in Norway, in a feasible, well-focused interdisciplinary project, and facilitates participation in major field campaigns, mutually adding value to them while benefiting from the logistics, high-quality platforms and broader multidisciplinary data sets. We hypothesise that 1) vertical mixing rates are enhanced in the absence of sea ice, and bring toward the upper ocean and sea ice a. more oceanic heat, leading to a positive feedback and causing more melting b. more CO2, reducing the air-sea pCO2 gradient and the CO2 uptake c. more nutrients to support increased net primary production 2) the dominant mixing mechanisms are a. breaking of near-inertial waves and upwelling due to passage of storms in the central Arctic b. tidally-induced mixing and coastal upwelling over the continental slope and shelves 3) an efficient vertical pathway of CO2 to the interior ocean is brine release from first year ice formation.