Abstract (eng)
This work describes progress towards a better description of the ongoing fundamental changes in the Arctic by (i) creating observation-based, up-to-date estimates of the Arctic water budget, (ii) validating historical energy and water budget simulations of state-of-the-art climate models, and (iii) developing new tools for the precise calculation of oceanic transports on various modeling grids. The first part of this doctoral thesis deals with the Arctic water budget as represented by observations. Firstly, river discharge into the Arctic Ocean is analyzed and by combining available gauge observations with reanalyses estimates, a new and up-to-date pan-Arctic observation based river discharge estimate is created. Through combination with atmospheric inputs from reanalyses, storage changes from the Gravity Recovery and Climate Experiment (GRACE) and oceanic volume transports from ocean reanalyses, the closure of the non-steric water volume budget is assessed. Finally, using a variational adjustment scheme best estimates for every budget equation term are provided, offering an up-to-date picture of the Arctic water budget and a solid foundation for the second objective of this thesis, the validation of climate models. The second part of this thesis analyzes relevant components of the Arctic energy and water budgets for climate models from the Coupled Model Intercomparison Project Phase 6 (CMIP6). Simulated long-term averages, trends and seasonal cycles of vertical fluxes at the surface as well as the top-of-atmosphere, lateral transports and storage rates in atmosphere and ocean are validated against our observation-based basis. Large inter-model spreads and systematic biases are found in the representation of long-term averages and annual cycles for several water and energy budget components. Surface freshwater fluxes into the Arctic Ocean tend to be overestimated by most CMIP6 models and about two thirds feature an early timing bias in the seasonal runoff cycle, attributed to an early snow melt bias and the lack of realistic river routing schemes. The net vertical energy flux out of the Arctic Ocean and poleward oceanic heat transports are systematically underestimated by all models. The latter show strong anti-correlation to the mean sea ice cover, atmospheric heat transports, and the long-term ocean warming rate. This strongly suggests that accurate depiction of the mean state is critical for realistic projections of future Arctic warming. This new and comprehensive overview of the models' performance, biases and their ability to reproduce the observed Arctic energy and water budgets should help to evaluate the reliability of the models' future projections. Another aspect of this doctoral thesis is dedicated to the technical development of new tools for the precise calculation of oceanic transports. Oceanic transports significantly influence the global climate, however their evaluation and validation based on reanalysis and model data is complicated by the distortion of the employed model grids and the diversity of different grid types used. Using two different methods, StraitFlux allows for precise calculations of oceanic fluxes through almost arbitrary sections, enabling straightforward comparisons with observed oceanic volume and energy transports at available sections such as the RAPID array or Arctic gateways. StraitFlux has been in use for transport studies at the Greenland-Scotland Ridge and the Indonesian Troughflow region and was essential in the calculation of Arctic lateral fluxes in this doctoral thesis.