Interannual to Decadal Sea-Level Variability in the Indo-Pacific Region : Development and Application of a High-Resolution Ocean Model
Strong sea-level trends that exceeded the global mean signal several times over, have been observed in the western parts of the tropical Pacific between the early 1990s and early 2010s. These trends can be attributed to decadal and multidecadal variability and are mainly due to steric variations driven by wind-stress anomalies. This doctoral thesis aims to broaden our knowledge about these trends. Specifically, the role of ocean–atmosphere heat and freshwater fluxes and the linkage between sea-level variability in the western tropical Pacific and the densely populated region of the Australasian Mediterranean Sea (AMS) is investigated. These issues are addressed using a framework of ocean-sea-ice models that includes a newly developed high-resolution configuration named NUSA20 to resolve the fine-scale bathymetry of the AMS. To facilitate sensitivity experiments where the realistic atmospheric forcing is replaced by a repeated annual cycle, the recently introduced repeated-year-forcing approach (RYF) is evaluated. Three RYF-experiments using different annual cycles are conducted. Although the specific choice might be subject to the research question, it is shown that the experiments based on the periods from 1st May to 1st April of the years 1984/85 and 1990/91 show only minor differences and are able to characterize the model drift and transients of the hindcast simulation. Using a series of sensitivity experiments to disentangle the impacts of momentum fluxes on one side and heat and freshwater fluxes on the other side on sea-level variability, it is shown that anomalous buoyancy fluxes amplify interannual sea-level variability in the eastern tropical Pacific and that both heat and freshwater flux anomalies trigger westward propagating sea-level anomalies in the central Pacific that dampen variability on interannual and decadal timescales in the western part of the basin. In the AMS decadal fluctuations of the large-scale climate modes of the Pacific are found to account the majority of the low-frequency sea-level variability in almost all deep basins of the region. Intrinsic variability is negligible except in the South China Sea, where it accounts for up to 25 % of the total decadal sea-level variability. Overall, this thesis provides an assessment and a practical application of the RYF-approach. Building on sensitivity experiments, corresponding reference simulations and a high-resolution configuration the importance of ocean–atmosphere buoyancy fluxes for sea-level variability and the close dependence of decadal sea-level variability in the AMS on low-frequency fluctuations of Pacific climate modes is demonstrated.