Abstract/Summary

Accurate forecasting of the ocean is crucial for numerous applications, including enhancing numerical weather prediction through coupled ocean-atmosphere systems. Upcoming satellite missions will provide measurements of ocean surface currents from space, offering the potential for significant improvements in forecasting accuracy. Data assimilation is the technique that integrates observations with numerical models to produce ocean forecasts. Existing ocean data assimilation systems rely on assumptions about velocity fields that are sufficient in the absence of ocean current observations. However, the future availability of satellite current measurements necessitates a reassessment of these assumptions and advances in the treatment of velocity fields within the existing systems. In this thesis, we propose a modification to NEMOVAR, the variational data assimilation system used by the NEMO (Nucleus for European Modelling of the Ocean) model, introducing alternative velocity variables to those currently used. The proposed variables are ageostrophic streamfunction and velocity potential. We design and test a novel transformation to these variables within both a simplified shallow water model and an idealised configuration of the NEMO model. This transformation is analysed both mathematically and numerically in the shallow water model. In implementing this transformation, we conduct an in-depth exploration of the boundary conditions associated with these new variables and investigate a numerical artifact in the solution. The transformation is further examined in the NEMO configuration, with a focus on integrating these variables into the NEMOVAR assimilation system. Assimilation experiments are performed to evaluate the impact of this new variable formulation on the assimilation of future ocean surface current observations. Results demonstrate that the proposed approach improves the accuracy of the analysis produced by the assimilation of such measurements. This thesis provides a comprehensive assessment of the theoretical and practical implications of adopting these alternative velocity variables and establishes a strong foundation for their future application within NEMOVAR.