Master thesis defense by Floriane Océane Sudre

Title: Sensitivity to Friction Changes: the Case of an Idealized Gullmarn Fjord

Abstract

The ocean is a vital component of the Earth’s climate. As such, the ocean can also act as a powerful climate metric for global climate changes. Nonetheless, how accurately ocean properties and ocean circulation can be assessed by existing modelling is still source of debates and subject of numerous research projects. The present Thesis aims to contribute to such debates by better understanding the role of friction. It does so through a modelled study of water circulation in a selected Swedish fjord.

Although they are often neglected or roughly parametrized in large scale ocean models, bottom, vertical, and lateral friction might have a non negligible impact on ocean circulation. For the scope of the present Thesis, the influence of changes in bottom friction, vertical eddy viscosity, and lateral eddy viscosity parametrization are considered independently of each other. To clearly identify the role of those processes, this work relies upon the General Estuarine Transport Model (GETM) as applied to an idealized fjord, which has been modelled on the approximate dimensions of the Swedish Gullmarn fjord.

On the basis of these experiments, the Thesis suggests that the idealized fjord circulation is most sensitive to horizontal viscosity variations especially at the eastern boundary. In turn, variations in bottom and vertical frictions are found to be less significant. Additionally, the horizontal eddy viscosity pattern is found to resemble salinity distribution more than the velocity distribution, suggesting that the forced tidal flow has less impact on the idealized fjord circulation than the density fluctuation.

This work is part of a preliminary study for an observational expedition taking place in the Swedish Gullmarn fjord. The main objective of this broader project consists in testing the validity of assumptions on friction in ocean models. Within such context, the present work brings evidences that more research needs to be done to enquire the role of friction on the estuarine circulation, especially the horizontal eddy viscosity. It further provides the basis for the design of a more realistic model of the Gullmarn fjord.