Tridib Banerjee

• Several oceanic processes depend delicately on mixing of fluid parcels, particularly across density surfaces because of its extremely small magnitude. Even a fractional deviation in its representation can therefore cause large errors in various other ocean modelling aspects like circulation or tracer distribution. Moreover, since this mixing is also vital in maintaining the global energy balance, its accurate representation is highly desirable. This thesis thus deals with the issue of spurious mixing (artificial mixing of numerical or non-physical origin) across density surfaces in general circulation ocean models. It explores ways to properly identify it and also to potentially mitigate it. The thesis predominantly evolves around Finite volumE Sea Ice-Ocean Model (FESOM2). It develops a split-explicit external model solver together with an asynchronous time-stepping procedure that supports Arbitrary Largangian Eulerian (ALE) coordinates. It also implements a few such ALE coordinates known to reduce spurious mixing across density surfaces. The thesis then further develops a diagnostic technique that provides semi-local in space and time estimates for such spurious mixing on any grid without operator splitting. The work shows the novel solver to be less dissipative and scale better at any given workload without the need for additional temporal-filtering subcycles. It also shows the novel diagnostic technique to provide a local decomposition of various spurious mixing components. It reports levels of spurious mixing across density surfaces for different cases and how it can be much larger than the physical mixing. Finally, it provides discussion on the future possibilities and objectives.