Interactions of tidally-induced flows with complex bathymetry often result in vertical mixing and locally enhanced turbulence, in addition to the formation of internal waves which can propagate away and break elsewhere resulting in further vertical mixing. Vertical mixing is important both for ocean dynamics and also as it can increase biodiversity and productivity by facilitating nutrient exchange between the deep and surface ocean.
More generally, turbulent mixing in the ocean is an important, challenging and widely investigated matter. Understanding and modelling of the processes by which energy is converted from larger scales into small scale mixing, and how this impacts upon ocean circulation, present major challenges within the field.
In order to improve the understanding of the influence of bathymetry in shallow coastal oceans on mixing, two dimensional vertical slice numerical simulations are presented conducted with both Fluidity and MITgcm. These simulations analyse the effect of the interaction of tidal currents with complex bathymetry such as sills and flooded river valleys. Simulating vertical mixing processes explicitly generally requires very high numerical resolution to properly capture the dynamics and to diagnose correct mixing rates. A potential approach to mitigate this is considered through the application of mesh adaptivity for Loch Etive, which is shown to reproduce dynamics well while using between one and two orders of magnitude fewer degrees of freedom than a fixed uniform mesh.
Idealised and more realistic simulations of the modern day real world fjord systems of Loch Etive and Idefjorden are first used to further validate model results and to better understand the processes which occur in these systems, including the influence of the second sill and the inter-sill spacing, before idealised simulations of the geological ancient are conducted. Targeted simulations of the geological ancient explore major stages in a transgression and regression during an icehouse and greenhouse world due to changing coastline geomorphology and the resulting
nutrient heterogeneity.