Modelling the Movement of Marine Particles in Discrete Time and Space with Application to Scotian Shelf Connectivity

Abstract

The complexity of ocean circulation makes it difficult to predict both the origin and destination of objects transported by ocean currents. Many practical applications (e.g., planning marine search and rescue operations, predicting the year class success of important fisheries, and responding to threats posed by oil spills and mines) require a Lagrangian approach to the modelling of fluid movement. Similarly, many scientific applications (e.g., understanding the connections between the spawning and nursery areas of marine organisms, estimating the residence time of deep ocean basins and the exchange between them) are sometimes tackled more effectively using a Lagrangian approach. One way of estimating where objects come from, and where they go, is based on integrating a stochastic differential equation for particle position. In this thesis, the effectiveness of two such methods are explored: one based on discretization in time (the conventional approach) and another based on discretization in space. The ability of the methods to deal with both Lagrangian chaos (evident in time varying flows without diffusion) and spatial gradients in diffusivity (leading to false aggregation of particles), is discussed using simple, idealized examples. The discrete-space method is then applied and evaluated for the Scotian Shelf-Gulf of Maine system using flow fields predicted by an ocean circulation model with a horizontal grid spacing of about 2 km. Positively and neutrally buoyant particles are tracked in both two and three dimensions. Connectivity between deep basins, and the shelf break, is quantified for the study area and the ability of the discrete space tracking scheme to predict the observed movement of near surface drifters is assessed.