The connected ocean: comparing inter-ocean transport at the surface and at depth

Abstract

The oceans form a global network of inter-connected basins, with currents transporting a range of tracers such as heat and pollution between them. Although the ocean dynamics play a key role in determining transport between basins, the basins are de ned by arbitrary geographical borders rather than physical connections. This thesis proposes a method of detecting dynamical boundaries and applies it to the ocean surface and interior. To achieve this, probabilistic models are used to summarise the transport described by vast datasets of water-following trajectories. Such models, known as Markov Chain models, have previously been used to map the distribution of plastic pollution yet are known to induce an artificial dispersion. First, a sensitivity analysis of the effect of model parameters on artificial dispersion is performed, to determine the optimal model set-up. Next, a global dataset of observed trajectories is used to detect dynamic transport barriers in two key areas of inter-ocean surface exchange: the Agulhas Current system and the North Atlantic inter-gyre transport barrier. Connectivity maps are introduced as maps of tracer destinations which highlight dynamical segregation between regions. For example, these are used to identify the source region for Agulhas Leakage and one-way equator-ward transport across the Gulf Stream. Using a new method to extract geostrophic motion from a trajectory dataset, the geostrophic contribution to inter-basin transport can be identified. Finally, connectivity maps are produced for the ocean interior using virtual particles released along isopycnals in the eddy-permitting ORCA025 ocean circulation model. The change in connectivity, in the two study regions, between two target isopycnals is the first step in creating a 3D border of the ocean basins. New basin definitions are then explained by transport processes relevant to the region. Future work should establish the link between dynamical boundaries and the distribution of heat and pollution.