Simulated Lagrangian pathways between the Leeuwin Current System and the upper-ocean circulation of the southeast Indian Ocean

The Leeuwin Current System, along the west Australian coast (22°S–34°S), forms a unique but poorly understood eastern boundary regime in which tropical waters flow poleward. Here we depict the three-dimensional paths connecting this eastern boundary system with the upper-ocean large-scale circulation around Australia based on selected trajectories from an online numerical particle tracking performed during the 1993/1997 integration of the 0.28° Los Alamos National Laboratory Parallel Ocean Program model. The simulated trajectories reveal a wealth of details about the regional circulation that are difficult to understand from observed and model Eulerian data alone. They reveal links between the Leeuwin Current, Leeuwin Undercurrent, Eastern Gyral Current, and zonal flows within the Subtropical Gyre. New findings include: a remote tropical source of the Leeuwin Current in the equatorial Indian Ocean, via the South Java Current; inshore (along the southern part of the North West Shelf) and offshore routes in the Indo-Australian Basin feeding the Leeuwin Current; strong exchange between the Leeuwin Undercurrent and adjacent Subtropical Gyre through a series of near surface eastward jets and deeper westward jets; and the tropical origin of the Eastern Gyral Current as a recirculation of the South Equatorial Current. We propose a current schematic summarising the links between the meridional boundary flows off Western Australia and the larger-scale circulation.