Online Lagrangian Particle Advection in MOM6

Virtual Lagrangian particles are useful for studying the pathways of water parcels through the ocean at multiple spatial scales. Lagrangian particles can also be used to study the transport of heat, carbon, micro-plastics and many other tracers. In fixed vertical coordinate models, all three components of the velocity are used to advect virtual Lagrangian particles. In MOM6, the vertical velocity is not defined, so an alternate method for particle advection is needed. Here we present a new method for online 3-D Lagrangian particle advection in a Lagrangian vertical coordinate ocean model. We have implemented this method in MOM6 and tested it in multiple idealized model experiments.

Our new method for particle advection mirrors MOM6's general scheme for vertical transport. Between vertical regridding steps, particles follow a fixed vertical level, which itself moves in the vertical direction. During regridding steps, particles maintain their position in physical space, and the grid mesh is moved around them. In this work, we present results of online particle advection in MOM6 in an idealized channel model. We show that online particle advection is associated with less spurious diapycnal particle transport.