Can Lagrangian Tracking Simulate Tracer Spreading in a High-Resolution Ocean General Circulation Model?

Abstract

To model tracer spreading in the ocean, Lagrangian simulations in an offline framework are a practical and efficient alternative to solving the advective–diffusive tracer equations online. Differences in both approaches raise the question of whether both methods are comparable. Lagrangian simulations usually use model output averaged in time, and trajectories are not subject to parameterized subgrid diffusion, which is included in the advection–diffusion equations of ocean models. Previous studies focused on diffusivity estimates in idealized models but could show that both methods yield similar results as long as the deformations-scale dynamics are resolved and a sufficient amount of Lagrangian particles is used. This study compares spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same ocean model. We use a global, eddy-resolving ocean model featuring 1/20° horizontal resolution in the Agulhas region around South Africa. Tracer and particles were released at one time step in the Cape Basin and below the mixed layer and integrated for 3 years. Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D horizontal distributions show no significant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved subdaily variability of the vertical velocities and the spatial variation of vertical diffusivity. This, in turn, has a small impact on the horizontal spreading behavior. The estimates of eddy diffusivity from particles and tracer yield comparable results of about 4000 m2 s−1 in the Cape Basin.

Vertical Structure and Seasonal Variability of the Inflow to the Lofoten Basin Inferred From High‐Resolution Lagrangian Simulations

“…The inclusion of vertical diffusion (as well as lateral diffusion) in Lagrangian models must address a proper choice of the stochastic model and its coefficients with respect to the unresolved nonlocal mixing, boundary effects, and spatially variable diffusivity; see, for example, Hunter et al (), Griffa (), and Berloff and McWilliams (). In a recent study, Wagner et al () evaluated the ability of off‐line Lagrangian simulations to reproduce spreading of a tracer patch simulated in‐line with the advection diffusion equation of the ocean model. They used a daily output of a high‐resolution model and a vertical diffusivity coefficient varying by 5 orders of magnitude with depth and seasonally.…”

“…We do not apply explicit lateral or vertical diffusion to avoid making trajectories too diffusive with respect to transport properties of the ROMS model solution used to force them. The question of adding additional diffusive term to compensate for the missed variability in off‐line Lagrangian simulation has been a subject of a discussion but is usually avoided in studies using high‐resolution model outputted at high frequency; see for example, Bower et al (), Gelderloos et al (), Rühs et al (), and in our previous study, Dugstad et al (), as well as van Sebille et al () and Wagner et al () and references herein. We will come back to this point in section .…”

They Came From the Pacific: How Changing Arctic Currents Could Contribute to an Ecological Regime Shift in the Atlantic Ocean

“…The fact that ARIANE (and other Lagrangian analysis packages) make use of preexisting model output makes it a powerful tool to answer questions where an online model run with passive tracers would be prohibitively computationally expensive (Van Sebille et al, ). However, small‐scale processes which are parameterized in the model, such as mixing and diffusion, cannot be directly assessed by Lagrangian analysis of advective pathways in a straightforward way (Wagner et al, ).…”

Nearshore Lagrangian Connectivity: Submesoscale Influence and Resolution Sensitivity

“…Similarly, we expect vertical advection to dominate over vertical diffusion in a submesoscale‐resolving simulation. Recently, Wagner et al () show that 3‐D Lagrangian trajectories (absent of diffusion) can well represent the horizontal spreading of a tracer advection‐diffusion model in a Δ x ≈5‐km simulation. Despite our choice to leave out diffusion, we note that application of Lagrangian trajectories that seeks more completeness (e.g., larval dispersal) should include diffusivity.…”