Can Lagrangian tracking simulate tracer spreading in a high-resolution Ocean General Circulation Model?

Wagner, Patrick, Rühs, Siren, Schwarzkopf, Franziska U., Koszalka, Inga Monika and Biastoch, Arne (2018) Can Lagrangian tracking simulate tracer spreading in a high-resolution Ocean General Circulation Model? [Poster] In: AGU Fall Meeting 2018, 10.-14.12.2018, Washington, D.C., USA.

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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 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 floats is used. This study compares the spreading of an Eulerian tracer simulated online and a cloud of Lagrangian particles simulated offline with velocities from the same 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 at a depth of 160 m and integrated for 3 years.

Large-scale diagnostics, like mean pathways of floats and tracer, are almost identical and 1D-horizontal distributions show no signigicant differences. Differences in vertical distributions, seen in a reduced vertical spreading and downward displacement of particles, are due to the combined effect of unresolved sub-daily 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 4048 m^2/s in the Cape Basin.

Document Type: Conference or Workshop Item (Poster)
Research affiliation: OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-TM Theory and Modeling
OceanRep > GEOMAR > FB1 Ocean Circulation and Climate Dynamics > FB1-PO Physical Oceanography
Date Deposited: 08 Jan 2019 10:21
Last Modified: 08 Jan 2019 12:31
URI: http://eprints.uni-kiel.de/id/eprint/45140

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