Simulation of Drifter Response to Ocean Convection

The objective of this project is to simulate Lagrangian drifter response to oceanic convection. A large-eddy simulation (LES) model is used to predict the fully-turbulent nonhydrostatic evolution of the oceanic flow fields that are typical of the Labrador Sea. These 4-dimensional fields have been used to force a variety of Lagrangian drifter models (LDM's). These simulations of LDM response have been used to test strategies and combinations of drifter types to best achieve the overall scientific objectives for the ONR Labrador Sea Convection Experiment.

Summary: LES has been used to show that isobaric (Rossby-type) drifters will sense mean fields for temperature and velocity that will be biased by the tendency for the fixed- depth drifters to seek out and maintain position in zones of horizontal convergence. This result is very important for (i) suggesting strategies for drifter deployment, and (ii) under standing the results from the ONR ARI on deep convection that is presently occurring (winter 1997). An important corollary result is that the isobaric float-observed fluxes may be corrected by a predictable structure function, calculated by large-eddy simulation.

Trajectories of Lagrangian Drifters in Convective Oceans

The following figure shows a 3-D plot of the locations of Drifter #1, and the temperature observed by the drifter at that location for 10000 time steps (one time step = 50 seconds, locations at every 20 time steps were plotted). Red indicates the drifter moves into a warmer area and blue indicated the drifter moves into a cold area. The drifter starts from the point with a "X" mark. Drifter #1 experiences cyclonic rotational downward movement during the 6-day period. The spiral movement starts near the surface as the drifter was caught in a diverging zone - a downward convective plume, as clearly shown by the temperature observation of the drifter.

Lagrangian Drifter

Trajectories of Isobaric Drifters in Convective Oceans

The following figure shows the trajectories of isobaric drifters at different depths. The isobaric drifters are caught in cyclonic rotations induced by the converging plumes. The size of the loops give the approximate scale of these sinking plumes, which is the range of 250-500 meters. The examination of the temperature measurements by the drifters confirm that the drifters are indeed arrested in the plumes for some time during the course of drifting.

Isobaric Drifters

Publications and Conference Presentations

Funding for this project has been provided by the Office of Naval Research (ONR)

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