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2002 LAPCOD Meeting
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Reconstruction of near-surface drifter trajectories in the Pacific Ocean with a hybrid model
Nathan Paldor, Yona Dvorkin, Arthur J. Mariano, Tamay Özgökmen, and Edward H. Ryan
The Hebrew University of Jerusalem
npaldor@rsmas.miami.edu(Abstract received 10/15/2002 for session D)
ABSTRACT
A hybrid Lagrangian-Eulerian model for calculating the trajectories of near-surface drifters in the ocean is developed in this study. The model employs climatological, near surface, currents computed from a spline-fit of all available near surface drifter velocities observed in the Pacific Ocean between 1988 and 1996. It also incorporates contemporaneous wind fields calculated by either the US Navy (Navy Operational Global Atmospheric Prediction System - NOGAPS) or the European Centre for Medium Range Weather Forecasts (ECMWF). The model was applied to 30 drifters launched in the Pacific Ocean in 3 clusters during 1990, 1993 and 1994 (no ECMF winds were available for this year). For 10-day-long trajectories the forecasts computed by the hybrid model are up to 164% closer to the observed trajectories compared to the trajectories obtained by advecting the drifters with the climatological currents only. The best fitting trajectories are computed with ECMWF fields that have a temporal resolution of 6 hours. The average improvement over all 30 drifters of the hybrid model trajectories relative to advection by the climatological currents is 21% but in the open ocean clusters (1990 and 1993) the improvement is 34% with NOGAPS winds and 42% with ECMWF winds. This difference between the open ocean and coastal clusters is due to the fact that the model does not presently include the effect of horizontal boundaries (coastlines). For unknown initial velocities the trajectories generated by the hybrid model are significantly more accurate than advection by the mean currents on time scales of 5-15 days. Given the real-time availability of NOGAPS wind stress and the existence of on-line atlases of near surface ocean currents, the model provides a practical tool for forecasting the trajectories of surface drifters. On time scales of 3-days significant improvement is achieved only if the drifters' initial velocities are known. The model's success in providing more accurate trajectories also indicates that drifters' motion can deviate significantly from the climatological current and that the instantaneous winds are more relevant to their trajectories than the mean currents. A possible interpretation of our results is that the winds' role in drifter dynamics is more significant than its role in the water velocity since drifters do not have to obey the continuity equation.
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2002 LAPCOD Meeting, Key Largo, Florida, December 12-16, 2000