Abstract:

The talk presents a hybrid, Lagrangian-Eulerian, model for calculating the trajectories of near-surface drifters in the ocean. 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 1994). 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. 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 include the effect of horizontal 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. Knowledge of the drifterSs initial velocity significant improves the model performance and for 3-day long trajectories the model results are nearly twice closer to observations than persistence and 3 times closer than climatology. 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 and effective tool for forecasting the trajectories of surface drifters. The modelSs success in providing more accurate trajectories also indicates that driftersS 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 winds affect drifter motion more than the water velocity since drifters do not obey continuity.

Reference:

A hybrid Eulerian-lagrangian model for predicting the trajectories of surface drifters in the Pacific ocean. Paldor, N., Y. Dvorkin, A.J. Mariano, T.M., Ozgokmen, and E. Ryan. EGS-AGU-EUG Joint assembly, 2003, (EAE03-A-11666).

Geophysical Research Abstracts, Vol. 5, 11666, 2003 © European Geophysical Society 2003

Links:
E03-J-11666.pdf