Simulated Drifter Research Articles

Use of simulated drifter tracks to investigate general transport patterns and residence times in the Coastal Transition Zone

General transport patterns and residence times in the upper 100 m of the coastal transition zone (CTZ) were studied using simulated Lagrangian drifter experiments. The circulation fields used for the drifter experiments were obtained from a regional primitive equation model that incorporates coastal geometry and bottom topography that is representative of the CTZ region. The simulated circulation fields show velocity patterns that are consistent with those associated with the filaments and jets observed in the CTZ region. Drifters were released at 880 points in the model domain in a pattern that bracketed, both horizontally and vertically, the region in which offshore‐flowing filaments were observed to form. At each location, drifters were released at depths of 30, 60, 90 m and tracked for 30 days. The depth of these drifters varied along their trajectories in response to the vertical velocities experienced by the drifters. An additional set of drifters was released at 30 m and was constrained to remain at this depth. The composite drifter trajectories show that transport patterns and residence times in the upper 100 m are determined by the proximity of the drifter release point to the offshore‐flowing filament. Drifters released inshore of the filament are transported southward with the mean flow of the model California Current. Drifters released near the region in which the filament originates are transported rapidly offshore. The point at which this difference in drifter fate occurs is approximately 100 km offshore. The time required for a drifter to be transported from the nearshore region to the outer boundary of the area covered in the CTZ model domain is approximately 20 days. Comparison of the free‐floating drifters with the 30‐m fixed‐depth drifters shows that the deeper drifters are displaced southward as they are transported offshore in the jet. The magnitude of the southward displacement for the 90‐m drifters is about 15 km, implying substantial cross‐jet exchange. Comparison of the simulated drifter trajectories with the trajectory followed by a drifter released during the 1988 CTZ field sampling program support the hypothesis that the deepening of the abundance maxima observed in the distributions of Dolioletta gegenbauri, juvenile forms of Euphausia pacifica and Eucalanus californicus resulted from these populations being downwelled as they were transported offshore by the flow associated with the filament observed in 1988.

Observed and simulated kinematic properties of Loop Current rings

Two rings, shed by the Loop Current in 1980 and 1982, were observed for several months by satellite‐tracked drifters to migrate across the Gulf of Mexico. The drifter path data have been inverted to obtain estimates of the paths of the centers of the two rings, ring shape, and the swirl velocities. Three drifters were deployed in the 1980 ring, and the analysis of that data set establishes the variability of the above kinematic estimates for one ring. A comparison of the analysis of data from both rings provides some idea on inter‐ring variability. Both rings impacted the Mexican continental slope at about 22.8°N, 95.5°W. After a brief adjustment period, both rings reestablished and maintained a vortex character for several months in the slope region while migrating slowly to the north. The paths of the centers of the two rings along the slope are virtually identical. The same analysis routine was applied to some simulated drifter data obtained from the Hurlburt and Thompson (1980) Gulf of Mexico primitive equation model. In the midgulf, the agreement between the observed rings and the simulated ring is good, although the former showed stronger interaction with the continental slope topography and/or circulation than was seen in the latter. Along the slope, the model ring kinematic characteristics were in extraordinary agreement with the observations.