Estimates of dispersion from clustered-drifter deployments on the southern flank of Georges Bank
Figure 2: General location of each of the 16 clusters deployed over the southern flank of Georges Bank. (140.4Kb)
Figure 3a: Near-surface (<=15 m) cluster tracks with the path of the centroid denoted in black. (238.4Kb)
Figure 4: Plots of σ against time showing cluster divergence (top panel) and convergence (bottom panel). (10.34Kb)
Figure 5a: Along-bank (dashed) and cross-bank (solid) displacement (km) from centroid vs. time (year-day) for all 14 near-surface clusters. (21.91Kb)
Figure 6: Drifter displacements (km) relative to cluster centroid, which is denoted by a black dot at the center of each panel. (34.28Kb)
Figure 7: Apparent eddy diffusivity (Kac) vs. cluster size (λ) including values from DL2001 and the theoretical estimate (Kae) of Okubo (1971). (13.56Kb)
Figure 8: Mean cross-bank displacement of drifters relative to the cluster centriod (dashed) and cross-bank velocity of the cluster centriod (solid) vs. time (yearday). (191.2Kb)
Figure 9: Modeled (solid lines) and observed (dashed lines) cross-bank (blue) and along-bank (magenta) displacement relative to centroid of cluster #3. (11.49Kb)
Figure 10: T/S relations of four patch-following operations conducted on cruises SJ9505, SJ9507, OC301, and OC303. (61.58Kb)
Figure 11: Cross-frontal density structure (contoured σT) with cross-bank trajectory (bold arrow) of each drifter as observed within three days of CTD section. (22.08Kb)
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Data from 16 clustered-drifter deployments are used to examine horizontal dispersion on the southern flank of Georges Bank. The spreading rates of all clusters have an average of 1.6 km d-1 with a standard deviation of 1.8 km d-1. Both "effective" and "apparent" diffusivities are calculated for each cluster. Their ranges (i.e., -54 to 757 m2 s-1 for effective diffusivity) are related to differences in cluster size and proximity to the tidal mixing front. Cross-bank convergence is documented for nearly 40% of the clusters. This occurs especially for clusters with centroids within 10 km of the tidal mixing front location, as deduced from conductivity, temperature, and depth transects (CTD) conducted concurrently with the cluster deployments. Estimates of turbulent dispersion (distinct from shear effects) are derived by the method of Okubo and Ebbysmeyer (Okubo, A. and Ebbesmeyer, C.C., 1976. Determination of vorticity, divergence, and deformation rates from analysis of drogue observations. Deep-Sea Res., 23, 349-352). The results reveal that the effects of horizontal shear are important in spreading of larger drifter clusters. Often the impact of shear is evidenced by the track of a lone drifter that separates from a cluster as it is entrained into the current of the shelf-edge front or the tidal mixing front. Cluster dispersion is time dependent as evidenced by a significant modulation of cluster size at the M2 tidal frequency. This modulation is due to the spatial variation of tidal currents over the southern flank of Georges Bank and is closely reproduced by immersing drifter clusters into the flow field of a Georges Bank tidal model.
Author Posting. © Elsevier B.V., 2006. This is the author's version of the work. It is posted here by permission of Elsevier B.V. for personal use, not for redistribution. The definitive version was published in Deep Sea Research Part II: Topical Studies in Oceanography 53 (2006): 2501-2519, doi:10.1016/j.dsr2.2006.08.004.
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