Quasi-Lagrangian structure and variability of the subtropical western North Atlantic circulation
A large body of quasi-Lagrangian trajectory (SOFAR float) data collected from 1976-1979 from 700 m and 2000 m in the western North Atlantic is examined, and it is shown that the character of the trajectories varies markedly over regions as small as a few degrees of latitude and longitude. Kinetic energy increases to the north and west in the basin at both levels. At 700 m a large northwest energy gradient is present between 30N and 31N at 70W. At 2000 m kinetic energy increases to the north and west with the largest gradients very near to the Blake Escarpment at the western boundary of the basin. At both levels the region in the vicinity of 28N, 70W appears to be locally a minimum of kinetic energy. At very low temporal frequencies the trajectories indicate that zonal motions are more energetic than meridional away from the western boundary in the thermocline. At the deeper level the trajectories appear to be influenced by the local bottom topography, though at 2000 m over very flat areas such as the Nares Abyssal Plain zonal motions appear to dominate over meridional at low frequencies as in the thermocline.
From the data it is possible to examine three regions of the subtropical western North Atlantic. North of approximately 32N and west of 60W, there is evidence of a westward recirculation in the thermocline, based on one very long trajectory. To the south of the recirculation regime in the greater MODE region (25-30N, 67-75W) there is evidence that individual fluid parcels in the thermocline undergo large rms displacements but small net displacements from their initial positions over times of the order of a year and may remain in this region-for a period of several years. To the south and east of the MODE region there is repeated evidence of the presence of a well-defined eastward flow in the thermocline of approximately 4 cm sec—1, extending zonally at least to the eastern edge of the sampled region.
At 2000 m, a southerly mean flow is present west of 67W and south of 32N, and there is indication that very near to the western boundary in this region the southerly deep transport can be sizeable. In many respects the 2000 m trajectories examined here from west of 67W are both qualitatively and quantitatively similar to the much larger body of trajectory data collected- during MODE-I and discussed by Freeland et al. (1975). Trajectories from east of 67W at 2000 m show much lower kinetic energy levels than their more western counterparts, and the trajectories of three instruments set in the Nares Abyssal Plain region also indicate the possibility of larger horizontal length scales for the energy containing eddies and weaker horizontal dispersion than in the greater MODE region.
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