Date of Award
Doctor of Philosophy in Oceanography
H. Thomas Rossby
A large body of quasi-Lagrangian trajectory (SOFAR float) data collected from 1976-1979 from 700 m and 2000 min 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, towards the Gulf Stream at both levels, and at both levels the region in the vicinity of 28°N, 70°W appears to be locally a minimum of kinetic energy.
At very low temporal frequencies the trajectories indicate that zonal motions are larger than meridional ones away from the immediate vicinity of the western boundary in the thermocline. At deeper levels the character of the motion appears to be influenced by the local bottom topography. Over flat areas, the deep trajectories are primarily zonal, as in the thermocline. North of the greater MODE region (25°-30°N, 65°-75°W) there is strong evidence of westward recirculation in the thermocline. To the south and east of the MODE area there is repeated evidence of a well defined eastward mean flow in the thermocline of about 4 cm/sec. Within the greater MODE region floats are more likely to undergo large rms displacements but small net displacements from their initial positions over the course of a few years.
From several simple numerical and analytical model experiments estimates are made of the times over which quasi-Lagrangian SOFAR floats are useful followers of truly Lagrangian fluid parcels. In the prototype wave field used here, it is found that in the thermocline the motion of a float is representative of fluid parcel motion over times greater than 100 days when the rms amplitude of the field is 20 cm/sec; adding a field of internal gravity waves to the low-frequency field lowers this estimate by at most 30%. In deeper waters floats follow fluid parcels longer than in the thermocline.
In order to aid the interpretation of quasi-Lagrangian trajectory data in the ocean the quasi-Lagrangian properties of two simple dynamically consistent Eulerian models have been investigated and compared with existing data when possible. The first model, baroclinic planetary waves on a sloping bottom, yields a correlation between temperature and north-south displacement along the path that is dependent upon the direction of the bottom slope and the vertical structure of the wave. Three examples are shown from the data which, when interpreted in the context of the model, are consistent with earlier results that the western boundary of the basin acts as a generation region of mesoscale activity.
The second model is a quasi-Lagrangian formulation of Eady's model of waves in a shear flow. For stable waves, the temperature and north-south displacement are n out of phase along float paths. For unstable waves the two are out of phase in a more complicated, depth-dependent fashion. Two examples from the data are shown, and it is demonstrated that some qualitative features of the model obtain, though in both cases the perturbations are too large to be adequately covered by linear theory.
Riser, Stephen C., "QUASI-LAGRANGIAN SIGNATURES OF THE NORTH ATLANTIC CIRCULATION" (1981). Open Access Dissertations. Paper 1599.