Moored velocity measurements on the edge of the Gulf-Stream recirculation
Additional Document Info
Mean velocities, eddy kinetic energies and time series of velocities at several depths are presented from moored current meters for the 15 month period May 1978 to July 1979 as part of the POLYMODE Local Dynamics Experiment, centered at 31N, 69 degrees 30’W. The mean velocities in the upper one kilometer are toward the southwest at 2.4 to 4.4 cm/s, consistent with Worthington’s (1976) description of the mean circulation of the North Atlantic. The abyssal flow is toward the east, directed toward shallower depths, at 2 cm/s. This deep flow agrees with neither Worthington’s description nor other direct current measurements along 70W. The record-averaged eddy kinetic energies are three times as large as those 300 km to the south during the MODE experiment. For periods between 880 and 2 days the vertical structure of the kinetic energy spectrum is independent of frequency in contrast to the MODE results. These results are consistent with those obtained at other longitudes when position in the mean circulation (relative to the Gulf Stream) is taken into account, indicating that this rapid increase in kinetic energy and change of spectral vertical structure in the region of return flow compared to the “mid-ocean” may be ubiquitous. Twice during the 15 month record strong surface intensified jets passed through the array and contribute substantially to the eddy kinetic energy. Averages excluding these two periods indicate, however, that the mesoscale eddies seen in between these events are still more energetic than those seen to the south. The abyssal flow is dominated, especially during the passage of the jets, by oscillatory flow consistent with the topographic Rossby wave description derived from the 1300 m SOFAR floats (Price and Rossby, 1982). In addition small scale eddies are apparent when the jets are present. Thus the rapid increase of eddy kinetic energy within the recirculation region appears, in part, to be associated with a different eddy structure.