Using moored array data collected across the Gulf Stream at 55 degrees
W, the author has investigated the dynamical signatures of the veloci
ty fluctuations at the thermocline (550 m) and near-bottom (4000 m) le
vels. The cross-stream amplitude and phase structures for motions with
period from 64 to 12 days, in a coordinate frame aligned with the ins
tantaneous stream, have a striking resemblance to those computed from
small amplitude instability theory. It is the sinuous or asymmetric mo
de that appears to dominate the observed fluctuations as is predicted.
In addition, the mean downstream velocity at the two levels is consis
tent with a two-layer model for the mean now in which the lower layer
has uniform potential vorticity. When fit to the data this model predi
cts a value for the rate of change of the Coriolis parameter appropria
te to this latitude, but one would expect it to be augmented by a fact
or of 2 or so by the significant slope of the bottom in this region.