Abstract
Using moored array data collected across the Gulf Stream at 55°W, the author has investigated the dynamic signatures of the velocity fluctuations at the thermocline (550 m) and near-bottom (4000 m) levels. The cross-stream amplitude and phase structures for motions with period from 64 to 12 days, in a coordinate frame aligned with the instantaneous stream, have a striking resemblance to those computed from small amplitude instability theory. It is the sinuous or asymmetric mode that appears to dominate the observed fluctuations as is predicted. In addition, the mean downstream velocity at the two levels is consistent with a two-layer model for the mean flow in which the lower layer has uniform potential vorticity. When fit to the data this model predicts a value for the rate of change of the Coriolis parameter appropriate to this latitude, but one would expect it to be augmented by a factor of 2 or so by the significant slope of the bottom in this region.
