Funding for this project is furnished by the Office of Naval Research
SYNOP experiment has revealed energetic deep water motions under the Gulf Stream (GS) related to deep eddy processes and topographic wave activity. Vertical coupling of the deep water velocity field with the Gulf Stream appears to be misrepresented in current model studies, being a potential source for discrepancies in the stability analysis of the GS system. The overall scientific goal of this project is to improve our skills in modeling large scale meandering and eddy processes in the Gulf Stream through better understanding of the vertical coupling processes.
The Potential Vorticity (PV) structure of the Gulf Stream was examined in order to provide improved fields for initializating models. High resolution CTD and velocity measurements across the GS were combined with the PV-Gradient Layered Model to obtain the best possible estimates of the Gulf Stream PV and to test the dynamical role of different PV features. Two dynamically-justified layered approximations of the GS PV were obtained for subsequent use in model studies of the Gulf Stream stability.
Our efforts have also focussed on studying bottom-intensified motions (Topographic Rossby Waves) and their coupling with the Gulf Stream.
Observational features of TRWs were based on about 5-year-long deep water velocity records obtained from SYNOP and SAIC experiments. Cross-slope and along-slope structure of topographically trapped motions could be resolved from these data. Gulf Stream eddy activity through the course of both experiments was determined from GS SST front positions obtained using AVHHR images (courtesy of GSO Remote Sensing group). Time series of the GS eddie energy along different sections were compared against TRW activity.
Significant changes of the bottom slope on scales of TRW wavelengths prohibit the use of the WKB approximation in the study region and place the applicability of the traditional ray-tracing technique here under question. Instead of separating the vertical and horizontal variables (which is not proper under these conditions) the lateral-vertical modes of topographically-trapped motions near Cape Hatteras were used to study the energy distribution cross-slope and in vertical. The Brink and Chapman technique involving realistic topography and vertical stratification was used for finding the modal structure.
Introduction of the mean flow changes the modal structure imposed by topography since the PV gradient of the mean flow bears its own modal structure intrinsic to the flow itself. When coupled, topographic and mean flow PV gradients introduce a unique combination of modes. Both stable and unstable modes are allowed depending on the mean flow PV pattern. Currently we are working on including the mean flow in the procedures for modal structure analysis. We use primitive equations rather than QG to allow finite slopes. The system is linearized over the mean flow.
Logoutov, O., G.Sutyrin, and D.R. Watts, 2001. Potential vorticity structure across the Gulf Stream: observations and a PV-gradient model. J. Phys. Oceanogr., 31, 637-644. (PDF)
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