CHARACTERIZATION OF 3-DIMENSIONAL LAGRANGIAN CIRCULATION ASSOCIATED WITH TIDAL RECTIFICATION OVER A SUBMARINE BANK

Two approaches to the quantitative characterization of Lagrangian circ ulation in three-dimensional (3D) time-periodic current fields are use d to describe major features of a model-derived flow field for barotro pic tidal rectification on Georges Bank. The first approach is an exte nsion of the 2D ''saddlepoint'' method used by Ridderinkhof and Loder to describe the vertically averaged Lagrangian circulation associated with various forcings over outer Gulf of Maine banks. Application of t he method to representative sigma (terrain-following) surfaces and com parison with particle trajectories in the 3D current field indicate th at the circulation saddlepoints and associated separation lines (betwe en recirculation and throughflow regions) in the vertically averaged f low over Georges Bank are qualitatively representative of the entire w ater column, but that there is significant vertical structure in the L agrangian circulation. In particular, the horizontal extent of the tid ally rectified clockwise gyre on Georges Bank is greater in the lower water column, due to an on bank component of near-bottom Lagrangian fl ow. The second approach-evaluation of the 3D Lagrangian residual displ acement held from particle tracking in the 3D current field-confirms t he presence of a significant Stokes velocity associated with tidal rec tification on Georges Bank. The around-bank component of the Stokes ve locity is counterclockwise with magnitude approximately one-third that of the mean Eulerian velocity arising from tidal rectification, there by partially offsetting the mean Eulerian velocity. The cross-bank com ponent of the Stokes velocity also generally opposes the mean Eulerian flow and has a comparable magnitude. It accounts for the onbank near- bottom Lagrangian flow on the southern flank and northern edge of the bank (and hence the expanded gyre in the near-bottom region), and illu strates the importance of including the tidal velocity in model studie s of Lagrangian circulation in tidally energetic regions.