A MODIFIED GALERKIN-SPECTRAL MODEL FOR 3-DIMENSIONAL, BAROTROPIC, WIND-DRIVEN SHELF CIRCULATION

We describe an efficient numerical scheme for calculating wind-driven currents on the continental shelf. Our scheme is based on the spectral approach introduced by Heaps and subsequently modified by Lardner. Th e basic idea behind Heaps' approach is to express the horizontal flow, u(x, y, z, t), as a linear combination of vertical structure function s, phi(r)(z), and then solve numerically for the temporally and horizo ntally varying coefficients. To obtain an accurate representation of w ind-driven flow, many phi(r) are often required. Following Lardner, we reduce this number by subtracting from u an analytically defined flow field, u(P), prior to its expansion in terms of the phi(r). Our choic e of u(P) is steady Ekman flow in water of finite depth. This particul ar choice includes, as a special case, the u(P) used by Lardner. Using an idealized basin and time-harmonic wind forcing, we compare the con vergence rate of the expansion of u - u(P) with u(P) taken to be (1) z ero, corresponding to Heaps' approach, (2) flow with, constant horizon tal shear stress through the vertical, corresponding to Lardner's rece nt suggestion, and (3) steady Ekman flow. We find that removal of stea dy Ekman flow generally leads to the most rapid convergence, particula rly when the water depth is much greater than the Ekman depth, a condi tion often found on the middle and outer continental shelf.