FY 1985

Atlas of tidal elevation and current observations on the Northeast American continental shelf and slope

Moody, J.A., B. Butman, R.C. Beardsley, W.S. Brown, P. Daifuku, J.D. Irish, D.A. Mayer, H.O. Mofjeld, B. Petrie, S. Ramp, P. Smith, and W.R. Wright

Bulletin No. 1611, U.S. Geological Survey, U.S. Government Printing Office, 122 pp (1984)

Measurements of sea-surface elevation or bottom pressure at 100 stations and measurements of current at various depths at 139 stations on the North American Continental Shelf from Cape Hatteras to the Laurentian Channel have been analyzed using either the harmonic or the response method for five tidal constituents: the semidiurnal tides M (12.42 hours), N (12.66 hours), and S (12.00 hours), and the diurnal tides K (23.93 hours) and O (25.82 hours). The amplitude and phase of elevation and of east and north current for the five constituents are presented in tables. The ellipse representation of the current is also tabulated. In addition, plates are presented showing tidal elevation and surface current (for M, N, S, K and O) and representative current ellipses (for M, K and O). This is the first compilation of tidal elevation over this region of the Continental Shelf for constituents other than M and of tidal currents at depths other than at the surface. The coamplitude and cophase lines for all semidiurnal tides are similar and indicate a co-oscillating tide in the Middle Atlantic Bight and on the Scotian Shelf, and a near-resonant tide in the Gulf of Maine. There is an amplitude minimum of the sea-surface elevation over Nantucket Shoals and Georges Bank. The diurnal tides (K and O) are dominated by an amphidromic point near Sable Island and a secondary amphidromic point in the Middle Atlantic Bight. Along the remainder of the shelf the diurnal tide seems to be a combination of Kelvin and shelf waves. The major axis of the M tidal current ellipses are generally oriented perpendicular to the local isobaths except near the coast and in the Great South Channel and Northeast Channel. In contrast, the major axis of the ellipses of the two diurnal constituents are generally oriented parallel to the isobaths. The amplitude of the M, K, and O surface current is weakest at the outer edge of the shelf, reaches a maximum midway across the shelf, and then decreases toward the coast. The variability of the tidal current estimates is largest near the shelf break where some contribution from a baroclinic tide is expected. Observations at five stations, where current measurements were made at 1 and at 10 to 20 meters above the bottom, were used to empirically determine the vertical structure of the M semidiurnal current near the bottom. The amplitude of the M bottom current is typically about 50 percent of the amplitude of the near-surface current. The empirical curve was used to estimate the bottom tidal currents (1 meter above the bottom) at 78 locations on the Continental Shelf where measurements were available only at 10- to 20-m above the bottom.