The kinematics and dynamics of the New England continental shelf and shelf/slope front

Abstract

A 37 day long field program was carried out in March 1974 on the New England continental shelf break to study the current and hydrographic structure and variability on the shelf and in the shelf/slope front. A second experiment was conducted in the shelf break region for one week in January 1975 to study frontal exchange processes.The mean currents during the March 1974 experiment all had a westward alongshore component, increasing in magnitude progressing offshore from 15 cm/sec to a maximum at the nearshore edge of the shelf/slope front of between 10 and 20 cm/sec, and decreasing in magnitude with depth. The current structure was such that the velocity vector rotated clockwise with depth in the shelf waters inside the front. The mean alongshore transport of shelf water was on the order of 0.4 Sverdrups through a cross-shelf transect south of Block Island. About 30% of the transport occurred in the wedge-shaped region offshore of the 100 m isobath and inshore of the front. Comparison of the observed mean currents with those predicted by the steady frictional boundary layer model of Csanady (1976) indicates that the model captures most of the essential features of the shelf circulation.The low frequency currents contain approximately 30% of the total current variance. An empirical orthogonal modal analysis indicates that for low frequency alongshore motions the whole shelf together with the water above the front moves as a unit and that the on-offshore currents are characterized by opposing flows at surface and bottom. The alongshore wind stress component is the dominant forcing term for these low frequency motions and for the subsurface pressure field as well. For motion with periods longer than 33 hours, the time derivative term in the cross-shelf momentum balance is comparable with the Coriolis term while the advective terms are 2 to 10 times smaller, on the average.The semi-diurnal tide is barotropic over the shelf with current magnitudes that increase almost by a factor of two between the shelf break and the inshore mooring 70 km shoreward. At the shelf break onedimensional continuity gives the correct relation between the surface tide and the semi-diurnal currents. The semi-diurnal tide is clockwise polarized. The diurnal tide is baroclinic, increasing somewhat toward the bottom, is less clockwise polarized than the semi-diurnal, and has tidal ellipses aligned with the isobaths. The diurnal tidal energy decreases toward shore.Inertial energy in the frontal zone is equal to the semi-diurnal tidal energy near the surface. The inertial energy decreases with depth and is an order of magnitude smaller further on the shelf. The inertial oscillations are shown to be highly correlated with the wind stress record, arising and decaying on a time scale of 3 to 4 days. The inertial oscillations are shown to be preferentially forced by wind stress events that have a large amount of clockwise energy at near inertial periods.The frontal zone is shown to be in near geostrophic balance with an anticipated vertical ...

The dynamics and kinematics of the coastal boundary layer off Long Island

“…As reference to the lower layer velocity relation (equation as those reported by Kundu (1976) and Flagg (1977) which have no obvious meteorological origins. In any case, it is clear that in coastal regions one might well encounter locally generated near inertial motions which exhibit strong spatial variability.…”

“…Whatever the origins, this intermittency results in a dissembling compleXity which can confound attempts to establish a causal relation between wind and wave. It is not uncommon to find no oscillations when they might have been expected (Webster, 1968) nor to suddenly observe fluctuations with no apparent cause (Kundu, 1976;Flagg, 1977).…”

“…Flagg (1977) and Smith et al (1978) in their respective studies of the New England and Scotian shelves. In the coastal boundary layer, however, this trend is reversed with depth as the effect of friction apparently becomes more important than shoaling.…”

“…Flagg (1977), and Mayer et al (1980), have shown inertial kinetic energy to decay rapidly in the shoreward direction thereby limiting significant levels to middle and outer shelf locations. Off the Oregon coast Hayes andHalpern (1976), andKundu (1976) have reported substantial near-inertial energy within -15 km of shore in relatively deep (order 100 m) water.…”

“…The observation that inertial energy substantially decreases in the shoreward direction has generally been interpreted asa manifestation of increased frictional influence as the water column shoals (eg. Flagg, 1977).…”