Current structure and mixing in the shelf/slope water front south of New England

Abstract

Horizontal and vertical currents in the front along the edge of the continental shelf south of New England during the late spring were measured by tracking special neutrally buoyant floats which tagged different water masses. The mean current (10–20 cm s−1) was westward along the shelf and was confined completely to the overlying shelf water with a geostrophic transport of about 0.7 sverdrups between the 80‐ and 300‐m isobaths. No mean cross‐shelf or vertical currents could be reliably measured. Higher‐frequency currents were detected and are described. Numerous conductivity‐temperature‐depth lowerings were made over the drifting floats to examine the interleaving process and possible mixing between shelf water and slope water. The layers are thinned by vertical shear, and they gain and lose both heat and salt on a time scale of 1–3 days. This plus the rich microstructure suggests the existence of vertical turbulent exchange between layers having a turbulent diffusivity of the order of 5 cm2 s. Estimates of net annual heat and salt input to the shelf are made, and they appear to be significant in determining the heat and salt budget of the shelf water mass.

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

“…A schematic of this description is shown in Figure 3.7. Our evidence for a velocity deficient zone under the front is sketchy at best but the scheme suggested here is also consistent with the behavior of the neutrally buoyant floats of Voorhis, Webb, and Millard (1976).…”

“…The frontal zone is the region of significant transition between the continental shelf and continental slope water masses. The early work of Bigelow (1933), Bigelow and Sears (1935), the more recent studies of Cresswell (1967), Boicourt (1973), , Wright (1976), andVoorbis, Webb, andMillard (1976), among others, the seasonal thermocline. This band is surrounded, inshore, above, and offshore by warmer waters.…”

“…Zenk and Briscoe (1974) have examined the summertime internal wave band at the 500m isobath offshore of the front with current meters located at 60 and 85 m. They found that the temperature spectra exhibits a cutoff at N, the local Brunt-Vaisala frequency, and the kinetic energy distribution scaled in the vertical in reasonable accord with linear internal wave theory. Voorhis, Webb, and Millard (1976), using neutrally buoyant floats placed in and near the frontal zone, found a significant amount of internal wave energy which was modulated at tidal and inertial frequencies. …”

Radium isotopes as tracers of coastal circulation pathways in the Mid-Atlantic Bight

“…The shelfbreak jet, a relatively small scale feature, was found to account for the majority of alongshelf flux. The resulting volume transport is considerably higher than transport estimates that are typically cited for the Mid-Atlantic Bight (Biscaye et al 1994a;Beardsley et al 1985;Ramp et al 1988), although they are similar to some of the larger estimates that are less frequently used in budget calculations (e.g., Burrage andVoorhis et al, 1976). The large alongshelf transport was found to be consistent with the unusual hydrographic structure, as predicted by the Yanovsky and Chapman model (1997).…”

Salinity maximum in the pycnocline of the Middle Atlantic Bight1,2

“…Therefore, it is possible that the pycnocline S-max, which is active for only part of the year, may provide about half of the annual salt requirement of the shelf water. Voorhis et al (1976) computed a slopeshelf salt flux of 1.74 g*crn-'* s-l by way of vertical turbulent exchange between interleaved layers. They pointed out that if this value represents an average annual flux, it would balance river input.…”