Shelf-slope water frontal structure, motion and eddy heat flux in the southern Middle Atlantic Bight

Abstract

Results of an analysis of SEEP-II temperature and velocity data to investigate the shelf-slope frontal structure, motion and eddy heat flux in the southern Middle Atlantic Bight are presented. In the winter and spring the inclination of the frontal boundary and the vertical velocity shear when averaged over periods greater than a week, approximately satisfy the Margules' equation. Associated with the locally wind driven cross-shelf excursion (∼ 20 km) of the foot of the front are vertical displacements of the frontal boundary due to vorticity constraints imposed by the bottom topography. Some of the smaller-scale frontal motions suggest wave-like disturbances propagating southward along the continental margin. In April 1988 the most severe wind event recorded during SEEP-II displaced the foot of the front into the upper slope region and injected shelf water into the interior of the slope water column. On the mid-shelf cross-shelf eddy heat fluxes are negligible, as temperature variations and cross-shelf velocities are in quadrature. At the shelfbreak, however, depth-averaged heat fluxes, approximately 4 × 106 W m−1 onshore, are significantly different from zero and comparable to the exchange estimated by Fairbanks using 18O tracer data. In the winter and spring the eddy flux is greatest near the frontal boundary in the lower water column and is dominated by a few large exchange events. In the summer the subtidal eddy flux is greatest just below the seasonal pycnocline in the upper water column, with a peak in the 5–11 day band that may be a contribution from frontal instabilities. Bottom Eulerian mean velocities are 1–2 × 10−2 m s−1 offshore over the shelf and slope. Measured eddy fluxes are adequate to achieve the advective-diffusive balance required to maintain the frontal position in steady state.