Current variability across the New England continental shelf-break and slope

Abstract

The year-long SEEP—I moored array of current meters across the continental shelf-break and slope south of Cape Cod indicates mean alongshore flow to the west across the array during both the winter and summer. The mean flow consists of a surface-intensified jet at the shelf-break, offshore flow across the shelf-break of slightly greater than 1 cm s −1, and the suggestion of weaker flow over the mid-slope than either seaward or shoreward of this region. There is an onshore increase of kinetic energy at diurnal and semidiurnal frequencies. In the inertial band there exists a winter-to-summer near-surface increase in variability, and a subsurface decrease. In the synoptic band there is a larger response over the shelf to the winter-to-summer decrease in wind stress. The spectra are generally red with enhanced near-bottom energy levels over the slope in summer, indicative of increased topographic Rossby wave activity. Spatially coherent wind-driven motion does not extend seaward of the shelf-slope frontal region. The current response to the alongshore wind is consistent with Ekman dynamics, and more significant during the winter, while during the summer the response is more coherent with the cross-shore wind. There is significant synoptic band coherence between the cross-shore component of the wind stress and the currents throughout the year. Only one (known) warm-core ring intersected the array throughout the year, however heightened and sustained topographic Rossby wave activity was evident in summer. These waves have a period of roughly 16 days, an offshore phase speed of ∼9km day −1 and a coherence scale of ∼160km. There is some evidence of refraction of the wave energy towards the along-isobath direction as the topography shoals, and no evidence of these waves shoreward of the mid-slope region.