Intrusions and Vertical Mixing at the Shelf/Slope Water Front South of Nova Scotia

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Abstract
Intensive profiling of the shelf/slope water boundary south of Nova Scotia has been made with a Batfish vehicle measuring physical and biological parameters consisting of chlorophyll a, zooplankton, conductivity, temperature, and depth. Continuous profiling (~30 h) through the boundary zone revealed intrusions at the subsurface front consisting of plumes of ascending slope water overlying descending surface coastal water both moving vertically and horizontally. Movements of the intrusions were coherent with M2 tidal cycles and exhibited localized turbulent mixing ascribed to shear instabilities. This mixing gives rise to a potentially efficient mechanism for vertical transport of heat and salt particularly as input to the Scotian Shelf. These intrusions also form inversion layers consistent with earlier observations which showed that these layers can decay over longer time scales by double diffusion processes. The vertical mixing at the subsurface front may also account for the anomalously high productivity along the front by means of continual replenishment of nutrients from below. Key words: physical oceanography, biological oceanography, Scotian Shelf, profiling