Cross-shelf exchange driven by dense flow down a canyon
Laboratory experiments investigated the dynamics controlling the cross-shelf exchange in a prograde sloping canyon induced by dense shelf water descending into the canyon. This thesis is motivated by the dispersal of dense water generated by polynyas on the Arctic and Antarctic continental shelves. Laboratory results corroborate prior numerical results suggesting that canyons are hotspots of cross-shelf exchange. When the dense water descends a canyon, it induces an onshore return flow of offshore water into the canyon. This return flow is initially driven by the dense water eddies descending the canyon and acting like a bucket brigade. At later times, another mechanism may also be at play where large dense cyclonic (anticlockwise) eddies on the northern continental shelf may pull more dense water out of the canyon producing a region of low pressure, near the canyon head, which induces an increase in ambient flow into the canyon. The Burger number (Rossby radius of deformation/canyon width) and the dense water source location with respect to the canyon head affect the offshore ambient water velocity up the canyon. Additionally, as the offshore water reaches the canyon head, the offshore water volume flux becomes larger than the dense water volume flux, possibly due to the low pressure region described above. Understanding these dynamics in the Antarctica region is of global significance for two main reasons: 1. The offshore flowing dense water forms Antarctic Bottom Water and thus affects the global meridional circulation; 2. The onshore heat transport induced by the return flow drives glacial ice melt and therefore contributes to sea level rise.
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Massachusetts Institute of Technology and Woods Hole Oceanographic Institution