Canyons Influence on Cross-shelf Exchange - When Dense Water Goes Down, Warm Water Comes Up

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

This project will examine the dynamics of the cross-shelf exchange in a slope canyon induced by dense shelf water descending into the canyon. The case to be examined in the Antarctica region is of global significance because the offshore flowing dense water forms Antarctic Bottom Water and thus affects the global meridional circulation, while the onshore heat transport induced by the return flow drives glacial ice melt and therefore contributes to sea level rise. These important processes will be investigated through a combined laboratory and numerical approach. The project will explore the leading order dynamics controlling the cross-shelf exchange of momentum, heat and tracers when dense water descends a canyon. Key locations to measure dense shelf water offshore transport and offshore water onshore transport will be identified to provide guidance to future field campaigns. This proposal will support the Ph.D. research of a graduate student, and other students will be involved through the PIs’ active lab-groups. The experimental work will utilize the Geophysical Fluid Dynamics Laboratory at WHOI. In Years 1 and 3, the Geophysical Fluid Dynamics Laboratory Open Week event will target undergraduate and graduate students, postdocs, K-12 students. This event will host 10 hands-on demonstrations and displays. Material from this project will be used in educational activities by the Co-PIs. Preliminary numerical modeling revealed two unexpected flows in the Antarctic coastal region. The first is a current on the continental shelf east of the canyon, which contributes to the cross-shelf exchange by transporting a third of dense water across the shelf-break upstream (from the perspective of the Kelvin wave propagation) of the canyon. The second is the onshore intrusion into the canyon of deep offshore water, some of which is able to reach the coast. The project will combine numerical modeling with laboratory tank experiments to test two hypotheses: first, that a substantial fraction of dense water generated on the shelf over the head of a canyon flows onto the upstream shelf and eventually moves offshore into the slope sea on the upstream side of the canyon; and second, dense water descending down a canyon induces an onshore return flow of deep offshore water in the canyon. It is also hypothesized that this onshore flow originates on the continental slope downstream of the canyon and exits the canyon onto the continental shelf upstream of the canyon. The Regional Ocean Modeling System (ROMS) will be used to simulate the formation and subsequent transport of Dense Shelf Water (DSW) generated in a coastal region, as well as the motion of ambient water, in an idealized continental shelf and slope system. Idealized laboratory experiments using a rotating glass tank with a very similar set up as the numerical simulations will be used. These experiments will test whether the assumptions and parameterizations used in the numerical model are influencing the dynamics of interest. In addition, agreement between the laboratory and numerical results will provide confidence that the relevant dynamics are correctly captured in the laboratory. This award reflects NSF's statutory mission and has been deemed worthy of support through evaluation using the Foundation's intellectual merit and broader impacts review criteria.