Circulation and Vorticity Dynamics of Laboratory Sill and Basin Flows

Woods Hole Oceanographic Institution, Woods Hole MA

Investigators

Abstract

Recent oceanic observations (e.g., Faroe passage and Denmark Strait) show time-dependent and steady components of the strait flows, and aspects of the downstream mixing. However, much less is know about the structure of the currents which feed the straits and how the strait flow is coupled to the upstream basin circulation. Second, recent theoretical and numerical work suggests a novel coupling mechanism, but there are no observational data and few laboratory experimental data with which to test and generalize these new ideas. The proposed comparisons between theory and laboratory experiments can be used to suggest future oceanic observations and possible overflow monitoring strategies. The intent is to provide as much information as possible to guide future ocean cruises, general circulation theory, and numerical climate modeling projects Laboratory experiments will be developed and conducted to investigate the coupling of circulation in a finite, rotating basin with the hydraulically controlled flow over a sill. This situation usually applies to layers of deep water passing from one deep ocean basin, which is filled by this water, to another basin through a col, or the deepest saddle point passage. In the downstream basin the water is typically less dense. The goal is to obtain quantitative measurements with which to examine the basin circulation, including any feeder currents, the potential vorticity dynamics of the coupled basin-strait system, and the hydraulic flow in the strait. A recent numerical study showed a novel, and as yet unexplained, potential vorticity control exerted by the strait on the basin. Exploration of this mechanism will be the starting point for this research. The proposed experiments will be conducted on the WHOI GFD Laboratory one- and two-meter turntables with tanks containing an upstream basin and a passage to a small catch basin. The work will first explore flows of water under air to permit investigation of flows with relatively small viscous effects in both the upstream basin and strait. It is also planed to pump salty water under a deep layer of fresher water (reduced-gravity configuration), where both increased rotational effects and the role of increased frictional effects in the upstream basin can be explored. Experiments with two active layers may be examined at later stages of the work. Various upstream geometric conditions will be investigated (e.g., flat and bowl-shaped basins) and the role of different mass source characteristics (e.g., boundary inflow and interior downwelling). In all cases layer depths and velocity distributions (with particle imaging techniques) will be measured within both the basin and passage for a wide range of relevant parameters. The experimental work will be parallel to, and compared with, similar theoretical and numerical work already underway at WHOI. Broader Impacts: Deep passages offer ideal points to monitor the overturning circulation. However, this can only be done properly if the dynamical connections between strait and basin flows are understood. This work will begin to answer some of these questions and should be of wide interest beyond the specific dynamics explored here. The PIs are both active in the MIT/WHOI Joint Program and the results of this research will quickly reach the classroom. The grant will also support a graduate student and so further contribute to broader educational goals. Lastly, the grant will generally support the WHOI Geophysical Fluid Dynamics Laboratory, one of few facilities of its type in the world. The WHOI GFD Lab has a long history of helping scientists and students from WHOI and around the world conduct experimental work on a broad range of fluid dynamics problems.