The dynamics controlling the entrainment in dense currents are fundamental to the formation, movement, and distribution of the densest water in the ocean; a cornerstone of the thermohaline circulation. However, the entrainment and mixing in these currents occurs at such small scales, and the flows are so rapid that full resolution of the dynamics is presently very difficult, if not impossible, in global ocean circulation and climate models. Consequently, the entrainment, and often the overflows themselves are not resolved and need to be parameterized.
Intellectual Merit : The existing parameterizations for entrainment in dense currents account primarily for the shear-induced entrainment at the interface between the dense flow and the ambient fluid. However, the turbulence generated by roughness elements at the bottom boundary, which produces an enhanced drag, is intense and cannot be ignored. In this project, it is hypothesized that for dense currents having a height much larger than the bottom boundary layer thickness, the turbulent eddies generated by the bottom roughness will play a role in homogenizing the dense current but will not contribute to entrainment of ambient waters within the dense current (i.e. will not contribute to changes in the water properties). Conversely, for dense currents having a height comparable to or smaller than the bottom boundary layer thickness, the turbulent eddies near the bottom should be large enough to entrain the ambient water lying above the dense current and should significantly influence the dense water properties. The effect of entrainment due to bottom roughness should therefore be included in the entrainment parameterizations, and the parameter regime in which bottom roughness is important be identified. This project will address this shortcoming with a combined laboratory and numerical study focused on entrainment and dense currents dynamics over a wide range of rough bottoms, in which the shape (circular, square, and triangular cross section), vertical extent, spacing (sparse vs. dense configuration), and spatial distribution (regular vs. irregular) of the roughness elements will be varied. The investigators will: (i) quantify the relationship between entrainment and the ratio of the dense current height to the bottom boundary layer thickness; (ii) determine the influence of the shape, vertical extent, spacing, and spatial distribution of the roughness elements on the thickness of the bottom boundary layer in which turbulent eddies are expected to develop; (iii) establish a new universal entrainment parameterization which takes into account the bottom roughness.
Broader Impacts : Overflows and dense currents are important aspects of the deep ocean circulation. As such, improved understanding of the dynamics of these currents when they flow over a rough bottom bathymetry, and the development of relationships between the entrainment and roughness parameters has the potential to change the way mixing is parameterized in these flows. In practice, results from this project could lead to advanced entrainment parameterizations, a more realistic location in the water column of important water masses in climate models, and hence, an improved prognostic power of these models.
This project will support the Ph.D. thesis work of a graduate student at UT San Antonio. The student will gain experience in collaborative research involving numerical models and laboratory experiments. Both PIs are deeply involved in teaching and advising and the results from this project will quickly find their way into graduate education. Cenedese is also on the faculty of the Geophysical Fluid Dynamics (GFD) Summer Program at WHOI. Over the course of this grant, it is expected that one or more GFD fellows will work on related projects under her guidance. Finally, the investigators plan to attract graduate and undergraduate guest students to work at WHOI on ~ 4 month projects in support of the proposed study. The experimental work at WHOI will also utilize the Geophysical Fluid Dynamics Laboratory, which has a long tradition of providing facilities, and assisting students and scientists from around the US and world to conduct fluid dynamics experiments in the areas of physical oceanography, geology, and bio-physical interactions. Videos of experiments and related numerical models will be presented on the web with associated explanations suitable for classroom use.
