In order to close the global overturning circulation of the ocean, the production and sinking of dense water at high latitudes must be balanced elsewhere by buoyancy gain and upwelling. Both these processes are intimately linked to diapycnal mixing, which implies that mixing processes are fundamentally important in the Earth's climate system. Additionally, vertical motion is a primary driver for the abyssal circulation, which implies that its spatial distribution must be known in order to understand and model the circulation. Observations collected during the last few decades indicate that the strongest mixing in the deep ocean is primarily found near the rough topography of mid-ocean ridges (MORs), which cover more than 50% of the entire sea-floor. Of particular interest are the many ridge-flank canyons (fracture zones) that incise the flanks of slow-spreading ridges every 50 km or so. Not only are these canyons the main sites of strong mixing, they are also strikingly regular and self similar, with persistent up-flank flows and overflows across the ubiquitous bathymetric hills that act as sills for the along-axial flows. In spite of the observational advances regarding the spatial distribution of mixing and the circulations in ridge-flank canyons, virtually nothing is known about the spatial distribution of vertical motion in the ocean. While current high resolution ocean models have made significant strides towards realism in many areas, the accuracy of the upwelling field remains as one of the important outstanding problems. The dynamics acting near MORs, and in particular the flows in the ridge flank canyons will be investigated by using numerical modeling and analysis of existing observations. In addition to improving our understanding of the lateral circulation and the mixing near rough topography, one could expect insights from this study to contribute towards developing more accurate representations of upwelling and the resulting abyssal circulations in ocean general circulation models.
Intellectual Merits: Turbulent flows are strongly dependent on the details of domain geometry. Even the highest resolution large-scale ocean models cannot adequately resolve all the fine-scale features of bottom topography. As such, parameterization and modeling of upwelling induced by rough topography poses a fundamental challenge. The self-similar, regular nature of the geometry of the ridge-flank canyons and the associated flows may greatly facilitate representation of the associated drag and mixing in large-scale models. The numerical modeling of these flows is a novel undertaking, which will reveal the importance of form drag, hydraulic control and time-dependent forcing in stratified flows over multiple sills.
Broader Impacts: The results from this project will be disseminated in the form of articles in journals, presentations in national and international scientific meetings, and via a project website. Ultimately, a better understanding of flow dynamics on MORs will lead to an improved understanding and modeling of the ocean general circulation. As ocean models are used to predict circulation patterns at a variety of scales, the results from this study may have the broader impact of aiding climate modeling, which can ultimately impact society by influencing policy. The results from this project may also help improve our understanding on the dispersal of hydrothermal "products", including heat, geochemical tracers and animal larvae. The study will provide support for both a PhD student and a female PhD recipient in physical oceanography.
