Boundary Layer Effects on Flow and Mixing in Deep Ocean Canyons

University Of California-San Diego Scripps Inst Of Oceanography, La Jolla CA

Investigators

Abstract

Overview: The main reasons to identify the spatial and temporal distribution of mixing and upwelling in the deep ocean are to understand its energy budget, to close the overturning circulation and to understand deep currents and the transport of heat, fresh water, dissolved carbon, geochemical tracers, and nutrients. Observations suggest that the majority of deep-ocean mixing happens over rough topography, like the flanks of the world's mid-ocean ridges. Though tidally-driven internal waves are the most commonly discussed explanation for this mixing, a number of observations suggest that mixing and upwelling within the ridge flank canyons is significant, and that it is not fully explained by internal wave processes. There are about 1,000 mid-ocean ridge flank canyons in the deep ocean, and the canyon to be studied in this project is representative of many or most of them. Therefore, if boundary layer processes are shown to be significant in this canyon, they are expected to be significant throughout much of the world's oceans. This study presents a novel hypothesis for the mechanism behind an important process (abyssal mixing), developed with physical models, and which can be feasibly and cleanly tested with an observational program. Intellectual Merit: A novel mechanism for abyssal mixing and upwelling is proposed: diffusion-driven boundary layers interacting with complicated topography. Though it occurs in conjunction with tidally-driven mixing, this mechanism may significantly change the expected distribution of deep-ocean mixing. This project investigates whether boundary layers are driving the circulation within a canyon in the South Atlantic using a series of modeling experiments to elucidate how these boundary layers interact with complicated topography and affect bulk properties of the ocean. The approach consists of a hierarchy of models with increasing complexity, from canyons with constant cross-sections to fully-realistic three-dimensional topography. A key deliverable is a set of metrics for the total impact of the boundary layers on the density field far from the boundary. These metrics will be related to the characteristics of the topography as a first step toward mixing parameterizations that do not collapse all topographic effects into a single roughness parameter. Broader Impacts: This project will improve our understanding of mixing and upwelling throughout the deep ocean. A key goal of the modeling portion of this study is beginning to develop mixing parameterizations for global models that take into account boundary layer and mixing processes within canyons and are based on both models and observations. This project will also form a central portion of the training of a postdoctoral researcher.