Towards a theory of the abyssal circulation

California Institute Of Technology, Pasadena CA

Investigators

Abstract

A revised picture is emerging of the deepest portion of the ocean's circulation. The mixing required to lighten dense bottom waters and that allows them to rise to mid-depth is strongly enhanced over rough topography. This project will work toward a dynamical theory that captures the interplay between the boundary transformations and interior flows and stratification. This work is important because the abyssal ocean has the capacity to store vast amounts of heat and carbon. A more complete understanding of what dynamical processes shape its circulation will help climate models capture its behavior and role in a changing climate. A pivotal role has been ascribed to the abyss in explaining the drawdown of atmospheric carbon dioxide during ice ages. Deeper insight into the dynamics of the abyssal circulation will also help understand better how it may have been different during these and other periods of Earth's history. The proposed work will build a hierarchy of dynamical models of the response of the abyssal circulation to bottom-intensified mixing on slopes with a focus on the two-way interaction between boundary layers on topographic slopes and the interior of the abyss. This interaction will be expressed explicitly using boundary layer theory. The approach will start with the local one-dimensional dynamics on slopes, then consider their coupling to the large-scale context in simple two-dimensional examples, and subsequently extend to idealized and increasingly realistic three-dimensional setups. This will connect recent work on bottom-intensified mixing on slopes to scaling theories for the abyssal overturning that are based on ocean basins with a flat bottom and vertical sidewalls. These dynamics will be explored using a planetary geostrophic model, whose reduced dynamics allow for direct dynamical insight and controlled numerical solutions that resolve all features, including the all-important boundary layers on topographic slopes. 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.