Where is the origin of the abyssal circulation?

This project will quantify volume, heat and salt transports by the mid-depth and abyssal circulations using approaches that are complementary to classical water mass analysis of hydrographic data. Virtual Lagrangian parcels will be tracked, primarily backward-in-time, using the three-dimensional velocity fields of two ocean state estimates: Estimating the Circulation and Climate of the Ocean (ECCO) and Southern Ocean State Estimate (SOSE). Lagrangian analysis provides a three-dimensional view of the circulation that captures the connections between the main cells of the meridional overturning circulation – the abyssal and mid-depth circulations. Lagrangian analysis will be used to isolate volume, heat and salt transport by individual components of the circulation. The northward branch of the abyssal circulation will be quantified by dynamical and tracer methods. Preliminary work has identified two apparently disconnected components of the abyssal circulation: a lower cell that extends into the abyssal basins, primarily in the Indo-Pacic, and a subpolar cell localized to latitudes poleward of 60S, associated with the prominent wind-driven subpolar gyres in the Weddell and Ross Seas. The innovation in the proposed research is to apply the powerful tools of Lagrangian analysis to two state-of-the-art estimates of the ocean circulation. Through unambiguous and reproducible conditional sampling, Lagrangian analysis can quantify the transport of mass, heat and salt by individual components of the overturning cells, isolating physical processes responsibile for these transports. The Lagrangian estimates of individual transports will be compared to approximations using Eulerian data. Additionally, the insight from the Lagrangian estimates will be used to investigate improvements to the more economical Eulerian approximations commonly used. The distributions of transit times between selected injection points in the Southern Ocean and target abyssal regions will be examined and these transit-time-distributions will be used to describe the timescales over which the abyssal ocean adjusts to surface changes and sequesters surface properties. 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.