Predicting Ocean Vertical Transport via Surface Coherent Structures

Vertical transport in the upper ocean impacts the surface mixing, advection of nutrients, and the ocean energy budget. Direct observations of significant and sustained vertical transport is difficult because vertical velocities in the ocean are often orders of magnitude smaller than horizontal velocities. Measurements of the vertical velocity field are not readily available, but surface velocity fields can be obtained from HF radar, satellite altimetry, and modeled horizontal velocity fields. While Eulerian analysis of these potentially noisy fields may highlight instantaneous convergence zones, a Lagrangian coherent structures analysis is more robust to noisy observational data and model parameter uncertainty. We correlate surface coherent structures to vertical transport below the surface to evaluate their capacity to predict regions of strong vertical transport. In particular, we compute Lagrangian metrics like the finite-time Lyapunov exponent field from the surface velocity and compare this with the corresponding local vertical subduction. This correlation is tested on a high-fidelity simulation of a sheared submesoscale flow and an operational ocean forecast. The identification of coherent structures provides a target zone for anticipated vertical transport that could be observed via Lagrangian floats.