Three key issues motivate a systematic re-examination of coherent vortex structures in Lagrangian float data: (i) a pressing need for detailed comparison of eddy parameterization schemes, as well as of eddy-resolving numerical models, against in situ observations; (ii) the recent recognition, based on satellite altimetry, that a large swath of variability previously attributed to Rossby waves instead appears due to an energetic nonlinear mesoscale eddy field, the interior signature of which is as yet largely unknown; and (iii) an increasing appreciation of the important but multifaceted role coherent eddies play in fluid transport and mixing, including eddies at scales too small to be accurately resolved by altimetry. These issues all point to the need for a quantitative and integrated study of the coherent eddy field and its large-scale impacts. At the same time, turbulence studies emphasize that the current ability to understand the oceanic vortex field is hampered by the lack of an objective and theoretically compelling avenue for inferring Eulerian "truths" from available Lagrangian data - that is, the inability to currently perform a formal census of vortex properties and populations from the Lagrangian platform.
This project will quantify vortex characteristics, populations, and dynamical impacts across a broad range of scales using the historical set of deep Lagrangian acoustically tracked floats. A series of recent advances by the PI and collaborators make such a study possible, providing a new and particularly direct link between Lagrangian observations and vortex dynamics. The starting point for the study is the ability to extract, objectively and with uncertainty estimates, detailed time-varying aspects of coherent eddies from individual Lagrangian trajectories. The first phase of this project utilizes this method to describe vortex currents as they appear in the data. In this second phase, inhomogenous Lagrangian observations will be used to form estimates of underlying Eulerian fields, via statistical inference backed by extensive testing within idealized quasigeostrophic simulations. The goal is to describe the vortex distribution as a geographically-varying random field controlled by a small number of parameters. The third and final phase quantifies impacts of the vortex field on the large-scale flow - by estimating the velocity and vorticity distributions as well as particle and vorticity transport associated with the eddies.
Intellectual Merit: This study will substantially extend and refine our understanding of the role of coherent eddies in the ocean. It will be an important step towards the larger goal of understanding and quantifying the impact of small-scale "eddy" processes on the large-scale circulation, by isolating one of the processes that make up the general phenomenon of "eddy fluxes". Addressing the connection to observations from satellite altimetry is particularly timely. The construction and testing of a Lagrangian vortex census statistical methodology is itself an important contribution.
Broader Impacts: A primary societal impact of this work is its potential impact on our ability to predict long-term climate variability by immediately increasing our ability to assess the fidelity of both eddy-resolving and non-eddy-resolving numerical models against in situ data. The postdoctoral researcher working on the project will benefit from exposure to novel analysis techniques and an important Lagrangian data set. Value-added versions of the float dataset, and all analysis software developed during this project, will be freely distributed to the community as described in the attached Data Management Plan; these open a new window into ocean dynamics.
