Eddy Vertical Structure and Variability: vortex evolution and the geography of geostrophic turbulence
Steinberg, Jacob Michael
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Glider observations of submesoscale and mesoscale eddy structure, behavior, and evolution are analyzed revealing both processes of eddy decay and statistical properties of geostrophic turbulence (i.e. mesoscale eddy-eddy interactions resulting in energy transfer across scales). These observations, made by Seaglider and Deepglider autonomous underwater vehicles, and their interpretation are split into three sections: 1) the observed evolution and decay of a single California Undercurrent eddy, 2) sampling platform biases and aliasing diagnosed via simulated glider sampling in high resolution ocean models, and 3) observations of geostrophic turbulence from eddy vertical structure in the North Atlantic (analysis of hundreds of full-depth profiles of isopycnal vertical displacement and geostrophic velocity). Three Seagliders, deployed off of the Washington coast in 2013, located a subsurface intensified California Undercurrent eddy completing repeat eddy-bisecting transects over a three month period. These transects were used to create and compare `snapshots' of eddy depth-radial structure at different times. The observed evolution of this eddy can be explained by both adjustment to new background conditions, weaker stratification at the eddy core depth and translation multiple degrees northward, and decay via lateral thermohaline intrusions. Eddy total mechanical energy, salt content, and the magnitude of the core potential vorticity anomaly all decreased while core spice and dissolved oxygen variance increased tenfold. While these coherent eddies are capable of translating hundreds of kilometers away from their formation sites, both adjustment and decay appear as relevant processes moderating their behavior. The idealized simulation of full-depth glider sampling in two high resolution ocean models was then carried out to quantify the accuracy of glider derived profiles of isopycnal vertical displacement and geostrophic velocity. These profiles are interpreted in the context of mesoscale eddy structure, interactions, and energetics. Glider vertical velocity, glide slope, and instrument noise were varied while simulating the sampling of hundreds of profiles in ROMS and HYCOM numerical simulations. The error in glider derived geostrophic velocity profiles, calculated as the difference from model velocity fields, is found to be on average less than 0.02 m/s. Projection of both model and glider derived profiles of displacement and velocity onto normal modes reveals the glider sampling framework to be capable of resolving model eddy vertical structure from the first through at least the tenth baroclinic mode, with increased resolution for steeper glide slopes. This glider analysis framework was then employed to explore eddy vertical structure from hundreds of full-depth temperature and salinity profiles completed by Deepgliders in the North Atlantic between 2014 and 2019. Gliders were deployed and piloted to five sites, each associated with varied levels of mesoscale eddy energy. At each site, gliders completed multi-month missions, the longest nearly 9 months, to observe eddy vertical structure and variability. Full-depth profiles permit the projection of displacement and velocity profiles onto dynamical modes and calculation of vertical wavenumber spectra of eddy potential and kinetic energy. These spectra, with a dominant fraction of energy in the first baroclinic mode, follow a linear log-log slope not different than the k^(-3) slope predicted by Charney  and reveal potential energy to dominate kinetic at modes three and higher. Displacements of order 100 m and velocities exceeding 0.05 m/s at depths greater than 2000 m were regularly observed suggesting bottom friction to be dynamically relevant in the dissipation of eddy kinetic energy. Subsurface intensified coherent vortices with core depths greater than 1000 m were also observed in multiple years with full-depth structure noticeable at the surface in concurrent altimetric observations and at the seafloor with order 100 m displacements. Away from sites of rough topography and boundary currents these full depth observations of eddy vertical structure, collected on a near-daily basis for months at a time, reveal geostrophic turbulence theory to well describe low mode mesoscale eddy energetics and evolution.
- Oceanography