Observations of Eddy Vertical Structure Variability in the North Atlantic and Energy Partitioning Across Vertical Modes

Observations of full depth mesoscale eddy vertical structure at five sites in the North Atlantic show spatial and seasonal variability in vertical wavenumber spectra of potential and kinetic energy, suggesting variability in kinetic energy pathways and routes to dissipation. Over the past five years, hundreds of full depth temperature and salinity profiles were collected by multiple Deepglider autonomous underwater vehicles on a near daily basis during multi-month missions at sites associated with varied levels of mesoscale eddy kinetic energy. Observations reveal eddy isopycnal vertical displacement and geostrophic velocity structure spanning the entire water column, with deep displacements often exceeding 200 m. Empirical Orthogonal Functions of geostrophic velocity profiles compare well with the vertical structures of flat-bottom, free-surface normal modes. These normal modes, and for comparison those altered by a sloping bottom and/or mean shear, were projected onto profiles of isopycnal vertical displacement and geostrophic velocity to estimate the modal partition of potential and kinetic energy. Vertical wavenumber spectra of potential and kinetic energy show spatial and seasonal variability in low mode energy, but at higher modes contain the k^-3 slope associated with the forward enstrophy cascade predicted by geostrophic turbulence theory. Exceptions include profiles made in less energetic regions where internal wave variability dominates. Simulations of glider slant profiling in high resolution ocean models and comparison between glider derived and model eddy vertical structure show that glider sampling can accurately resolve barotropic and baroclinic mesoscale eddy vertical structure through at least the tenth baroclinic mode.