Oceanic Geostrophic Turbulence Inferred From Vertical Structure Observations

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Appreciation that the ocean is full of eddies on a vast array of scales began with the discovery of what are commonly referred to as mesoscale eddies: those with horizontal scales of order tens to about 100 km. These eddies are geostrophic (a state where the pressure gradient force from density gradients is in balance with the Coriolis effect from the rotation of the earth) in contrast to meteorological eddies like tornadoes which are significantly ageostrophic. Because they are the most energetic features of oceanic motion, understanding how these geostrophic eddies work is key to understanding and predicting circulation, the distribution of properties, and biomass in the ocean. Testing theories of geostrophic turbulence will stimulate new work to better understand oceanic states in the past and to predict their future. Laws for the dependence of energy wavenumber spectra implied by geostrophic turbulence were proposed on theoretical grounds half a century ago. These distinguished between an inverse cascade transferring energy to larger scale and a forward cascade transferring enstrophy (the energy of rotating motions) to shorter scale. While the existence of an enstrophy cascade has been identified in spectra of trophospheric wind observations, evidence supporting such a cascade in the ocean is scant and contradictory. Recent observations of the depth structure of current and vertical displacement at the Bermuda Atlantic Time Series (BATS) site seem to confirm the scaling predictions for both the inverse energy and forward enstrophy cascade portions of the wavenumber spectrum. These preliminary observations will be extended and generalized by sampling four new sites with distinct eddy energy and latitude regimes. This project will train a graduate student in the use of the new glider technology and join the cadre of young scientists being trained to use autonomous platforms for oceanographic research.The initial observations at the Bermuda Atlantic Time Series (BATS) site show potential energy exceeds kinetic energy by more than an order of magnitude in the steeper portion of the spectra, implying vortex stretching dominates relative vorticity in potential vorticity fluctuations in the enstrophy cascade. Temporal variations on scales of several weeks suggest that the transition wavenumber between the two slope dependences varies as the inverse cube root of vertical wavenumber which nearly preserves enstrophy regardless of eddy energy. The goal of this project is to test if: 1) energy and enstrophy transfer portions of the wavenumber spectrum of geostrophic turbulence are a general feature of ocean eddies; 2) potential energy consistently exceeds kinetic energy in the oceanic enstrophy cascade wavenumber range; and 3) enstrophy for geostrophic eddies is self-limited to modest Rossby number. To accomplish these goals, Deepglider repeat survey missions in distinctly different eddy energy and latitude regimes in both the highly energetic Northwest Atlantic and the eddy desert of the Gulf of Alaska in the Northeast Pacific will be conducted. Missions will rely on chartered small boats or, in one case, on ancillary use of a large vessel. The approach at each site is to survey from surface to bottom a roughly 100 km by 100 km region of ocean along a common track every three weeks or so for a year. By comparing estimates from BATS and four new sites, at least a preliminary climatology of the geostrophic eddy inverse energy and forward enstrophy cascades will be created. These calculations will provide rich targets for new theories and numerical models of the ocean.