Interpreting Energy and Tracer Spectra of Submesoscale Turbulence

The goal of this presentation is to use observations of the submesoscale upper ocean to test geostrophic turbulence theories as well as to detect signatures of ageostrophic classes of motion like internal waves. Observations from two different regions are used: the Oleander data set from the Gulf Stream region and the Spice data set from the eastern North Pacific in the quiescent subtropical gyre. In the Gulf Stream region, we find steep kinetic-energy spectra at scales between 200 km and 20 km, consistent with predictions of interior quasi-geostrophic turbulence theory. At scales below 20 km, the spectra suggest that internal waves dominate the energy. This extends and confirms previous analysis of the Oleander data set. In the subtropical North Pacific, the energy spectra are flatter, but inconsistent with predictions of both interior and surface quasi-geostrophic turbulence theories. Ageostrophic flows, most likely internal tides, may contribute significantly to the energy throughout the submesoscales. Even more surprisingly, spectra of temperature variance along isopycnals, where internal waves are essentially filtered out, are also inconsistent with predictions of geostrophic-turbulence theories. These discrepancies suggest that these theories may apply to the upper ocean only in strong currents. Reasons for the failure of geostrophic-turbulence theories away from strong currents will be discussed.