Surface Ocean Enstrophy, Kinetic Energy Fluxes, and Spectra From Satellite Altimetry

Spectra and fluxes of enstrophy and kinetic energy (KE) are estimated in different parts of the midlatitudinal oceans using geostrophic currents derived from altimetry data. The presence of a strong inverse flux of surface KE is confirmed at scales larger than approximately 200 km, whereas a robust enstrophy cascading regime, accompanied by an approximate k(-3) KE spectrum, is observed from about 200 to 100 km. The character of fluxes and spectra is shown to compare favorably with those from a comprehensive Earth system model. In addition, as gridded altimeter data are affected by smoothening and interpolation, the qualitative robustness of the results is verified by sensitivity experiments using space and time-filtered output from the Earth system model. Given the rotational character of the flow, this large-scale inverse KE and smaller-scale forward enstrophy transfer scenario is consistent with expectations from three-dimensional rapidly rotating and strongly stratified turbulence studies as well as detailed analyses of spectra and fluxes in the upper-level midlatitude troposphere. In further accord with results from the atmosphere, decomposing the currents into stationary and eddy components (demarcated here by variability greater and less than 100 days, respectively), it is seen that, in addition to the eddy-eddy contribution, the stationary-eddy and stationary-stationary fluxes play a significant role in the inverse (forward) flux of KE (enstrophy). Thus, it is quite possible that, from about 200 to 100 km, the altimeter is capturing the rotationally dominated portion of a surface oceanic counterpart of the upper tropospheric Nastrom-Gage spectrum.