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

Abstract

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.

Spatiotemporal Variability of the Global Ocean Internal Processes Inferred from Satellite Observations

“…By diagnosing the spectral kinetic energy (KE) flux in the South Pacific Ocean from satellite altimetry measurements, Scott and Wang (2005) found that the oceanic KE is extracted at small spatial scales near the Rossby deformation radius, and transferred back to larger scales through an inverse/upscale KE cascade, consistent with the classical quasi-two-dimensional turbulence theory (e.g., Charney 1971;Salmon 1980). Such inverse KE cascades in the wavenumber space are later confirmed in a vast number of follow-up studies using observations and numerical models in different parts of the World Ocean (Schlösser and Eden 2007;Scott and Arbic 2007;Capet et al 2008;Klein et al 2008;Tulloch et al 2011;Venaille et al 2011;Wang et al 2015;Sasaki et al 2017;Khatri et al 2018). Recently, Arbic et al (2012Arbic et al ( , 2014 extended the spectral analysis to the wavenumberfrequency domains, and demonstrated the existence of a temporal upscale cascade of surface oceanic KE.…”

Diagnosing Cross‐Scale Kinetic Energy Exchanges From Two Submesoscale Permitting Ocean Models

“…The results from Scott and Arbic (2007) are consistent with the proposed modification to geostrophic turbulence by Scott and Wang (2005). More recent literature (Schlösser & Eden, 2007;Sasaki et al, 2017;Tulloch et al, 2011;Aluie et al, 2017;Brüggemann & Eden, 2015;Kjellsson & Zanna, 2017;Khatri et al, 2018) have also shown that an inverse cascade of energy mostly dominates the surface ocean at scales larger than R d .…”

Seasonality of surface stirring by geostrophic flows in the Bay of Bengal

“…While we anticipate exponential separation in an enstrophy cascading regime, it should be remembered that this is under the assumption of an inertial range with a constant enstrophy flux [47]. Computation of the enstrophy flux using altimeter data does show a dominant forward enstrophy regime at these scales, but the enstrophy flux is not constant, and is actually scale dependent [39]. Therefore, the powerlaw growth of RD is not inconsistent with a forward enstrophy transfer regime.…”