Near-surface ocean kinetic energy spectra and small scale intermittency from ship data in the Bay of Bengal

Horizontal currents in the Bay of Bengal were measured on eight cruises covering a total of 8600 $km$ using a 300 $kHz$ Acoustic Doppler Current Profiler (ADCP). The cruises are distributed over multiple seasons and regions of the Bay. Horizontal wavenumber spectra of these currents over depths of 12--54 m and wavelengths from 2--400 km were decomposed into vortical and divergent components assuming isotropy. An average of across and along track spectra over all cruises shows that the spectral slope of horizontal kinetic energy for wavelengths of 10--80 $km$ scales with an exponent of $-1.7 \pm 0.05$, which transitions to steeper scaling for wavelengths above 80 $km$. The rotational component is significantly larger than the divergent component at scales greater than 80 $km$, while the two are comparable for smaller wavelengths. The measurements show a fair amount of variability and spectral levels vary between cruises by about a factor of 5 over 10--100 $km$. Velocity differences over 10--80 $km$ show probability density functions and structure functions with stretched exponential behavior and anomalous scaling. Comparisons with the Garrett-Munk internal wave spectrum indicate that inertia-gravity waves account for only a small fraction of the kinetic energy except at the smallest scales. These constraints suggest that the near-surface flow in the Bay is primarily balanced and follows a forward enstrophy transfer quasigeostrophic regime for wavelengths greater than approximately 80 km, with a larger role for unbalanced rotating stratified turbulence and internal waves at smaller scales.

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