Mesoscale to submesoscale wavenumber spectra in Drake Passage

This study discusses the upper-ocean (0‐200m) horizontal wavenumber spectra in the Drake Passage from 13yr of shipboard ADCP measurements, altimeter data, and a high-resolution numerical simulation. At scales between 10 and 200km, the ADCP kinetic energy spectra approximately follow a k 23 power law. The observedflows are more energetic at the surface, but the shape of the kinetic energy spectra is independent of depth. These characteristics resemble predictions of isotropic interior quasigeostrophic turbulence. The ratio of across-track to along-track kinetic energy spectra, however, significantly departs from the expectation of isotropic interior quasigeostrophic turbulence. The inconsistency is dramatic at scales smaller than 40km. A HelmholtzdecompositionoftheADCPspectraandanalysesofsyntheticandnumericalmodeldatashowthat horizontally divergent, ageostrophic flows account for the discrepancy between the observed spectra and predictions of isotropic interior quasigeostrophic turbulence. In Drake Passage, ageostrophic motions appear to be dominated by inertia‐gravity waves and account for about half of the near-surface kinetic energy at scales between 10 and 40km. Model results indicate that ageostrophic flows imprint on the sea surface, accounting for about half of the sea surface height variance between 10 and 40km.

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