Decomposing kinetic energy along Line P in the Pacific Ocean




Wang, Manman

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The upper ocean is host to overlapping vortical and internal waves dynamics over the submesoscales (10-100km), both of which are poorly represented in eddy-resolving ocean models. We analyze upper-ocean (0-200 m) horizontal-wavenumber spectra along Line P in the North Pacific subpolar gyre from shipboard ADCP measurements in February and June (2013-2015), and compare them to spectra from a 1/36th degree numerical simulation output. At scales between 10 and 100 km, the ADCP along-track (Cv) and across-track (Cv) kinetic energy spectra approximately follow power laws of k-2 and have a ratio R = Cv/Cu - 1. For purely non-divergent motions, the order of the power law and R should be the same, so divergent motions are evident. A Helmholtz decomposition estimates the fraction total kinetic energy that is contributed by internal-wave and vortex components. Vortex components follow a power law of k-2 with ratio R-2, consistent with predictions for a non-divergent flow, while internal waves are mostly consistent with the Garrett and Munk internal wave model. There are modest seasonal changes; vortical motions are slightly stronger in February than in June, whereas the amplitudes of the internal wave component increases in June. Depth variability of non-divergent vortical flows shows that at low wave-numbers energy decreases and that the kinetic energy spectra are bluer with depth, inconsistent with predictions from surface quasi-geostrophic theory of redder spectra with depth. Conversely, in the simulation the depth variability of the decomposed vortex components is in agreement with predictions of surface quasi-geostrophic theory. The simulations had very weak internal waves fields.



Kinetic energy spectra, Line P, decomposition