Date:	Thu, March 28, 2019
Time:	14:15
Place:	Research I Meeting Room

Abstract: The energy transfers between balanced and unbalanced motions is discussed in context of the ocean and its energy cycle. Balanced motions, for instance mesoscale eddies generate unbalanced motions, such as internal gravity waves by spontaneous emission, among other processes. The exact mechanism of wave generation, how- ever, is not well understood and hence not well represented in ocean models. This is hindered to an extent by the challenge of separating the flow field into balanced and unbalanced motions. We achieve this separation using the non-linear initialization procedure proposed by Machenhauer in 1977. Results show that gravity wave emission becomes increasingly stronger going towards a \(\mathrm{Ro} = O(1)\) regime. The kinetic energy tied to the unbalanced mode scales close to \(\mathrm{Ro}^2\) with \(\mathrm{Ro}\) being the Rossby number. This \(\mathrm{Ro}^2\) scaling, however, is appropriate for the first order Machenhauer decomposition only. The scaling becomes \(\mathrm{Ro}^4\) for second order decomposition and so forth. Furthermore, internal gravity waves dissipate predominantly through small-scale dissipation, which emphasizes their role in the downscale energy transfer.

In additional experiments, we diagnose gravity wave emission in numerical simulations of lateral and vertical shear instabilities of geostrophically balanced flow, diagnosed up to fourth order in the Rossby number \(Ro\). While we find evidence for spontaneous gravity wave emission from balanced flow in a single layer model with large lateral shear and large Ro, a vertically resolved model with moderate velocity amplitudes appropriate to the interior ocean shows hardly any wave emission. This result suppresses the importance of spontaneous wave emission of balanced flow in the classical sense, but calls for the investigation of wave generation during frontogenesis creating strongly sloping isopycnal.