Reports

Hi, my name is Thomas, I am a PhD Student in subproject L2 “The interior energy pathway: internal wave emission by quasi-balanced flows” at the Max-Planck-Institute for Meteorology. This project tries to answer the question to what extend spontaneous wave generation contributes to the ocean’s route to dissipation.

The ocean’s route to dissipation is not yet fully understood. Both the large-scale currents and the meso-scale eddies in the ocean are essentially balanced. Moreover, those eddies tend to transfer energy upward towards larger scales. So the question arises how the energy is transported from the large scales to the small scales, where the energy can be dissipated. Spontaneously emitted waves can be refracted by the eddying flow and captured later. Wave capture is a possible way to transfer energy to smaller scales. I am contributing to this question by identifying spontaneously emitted waves in an OGCM.

To identify spontaneously emitted waves in a more or less realistic setting I use a high resolution global ocean model. The model runs in two configurations: One is a realistic setting forced by 6-hourly atmospheric fluxes obtained by reanalysis data and a second one with temporally constant forcing. By comparing the two simulations I found wavelike structures which are not generated by external forcing but by the eddying flows itself. The properties of these structures identify them as gravity waves which are likely to be generated by spontaneous emission. Further analysis may show to what extend these waves contribute to the ocean’s route to dissipation.

I am a PhD student in the subproject L2 at Jacobs University Bremen under supervision of Prof. Marcel Oliver.

Our role in the subproject can be briefly explained as follows: In large-scale ocean models, the ocean circulations are essentially balanced; however, this balance breaks down in small scales due to spontaneous generation of inertia-gravity waves by quasi-balanced circulations, and waves are maybe re-captured in later times. This spontaneous emission and wave capture is considered to contribute to the energy transfer from the essentially balanced large-scale circulation and mesoscale eddy fields down to smaller scales, which is a route to dissipation.

To analyse the role of inertia-gravity waves in interior dissipation, a reasonable approach is to diagnose the inertia-gravity waves by splitting the flow field into balance and imbalance components, which are the ocean circulation and the inertia-gravity waves, respectively. This balance-imbalance decomposition can be achieved by some diagnostic tools such as linear time filters, balance relations, and optimal potential vorticity balance. In this project, we want to provide a new numerical algorithm to separate spontaneously generated imbalanced flows from the vertical flows depending on a prior work called „optimal balance“.

We are currently working on application of the optimal balance algorithm to the shallow water model and the primitive equations will follow. The “optimal balance” algorithm is interesting to us not only for practical aspects but also mathematical features, so that we extensively worked on asymptotics-preserving schemes on a finite dimensional model in the algorithm. There are several other theoretically open questions, which are standing for the algorithm as its existence and uniqueness, to be considered.