L2: The interior energy pathway: internal wave emission by quasi-balanced flows

Horizontal velocity divergence (top) and geopotential at 130 mb on day 11 of the T126 model simulation. Vectors show the horizontal wind. Inertia-gravity waves are described by the velocity divergence. The quasi-balanced flows that emit the waves are described by the geopotential contours and wind vectors. From O'Sullivan and Dunkerton (1995).

Principal investigators: Prof. Jin-Song von Storch (Max Planck Institute for Meteorology), Prof. Gualtiero Badin (Universität Hamburg), Prof. Marcel Oliver (Jacobs University)

Both the large-scale currents and the meso-scale eddies in the ocean are essentially balanced, with geostrophic or gradient-wind balances in the horizontal and hydrostatic balance in the vertical. Moreover, meso-scale eddies tend to transfer energy upward towards larger scales. This situation points to the challenging question of what are the ocean's routes to dissipation that are needed for the interior general circulation, including its related eddy fi eld, to reach an equilibrium state in the presence of the constant atmospheric forcing. This subproject contributes to answering this question by exploring the route to dissipation via spontaneous wave generation: Eddying flows spontaneously emit internal waves. The waves, once generated, are refracted by the eddying flow, and may be captured later. Wave capture is characterised by an exponential increase in wavenumber and wave amplitude and an exponential decrease in the intrinsic group velocity. While capture is well understood theoretically for various types of flows, it is still open whether it occurs in the real ocean. By spontaneous emission and subsequent interaction with the emitted waves, energy is transferred from meso-scale eddies to smaller scales where instabilities and smallscale turbulence complete the downscale cascade to dissipation.

Quantifying this route to dissipation is central for the present CRC. Speci cally, this subproject aims
to address the following questions:

  • How important is the route to dissipation via spontaneous wave generation and wave capture?
  • What are the key factors that control the internal wave emission by quasi-balanced and
    eddying flows and the subsequent interaction between waves and flows?
  • Is it possible to formally characterise the internal waves emitted by a turbulent geostrophic flow?

We will address these questions using a combination of theory, conceptual models with idealised configurations based on rotating shallow water equations, and existing and specially designed numerical experiments with an ocean general circulation model based on the primitive equations.