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SUMMARY:2d-3d energy exchanges in a homogeneous thin-aspect-ratio ocean - 
 Straub\, D (McGill)
DTSTART:20081212T120000Z
DTEND:20081212T123000Z
UID:TALK15634@talks.cam.ac.uk
CONTACT:Mustapha Amrani
DESCRIPTION:Because of its thin aspect ratio\, large scale ocean circulati
 on is generally assumed to obey hydrostatic dynamics. In the idealized cas
 e of a homogeneous (unstratified) fluid\, this implies that the horizontal
  pressure gradient force is independent of depth. Homogeneous models of oc
 ean circulation typically assume the flow to be two dimensional. However\,
  it can be shown that such a two dimensional flow is unstable to 3d pertur
 bations. Following saturation of the 3d perturbations\, a 2d-to-3d energy 
 transfer persists. After a brief description of the instability mechanism\
 , we i) test its robustness to nonhydrostatic effects and ii) address its 
 potential role in the damping of midlatitude ocean gyres.\n\nInitially 2d 
 turbulence in a Boussinesq fluid is shown numerically to be unstable to 3d
  perturbations. Although non-hydrostatic pressure effects act to slow the 
 growth\, a robust growth in nonetheless observed even at unit aspect ratio
 . The perturbations are found to saturate at a level proportional to the d
 omain aspect ratio. Thus\, in the thin aspect ratio limit\, nearly 2d dyna
 mics are recovered --- provided the 3d modes are not externally forced.\n\
 nWe then consider the classic wind-driven ocean gyre problem for a homogen
 eous thin aspect ratio fluid on a beta plane. A large-scale stochastic for
 cing is applied to the 3d modes. This produces a background sea of near-in
 ertial oscillations which interact with the depth averaged flow. It is fou
 nd that 2d-to-3d energy transfers coupled with a forward cascade of 3d ene
 rgy provides a robust dissipation mechanism for the gyres. We speculate th
 at similar dynamics may also apply in a stratified ocean. 
LOCATION:Seminar Room 1\, Newton Institute
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