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Center for Nonlinear Studies |
The general circulation of the oceans, including basin-filling gyres and energetic currents and eddies is thought to be largely determined by a 'balanced dynamics', in which the main force balance in the horizontal is between the pressure gradient and Coriolis forces. Superposed on this circulation is a variety of unbalanced motion, including near-inertial oscillations. These occur near the Coriolis frequency and are associated with relatively large horizontal and small vertical length scales. Because the geostrophic flow evolves on a slower time scale, the inertial modes are not generally thought to strongly influence its dynamics. This separation of time scales is, however, minimal in western boundary regions and other areas where the geostrophic flow is particularly energetic.Here we numerically consider the mid-latitude wind-driven ocean circulation problem in an idealized setting. Examples are given where the near-inertial modes can act as an important energy sink for the general circulation. In particular, we consider the unstratified hydrostatic equations, for which the vertically averaged flow is analogous to geostrophic flow in a stratified ocean, and for which the 3d modes correspond to inertial oscillations in the linear limit. When the 3d energy is held at a significant (but small) background level via an external forcing, 3d modes can serve to extract (or add) energy to the 2d flow. Possible implications for ocean dynamics in a more realistic setting are also discussed.
Host: Balu Nadiga, CCS-2, balu@beasley.lanl.gov