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The response of convection driven by surface buoyancy fluxes to surface mechanical forcing

  • Author(s): Matusik, Katarzyna Ewa
  • et al.
Abstract

Experiments have been performed to gain insight into the response of a surface buoyancy flux-driven circulation to a localized input of mechanical energy. The mechanical energy is provided via a surface stress imposed along the same boundary as the buoyancy forcing. Both the case of the surface stress running concurrently to the sense of the circulation imposed by the buoyancy forcing as well as against it are explored. The nondimensional ratio of mechanical to buoyancy forcing, S[delta]f, is used to distinguish between distinct regimes that exist for each experiment set, and is maintained at O(1). In the case where surface buoyancy forcing and surface stress support a circulation in the same direction on the upper boundary, the overarching result is that the surface stress induces small-scale turbulence once the shear is strong enough to overturn the local stratification, corresponding to the threshold of S[delta]f > 0.42. The experiments are compared to numerical results of the recycling box model of Hughes et al. (2007), with varying mixing in the vertical. Experiment measurements agree satisfactorily with theory, and outline the response of the system to various mixing depths. If the stress is in competition with the buoyancy forcing on the upper boundary, experiments reveal a suite of regimes in S[delta]f in which the flow response varies. Briefly, full-depth buoyancy-driven convection persists for S[delta]f < 0.20, where the surface stress acts passively on the underlying circulation. For 0.20 < S[delta]f < 0.50, a competitive regime develops in which the flow periodically adjusts to the rivalry of the two forces by altering both the thickness of the boundary layer and the strength of the circulation. Once mixing develops for S[delta]f ≥ 0.50, the boundary layer flow competes strongly with the buoyancy-driven circulation, which is made shallow through an input of stabilizing buoyancy flux into the plume via mixing. For S[delta]f > 0.87, a two-cell circulation develops, featuring a shallow stress-driven cell overlying a deeper buoyancy-driven cell. The experiment results are applicable in the oceanographic context to imposed wind stress on the ocean surface simultaneously subject to surface buoyancy fluxes

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