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Dynamics of the Bottom Boundary Layer on the Northern California Shelf

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Final rept.

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Time-series measurements of velocity, temperature and conductivity on the northern California shelf during two winter seasons permit an observational test, in vertically integrated form, of a simple set of sub-inertial momentum and heat balances for the bottom boundary layer, which have resulted from recent theoretical work. These are 1 an along-isobath momentum equation that reduces to a classical Ekman balance 2 a cross isobath momentum equation in which the Ekman balance is modified by a buoyancy force caused by distortion of the isopycnal surfaces within the boundary layer and 3 a heat balance in which variability of temperature is produced by cross isobath advection. The measurements confirm the importance of buoyancy in the cross isobath momentum equation, and, as has recently been predicted theoretically, they indicate that buoyancy is a dominant effect when the boundary layer is thick, which typically occurs during downwelling-favorable flows. An Ekman balance describes sub inertial fluctuations in the along-isobath momentum equation with moderate success. The mean along-isobath momentum equation is significantly influenced by a buoyancy force caused by an along-isobath temperature gradient, and along-isobath advection is as important as cross isobath advection in the heat balance. Thus along-isobath variability of temperature, which has been neglected in most theoretical work, is fundamental to the structure of the velocity field and the evolution of the temperature field. The most striking feature of the measurements is a persistent cross isobath mean flow, which is offshore below 20 m above bottom, onshore above 20 m, and uniform above 30 m. The transport produced by this flow, relative to the unsheared flow above 30 m, is much larger than the Ekman transport associated with the mean along-isobath bottom stress.

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  • Physical and Dynamic Oceanography

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