Dynamics of the Bottom Boundary Layer on the Northern California Shelf

Abstract

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 subinertial 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 classic 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 subinertial fluctuations in the along-isobath momentum equation with only moderate success. In contrast to idealizations made in most theoretical work, a buoyancy force caused by an along-isobath temperature gradient is as important as bottom stress in the mean along-isobath momentum equation, and along-isobath advection is as important as cross-isobath advection in the heat balance.