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JOURNAL OF GEOPHYSICAL RESEARCH, VOL. 108, NO. C3, 3089, doi:10.1029/2002JC001392, 2003

Interaction of lateral baroclinic forcing and turbulence in an estuary

Jessica R. Lacy

Coastal and Marine Geology, U.S. Geological Survey, Menlo Park, California, USA


Mark T. Stacey

Department of Civil and Environmental Engineering, University of California, Berkeley, California, USA


Jon R. Burau

California District, U.S. Geological Survey, Sacramento, California, USA


Stephen G. Monismith

Environmental Fluid Mechanics Laboratory, Department of Civil and Environmental Engineering, Stanford University, Stanford, California, USA


Abstract

Observations of density and velocity in a channel in northern San Francisco Bay show that the onset of vertical density stratification during flood tides is controlled by the balance between the cross-channel baroclinic pressure gradient and vertical mixing due to turbulence. Profiles of velocity, salinity, temperature, and suspended sediment concentration were measured in transects across Suisun Cutoff, in northern San Francisco Bay, on two days over the 12.5-hour tidal cycle. During flood tides an axial density front developed between fresher water flowing from the shallows of Grizzly Bay into the northern side of Suisun Cutoff and saltier water flowing up the channel. North of the front, transverse currents were driven by the lateral salinity gradient, with a top-to-bottom velocity difference greater than 30 cm/s. South of the front, the secondary circulation was weak, and along-channel velocities were greater than to the north. The gradient Richardson number shows that stratification was stable north of the front, while the water column was turbulently mixed south of the front. Time-series measurements of velocity and salinity demonstrate that the front develops during each tidal cycle. In estuaries, longitudinal dynamics predict less stratification during flood than ebb tides. These data show that stratification can develop during flood tides due to a lateral baroclinic pressure gradient in estuaries with complex bathymetry.

Published 20 March 2003.

Index Terms: 4235 Oceanography: General: Estuarine processes; 4568 Oceanography: Physical: Turbulence, diffusion, and mixing processes; 4528 Oceanography: Physical: Fronts and jets.

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Citation: Lacy, J. R., M. T. Stacey, J. R. Burau, and S. G. Monismith (2003), Interaction of lateral baroclinic forcing and turbulence in an estuary, J. Geophys. Res., 108(C3), 3089, doi:10.1029/2002JC001392.