A description of subtidal pressure field observations on the Northern California Continental Shelf during the Coastal Ocean Dynamics Experiment

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Abstract
During the Coastal Ocean Dynamics Experiment (CODE) along the northern California continental shelf in the spring and summer of 1981 and 1982, the first comprehensive effort to obtain all of the observations necessary to infer pressure fluctuations throughout the water column was made. This is a description of the subtidal fluctuations in measured bottom pressure, internal pressures inferred from moored conductivity and temperature observations, and the water column total pressure anomalies computed from a combination of the two components. The pressure field during CODE 2 (April‐July 1982) is found to be more energetic than that in CODE 1 (April‐July 1981), with more than 70% total pressure variance concentrated at frequencies of less than (15 days)−1. The spatial structure of the pressure fluctuation field is characterized by coastal intensification and a tendency for poleward phase propagation. Like the pressures, most of the CODE across‐shelf pressure difference variance is found at the lower frequencies, with more than 60% of the total across‐shelf pressure difference variance at frequencies of less than (15 days)−1 The CODE across‐shelf pressure differences are found to be in geostrophic balance with both alongshelf currents and the total alongshelf transport. The alongshelf pressure difference variance is generally found to be concentrated at the lower frequencies much like that of the across‐shelf pressure differences. Most of the pressure and pressure difference variance is found to be highly coherent with wind stress. The 20‐day band pressure variability is apparently related to the response of the pressure field during so‐called “relaxation events.” The barotropic component of the alongshelf geostrophic transport (i.e., across‐shelf pressure difference) has been found to be almost perfectly coherent with regional wind stress τy and to propagate poleward within the CODE region at the same rate as τy and the alongshelf pressure gradient is very important in accelerating the alongshelf flow. The 5‐day band pressure variability in the CODE region appears to result from local wind stress fluctuations. The alongshelf pressure gradient was found to be of secondary importance relative to the wind in accelerating the alongshelf flow.