Surface-following buoys are widely used to collect routine ocean wave measurements. While accelerometer and tilt sensors have been used for decades to measure the wave-induced buoy displacements, alternative global positioning system (GPS) sensor packages have been introduced recently that are generally smaller, less expensive, and do not require calibration. In this study, the capabilities of several GPS sensors are evaluated with field observations in wind-sea and swell conditions off the California coast. The GPS buoys used in this study include Datawell Directional Waverider and Mini Directional Waverider buoys equipped with a specialized GPS Doppler shift sensor, and a low-cost experimental drifter equipped with an ''off the shelf'' GPS receiver for absolute position tracking. Various GPS position receivers were attached to the Waverider buoys to evaluate their potential use in low-cost wave-resolving drifters. Intercomparisons between the Datawell GPS-based buoys, the experimental GPS drifter, and a conventional Datawell buoy with an accelerometer-tilt-compass sensor package, show good agreement in estimates of wave frequency and direction spectra. Despite the limited (several meters) absolute accuracy of the GPS position receivers, the horizontal wave orbital displacements are accurately resolved, even in benign (significant wave height less than 1 m) swell conditions. Vertical sea surface displacements were not well resolved by the GPS position receivers with built-in or small patch antennas, but accurately measured when an external precision antenna was attached to the drifter. Overall, the field tests show excellent agreement between Datawell buoys using GPS and motion-sensor packages, and demonstrate the feasibility of observing ocean surface waves with lowcost GPS-tracked drifters.

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Physical and biological fields in the coastal transition zone off northern California were measured during February, March, May and June 1987 in an extended alongshore region between 60 km and 150 km offshore. The spring transition, as seen in coastal sea level and winds, occurred in mid‐March. Surface variability during the two spring cruises was stronger and of larger scale than that seen during the two winter cruises. An equatorward‐tending current, flowing along the boundary between low steric sea level inshore and high steric sea level offshore, dominated both the directly‐measured (acoustic Doppler current profiler) and geostrophic current fields during spring. Current jets of comparable strength directed both offshore and onshore were seen off Cape Mendocino and Point Arena; these evolved significantly in the 3 weeks between cruises. Inshore of the current, properties associated with upwelled water were found near the surface, including low temperature and high salinity, nutrients and chlorophyll; offshore of the current, waters were warmer, less saline, lower in nutrients and more oligotrophic. Geostrophic and directly measured volume transports in the current were about 2–3 Sv. Isopycnals inshore of the spring upwelling front were displaced vertically by O(40–80 m) from their depths during the winter survey; these displacements extended deep into the water column and were largely independent of depth between 100 and 400 m. Surface mixed layers tended to be deep in winter and shallower inshore of the upwelling front in spring. A connection between the equatorward‐tending frontal jet off northern California and the more well‐studied California Current further south is suggested by the similarity of their transports and of their dynamic height values.

Shipboard conductivity‐temperature‐depth, acoustic Doppler current profiler, and continuous temperature and salinity observations were made in the coastal transition zone off Point Arena, California (39°N, 124°W), during June 15–28, 1987, to describe the hydrographic structure and velocity fields associated with the cold filaments found there. An adaptive sampling plan was used to measure the properties of these filaments, guided in real time by satellite Advanced Very High Resolution Radiometer sea surface temperature imagery and feedback from the in situ sensors. The primary feature observed was a large, cool (12.0°–13.5°C), salty (32.7–33.0 psu) filament which extended over 200 km offshore from Point Arena and exceeded 500 m depth. This feature was bounded in the offshore direction by a continuous equatorward meander, with offshore velocities (60–87 cm s−1) on the northern edge of the cool filament and onshore velocities (69–92 cm s−1) along the southern edge, and persisted for at least 3 weeks. A second feature was advected into the study area from the north by an anticyclonic eddy offshore and later merged with the Point Arena filament. Smaller (30 km wide by 50–100 km long by 50–100 m deep) very cold (10.0°–12.0°C) high salinity (>33.0 practical salinity units (psu)) features were observed within the Point Arena filament, but persisted for only 6–10 days. The net volume transport of the larger feature was offshore at ∼3 × 106 m3 s−1 and suggests it was fed by an inflow to the region from the north. The smaller features were correlated with bursts of equatorward wind stress on an event by event basis but not with times of large‐scale wind stress convergence. Surface drifters deployed during the experiment closely followed the surface dynamic topography. Some of the drifters followed the path of the offshore meander, while others moved south inshore.

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