In shallow nearshore areas of continental shelves, water temperature and ocean circulation fluctuations have large impacts on coastal ecosystems. Previous studies show substantial fluctuations in sea level, along-shelf velocity, and water temperature along the West Coast of North America are due to coastal-trapped waves. Coastal-trapped waves are very different from the waves beachgoers see. The waves visible at the beach are surface gravity waves, which have wavelengths of ~1-100 meters and wave periods of a few seconds, and the restoring force in that case is gravity. In contrast, coastal-trapped waves can have wavelengths of thousands of kilometers, wave periods of days to weeks, and in this case the restoring force is partly due to the Earth’s rotation (i.e. the Coriolis force). Coastal-trapped waves change sea level along the coast by ~1-10 cm and drive currents on the continental shelf. Coastal-trapped waves are mainly driven by wind fluctuations, and are a mechanism by which winds in one location influence the coastal ocean in other locations thousands of kilometers away. For example, wind fluctuations off Baja California in Mexico have been observed to cause transport of cold water into the nearshore Southern California Bight. This kind of cold water supply may bring nutrients to kelp forests in Marine Protected Areas, including along the mainland Santa Barbara Channel and around the nearby Northern Channel Islands, which have been referred to as America’s Galapagos for their high species diversity. 

This research project was designed to determine the effect of wind fluctuations in Baja California on the ocean near Point Conception, California, U.S.A. In particular, we studied the nearshore pressure, water velocity, water temperature, and stratification - the vertical “layering” of warm to cold water from the surface to the seafloor. We found that the variations in pressure, velocity, and temperature on time scales of 10-25 days are highly coherent across the entire Santa Barbara Channel and around the Northern Channel Islands. These fluctuations are best correlated not with local winds, but with winds off Baja California, hundreds of kilometers to the south. The lag time for a pressure, velocity, temperature, or stratification fluctuation to appear in the Santa Barbara Channel after a wind event off Baja is ~2.5 days, consistent with existing models of coastal-trapped wave speeds. This indicates that on time scales of several days to several weeks, winds in Mexico affect what happens to the water temperature off Southern California.

We were also interested in the previously published suggestion that coastal-trapped waves could affect the frequency and strength of cold-water supply by tides to kelp forests. To determine this, we first had to characterize these cold-water events, or “internal tides.” We found that most of the cold water events at kelp forests in Marine Protected Areas in the Santa Barbara Channel and around the Northern Channel Islands arrive either every ~12 hours (semidiurnal) or every ~24 hours (diurnal), depending on the site. We determined that the diurnal internal tidal temperature changes were in phase with the local wind forcing and consistent over the 24 mooring sites in the study area. This suggests that these diurnal temperature oscillations are dominantly forced by local diurnal winds, i.e. the seabreeze, and not dominated by oceanic tidal forcing. In contrast, the semidiurnal temperature oscillations vary in their phasing over the study area. The semidiurnal internal tides are also much stronger at the Channel Islands than along the mainland, consistent with an existing numerical model suggesting internal tides are generated offshore of the Islands by the surface, or barotropic, tide. We found that the water temperature, the vertical stratificatio...

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