Detecting upwelling along the central coast of California during an El Niño year using HF-radar

Surface currents in the vicinity of Granite Canyon, California (36°25.9′N, 121°55.0′W), were measured hourly using HF-radar in 1990-1992. The 1990 data revealed the M₂ and S₂ semi-diurnal tidal constituents. These high frequency components were removed from 6-month records taken during part of the upwelling season of 1991 and 1992. Daily and weekly variations in current speed and direction were generally similar in 1991 and 1992 even though 1992 was an El Niño year. Correlation analysis revealed that in both years daily and weekly variation in currents were mostly explained by corresponding changes in the alongshore component of the wind stress, indicating the effects of coastal upwelling. Variation in sea surface temperatures adjacent to the coast were correlated with the currents generated by coastal upwelling in 1991, but not in 1992. These observations are consistent with the hypothesis that during an El Niño event the water is upwelled to the surface from above a depressed thermocline. In 1992, at Granite Canyon, normal coastal upwelling was superimposed upon the El Niño event of that year.