Low-frequency waves in the Santa Barbara Channel observed by high-frequency radar

Three years of high-frequency radar observations in the Santa Barbara Channel reveal sequences of alternating cyclonic and anticyclonic vortices propagating westward with a period of approximately 2 weeks. The sequences last up to a few months and occur intermittently throughout the year. The velocity distributions of cyclones and anticyclones are antisymmetric with a mean propagation speed of similar to5 km d(-1) and relative vorticity of order 0.1 f. Complex empirical orthogonal function decomposition in the 10-20 day passband partitions variance into two dominant modes. The first mode represents the vortices with an average period of 14.4 days. The second mode represents alongshore fluctuations with an average period of 13.3 days. Amplitude functions of the two modes correlate with in situ current time series at 5 and 45 m depths obtained from moorings at the east and west channel entrances, suggesting coupling with the larger-scale circulation of the northern Southern California Bight. The observations support interpretation of mode 2 as a coastal trapped wave since the period and alongshore flow are consistent with previous observations and predictions. We hypothesize that mode 1 is a resonant response of the Santa Barbara Basin in the form of a trapped topographic Rossby mode. The spatial mode resembles the predicted pattern based on a simplified analytical model of the fundamental mode. The amplitude of mode 1 is consistently large when the frequencies of modes 1 and 2 converge, suggesting that the vortices are a basin-scale resonant response to coastal trapped waves.