An internal tidal bore regime at nearshore stations along western U.S.A.: Predictable upwelling within the lunar cycle

Abstract

Nearshore upwelling due to predictable large internal bores may be a widespread phenomenon along the west coast of North America. Internal tidal bores (breaking internal tidal waves) cause drops in surface water temperature that last for 2–9 days. Negative surface water temperature anomalies (anomaly = daily datum minus day-of-the-year average) often reflect large internal tidal bores. These anomalies are predictable within the lunar cycle in spring and summer (but not in winter and fall) at a Southern California shore-station, the Scripps Institution of Oceanography Pier [Pineda (1991) Science, 253, 548–551]. The internal tidal bore hypothesis has been invoked to predict and explain this result: anomalies are predictable (on average) within the lunar cycle because of a lunar or spring-to-neap cycle in internal bore activity. The anomalies are more predictable in spring and summer than in fall and winter because internal waves are most energetic when the water column is well stratified. Long time series (18–64 years) of surface water temperature from 10 U.S. west coast shore stations from Oceanside in southern California to Neah Bay in Washington, were analyzed to evaluate the generality of the Scripps results. Nine stations showed that temperature anomalies are predictable on average within the lunar cycle in spring or summer, but not in fall and winter. There is high variability in the magnitude and variance of the average anomaly among stations. In general, for spring and summer, the most negative anomalies tend to occur on days 7–12 ( - ) and 19–24 ( - ) of the lunar cycle (day one being day after the new moon ▪). Few most-negative-anomalies occurred around new moon or full moon. The Farallon Islands station showed a more random distribution of anomalies within the lunar cycle during spring and summer.