Bottom water renewal in the Santa Barbara Basin

Abstract

We use quarterly full water depth occupations of a station at the center of the Santa Barbara Basin to estimate the timing, frequency, and magnitude of basin renewal events over the period 1986–2000. Of 58 occupations we have identified 11 events (in 11 of the 15 years) that are highly suggestive of recent bottom water renewal, with periods of annual flushing interspersed with relatively stagnant periods. All but one of these events occurred in winter or spring, with the spring events having the largest changes in bottom water properties (cooling and oxygenation). The event recorded in April 1998, during a period of rapid transition from El Niño‐influenced to La Niña‐influenced local conditions, revealed the potential magnitude of a full basin flushing: an increase in bottom water oxygen content of more than 0.4 mL/L and a cooling of nearly half a degree over 2 months. Water property profiles and near‐surface circulation patterns suggest that sufficiently dense water at the western sill, combined with strong flow into the basin from the northwest, may be key prerequisites to renewal. Interannual variability in the strength, duration, and timing of coastal upwelling and in regional circulation and hydrography, forced largely by the local impact of El Niño events, modulates the flushing potential of the basin. The Santa Barbara Basin apparently does not get flushed every year, and renewal events are not restricted to winter‐spring. Since the frequency and intensity of basin renewal events can greatly impact local denitrification rates, benthic community structure, and underlying sedimentary processes, we recommend that paleoceanographic interpretations of the local sediment record take account of this seasonally and interannually varying cycle of bottom water renewal.