Temporal variability in currents and the benthic boundary layer at an abyssal station off central California

Current meter records from seven sequential moorings were collected during 1993–1996 at an abyssal station off central California (Sta. M; 4100 m depth). The entire 214-yr time series of the flow at 600, 50, and 2.5 m above bottom (above, near the top of, and within the benthic boundary layer) were analyzed for mean flow statistics and subtidal and tidal variability. The mean current vectors at each altitude were southward at <1 cm s-1, although not significantly different from zero. Spectral analysis revealed energetic mesoscale and seasonal oscillations in the flow, indicated by peaks in principal axis magnitudes at periods of 50–175 days. Tidal (mainly semidiurnal) and inertial forcing contributed to flow variability and influenced the mean principal axis orientation calculated for each record. The highest flow speeds were recorded at 50 m above bottom (with an average of 3.8 cm s-1; maximum 18.2 cm s-1). An examination of monthly-averaged current vectors at this altitude revealed that flow was to the south during the months of highest flow speed (April 1994 and 1995 and October 1994) and to the north and west during the months of lowest flow speed (July 1994 and August 1995). es of light transmission and potential temperature, collected intermittently during the 214-yr period, showed that the benthic mixed layer at Sta. M extended on average 40 m above bottom (range 15–80 mab). Evidence for local resuspension of recently deposited detritus came from time-lapse photographs of the sea floor that showed a period of near-bottom turbidity that corresponded to a period of high near-bottom flow. However, rough estimates of friction velocity indicated that the bed stress usually was too low at Sta. M to cause local resuspension of sediments. We hypothesize that the observed benthic mixed layers contained suspended particles advected from more energetic areas to the north of Sta. M.