Variability in two deep coastal basins (Santa Monica and San Pedro) off southern California

Time series of midbasin hydrographic samples and of an array of moored temperature-velocity sensors are used to examine subtidal to interannual scale variability in two ∼900-m-deep basins off the southern California coast. The deepest sill that allows entry into the basins is at 740 m, and the two basins, Santa Monica and San Pedro, are separated by a sill about 40 m high. The mean flow 40 m above and 60 m below the 740-m sill depth is in the counterclockwise direction and the magnitude is ∼0.5–1.0 cm s−1. The data indicate that the waters at deeper levels of the basins are neither quiescent, at one extreme, nor are they filled with random noise, at the other extreme. Rather, at all scales examined, the variability is significant and much of it is spatially coherent. The amplitude of the subtidal velocity fluctuations is ∼4 cm s−1, almost an order of magnitude greater than that of the mean flow. The amplitude of the subtidal temperature fluctuations is ∼0.1°C at 700 m and ∼0.05°C at 800 m. The temperature variance is roughly a factor of 2 smaller at midbasin than on the basin perimeter. This is not the case with the velocity variance, which is generally greatest at midbasin and on the coastal edges of both basins. Much of the subtidal variability in both temperature and velocity appears to be associated with topographic waves that travel counterclockwise around the pair of basins. The observed period of the waves (∼10–20 days) is roughly consistent with that predicted by a barotropic model of free waves in an elliptical basin (Mysak, 1985). Comparison of time series of currents in the upper (<200 m) water column with those in the lower water column suggests that the waves are forced by the circulation in the upper water column, which is directed over (rather than around) the basins. Seasonal variability was observed in both temperature and velocity fields as deep as 800 m (i.e., below the depth of the deepest sill) along the perimeters of both basins and at midbasin in Santa Monica but not in San Pedro. Temperature increases and salinity and density decrease from spring to fall, consistent with the seasonal development of the poleward flowing California Countercurrent in the upper water column. The seasonal cycle of water properties has significant spatial variability over the basins: the amplitude is a factor of 2 greater on the seaward side of Santa Monica basin than at midbasin and on its coastal edges, and on the edges of San Pedro basin. The variance of the subtidal velocity and temperature fields also exhibit a seasonal signal: maximum variance occurs during fall; minimum variance occurs during spring. Interannual variability dominates the 4-year time series of water properties of the near-bottom water in each basin, and differs somewhat between the two basins. The interannual variability is dominated by events associated with renewal of basin bottom water. Two such events were identified: a minor one in 1987; and a major one in 1988, each beginning in May of that year. In both cases, the actual renewal occurs in less than a month, and the most rapid return to near-normal conditions occurs over a period of about 3–5 months.