Circulation over the Santa Monica-San Pedro Basin and Shelf

Abstract

The spatial and temporal structure of the circulation in Santa Monica-San Pedro basin and over the adjacent mainland shelf has been described in a series of experiments that included moored arrays of currents meters, hydrographic surveys, satellite-derived sea surface temperature maps and Lagrangian drifter deployments. This basin, located in the Southern California Bight adjacent to the coast, is roughly 100km long, 40km wide and 900m deep. From the sea surface to a depth of about 250m, the basin is open to the San Diego Trough to the southeast, the Santa Barbara Channel to the northwest, and the Santa Cruz basin to the west. The resulting data set is the first spatially comprehensive data set over the continental slope of the US west coast, the first comprehensive data set in a coastal region of such complicated topography, and the first that includes detailed measurements over a semi-enclosed shelf, namely, Santa Monica bay. The most significant contributions of the research based on this data set are: (1) demonstration of the alteration of a low mode coastal-trapped wave by a sharp bend in topography; (2) demonstration that fluctuating currents on a semi-enclosed shelf can be driven by the flow along its open boundary; (3) demonstration that topographic waves exist in the completely enclosed portion of coastal basins. In addition, the research allowed an order of magnitude improvements in regional knowledge of both the seasonal and subtidal scale fluctuations within the Southern California Bight. For example, on seasonal scales, the winter surfacing of the California undercurrent (the dominant feature of the seasonal mean flow field) as well as the continuity of the undercurrent in a spatially limited region were addressed. The existence of an equatorward undercurrent at mid water column depths (300–500m) over the continental slope during the late summer to fall season was also documented. On subtidal scales, the data demonstrated that the velocity field is far from “quiescent” over the basin, even below the depth of the deepest basin sill. Spatially-organized subtidal fluctuations occur at all depths and the time scale of the dominant fluctuations (∼ 20–30d) is a factor of two or more longer than that typical of most coastal shelves. The fluctuations have a subsarface maximum during most of the year and the maximum is best developed in late summer and over the mainland slope of the basin. Propagation characteristics south of the basin and within the basin along its coastal perimeter are consistent with those of first mode freely propagating coastal-trapped waves at these long periods. The long period waves enter the basin from southeast and then appear to pivot conterclockwise around the basin, so that at least some porion of the waves exits the basin over its western sill. The phase speed of the waves is reduced by an order of magnitude over all but the coastal slope of the basin. Below sill-depth velocity and temperature fluctuations appear to be the signature of freely propagating basin-scale topographic waves driven by the long period upper water column fluctuations. The waves travel counterclockwise around the lower basin at speeds about 25cm s −1. Current fluctuations over the adjacent inner Santa Monica shelf have significant variance at shorter periods as well as at the longer periods observed over the basin and its slopes. These fluctuations are driven predominantly by boundary forcing over the outer shelf and only secondary by local wind stress. Wind data suggests that although local upwelling within the Bight is extremely limited from late spring through fall, strong upwelling events occur routinely in winter early spring. During the summer to fall months effects of upwelling may be experienced within the Bight, but these effects are primarily the results of upwelling outside the Bight in combination with lateral advection. During periods of strong upwelling outside the Bigth, outflow from Santa Monica basin appears to be preferentially directed south of the Channel Islands rather than through the Santa Barbara Channel.