Characteristic patterns of the circulation in the Santa Barbara Channel

Abstract

The subtidal near‐surface circulation in the Santa Barbara Channel (SBC) and on the shelf north of Point Conception is described based on observations obtained during the 3‐year period from 1993 to 1995. Near‐surface currents in the channel are a superposition of a larger‐than‐SBC scale flow and a cyclonic circulation of variable intensity located inside the channel. On seasonal timescales the larger‐than‐SBC scale flow near the surface is equatorward in spring and poleward from summer through winter. The increase in equatorward flow in spring occurs concurrently with the increase in equatorward wind stress and the decrease in near‐surface temperatures and synthetic subsurface pressures (SSPs). The flow reverses in late spring, simultaneously with the increase in the along‐channel SSP difference and months before wind stress has reached its peak. The period during which the cyclonic circulation within the SBC is strongest coincides with the period of strongest poleward flow through the eastern entrance. A synoptic description of the circulation in the SBC is presented in terms of six characteristic patterns, labeled Upwelling, Relaxation, Cyclonic, Propagating Cyclones, Flood East, and Flood West. An analysis of the 5‐ and 45‐m currents into empirical orthogonal functions (EOFs) isolates 50% (53%) of the 5‐m (45‐m) low‐frequency current variance into three (two) modes. Combining these modes with the mean current fields, the modes have spatial patterns that correspond to the characteristic flow patterns subjectively deduced from inspection of daily averages of the near‐surface currents. From late spring through fall the two largest 5‐m current modes produce a repeating pattern of circulation in which the sequence of four states (namely, Upwelling, Cyclonic, Relaxation, and Quiescent) is traversed roughly every 16 days. In addition to the large‐scale cyclonicity in the central SBC, smaller cyclonic eddies form frequently in the eastern channel and travel toward the west with an average speed of 0.06 m s−1. The patterns described here develop in response to the wind stress and the along‐channel SSP gradient.