A curious relationship between the winds and currents at the western entrance of the Santa Barbara Channel

Abstract

Relationships between the surface winds and Acoustic Doppler Current Profiler (ADCP) currents at 20 levels (25 to 329 m) at the western entrance of the Santa Barbara Channel (SBC) at National Data Buoy Center (NDBC) buoy 46054 are evaluated for the 1 year period from 1 June 1996 through 31 May 1997 using a relatively new technique for correlating vectors. Gaps in the data were filled and the resulting time series examined to insure that the statistical properties of the edited data had not been significantly altered. Vertical current profiles, averaged over the year, indicate that the mean signal, although well‐defined, is smaller than the variability about the mean. Vertical profiles of current speed and direction suggest the possibility of at least a two‐layer system of circulation in the SBC with flow between 25 and 180 m being to the SSW and flow from 260 to 329 m being to the ENE, in agreement with previous results. Because of the existing dynamical balances, the currents are oriented approximately SW/NE, and the direction of the vertical current shear (i.e., the thermal wind) is essentially constant with depth. Thus warmer waters lie to the north and west, and colder waters, essentially south of the buoy, are consistent with observed cyclonic circulation at the western entrance of the channel. Vector correlations over the entire year indicate that the winds and currents are poorly correlated, consistent with previous results. However, correlations between the winds and currents below180 m increase noticeably with increasing depth, a result which was not anticipated. The surface winds and near‐surface currents in the SBC have repeatedly shown a seasonal connection between January and April and often into May. Our results show the same pattern and further quantify the strength of this relationship. Contrary to previous results, however, we find a relationship between the winds and currents during this period that extends well below the near surface region down to at least 300 m. Vector correlations between the currents per se at different levels show quite different patterns over time suggesting the influence of independent in‐water sources of variability. Finally, lagged vector correlations were calculated at four depths (21, 41, 121, and 313 m) for two 3‐month periods, the first from October through December 1996, a period when a strong connection between the winds and currents was not expected, and the second, during the spring (February through April 1997), when it was expected. For the first case, correlations increased after the first inertial period at 25 m and 41 m. At 313 m an immediate response (i.e., maximum response at zero lag) was observed with maximum correlations decaying rapidly over the first inertial period. During the spring, an immediate response at all four depths similar to the response observed at 313 m during the first period was observed. These results imply the importance of the inertial period as a relevant time scale. Satellite imagery reveals a major frontal zone often located in the vicinity of the buoy. Vertical sections of temperature and velocity along the axis of the SBC from the Princeton Ocean Model suggest that upwelled waters flowing into the channel from the NW are “subducted” below the warmer waters that flow through the channel from the east at a location probably not far from the surface front seen from space. These model results are consistent with the argument that the higher correlations below 180 m together with a rapid response to wind forcing occur because the deeper waters in the SBC have had recent contact with surface winds prior to their entry into the SBC (i.e, good short‐term memory). Lower correlations at intermediate depths (∼25–180 m) for waters flowing to the west may occur because they do not have direct contact with the surface wind due to the presence of a shallow (<25 m) surface layer and that their motion is partially guided by the channel itself and driven by advective forces outside the SBC.