Sensitivity of Coastal Currents near Point Conception to Forcing by Three Different Winds: ECMWF, COAMPS, and Blended SSM/I-ECMWF-Buoy Winds

摘要

Previous observational and modeling studies have indicated the importance of finescale winds in determining the circulation near Point Conception in the Santa Maria Basin (SMB) and the Santa Barbara Channel (SBC), California. There has not been a systematic attempt, however, to analyze and quantify the sensitivity of the near-surface circulation to different wind data. Here, a regional circulation model of the SMB and SBC is driven using three wind datasets: the European Centre for Medium-Range Weather Forecasts (ECMWF; ≈ 110 km x 110 km horizontal grid), the Coupled Ocean-Atmosphere Mesoscale Prediction System (COAMPS; 9 km x 9 km horizontal grid), and a blended wind product that combines Special Sensor Microwave Imager (SSM/I), ECMWF, and buoy and coastal wind data and that is referred to as SEB. A springtime period (March-May 1999) in which equatorward wind dominates and wind stress curls are strong is chosen for the study. Two groups of experiments are conducted: with and without assimilating moored temperature observations. The focus is on long time scales of greater than weeks and on mean currents. Comparisons between these experiments and between model and observation show that the circulation driven by the ECMWF wind is much weaker than that by the other two winds. On the other hand, the SEB dataset shows locally intensified wind stress curls behind capes and coastal bends, whereas these wind stress curls are weak in COAMPS. It is found that these small-scale variations in the wind field force alongshore inhomogeneous pressure gradients that in turn can significantly affect the near-coast currents. The result is that modeled currents forced by SEB agree better with observations than do those produced by COAMPS. Empirical orthogonal function analyses were conducted on the near-surface currents, sea level, wind, and wind stress curl. The mode-1 current (≈40%) is unidirectional (i.e., generally equatorward or poleward) and correlates well with the mode-1 wind (≈90%). The mode-2 current (≈20%) is cyclonic in the SBC and poleward inshore and equatorward offshore in the SMB; it correlates well with mode-1 sea level (≈70%), which suggests that mode-2 currents are driven by the pressure gradient. It is significant that neither mode-2 current nor mode-1 sea level correlates well with mode-1 wind stress curl (≈70%); rather, they correlate well with the time integral of the mode-1 wind stress curl. These conclusions support a previous theoretical idea that cyclonic circulation in the SBC and the inshore currents of the SMB are both driven by alongshore pressure setup induced by the time integral of the wind stress curl, rather than by the wind stress curl itself. This idea of a pressure setup is consistent with the differences found between the currents driven by COAMPS and SEB winds.