Fluctuations in alongshore wind stress &tgr; calculated from Fleet Numerical Oceanography Center geostrophic winds along the west coast of North America are relatively energetic at alongshore wave numbers ‖l‖≤1.1¿10-3 cpkm and frequencies 0.0250) dominates the summer &tgr; fluctuations for ω<0.1 cpd. In contrast, poleward propagation strongly dominates the &xgr; fluctuations in both seasons, but more so in summer. The largest coherence between &xgr; and &tgr; is observed in that part of (l, ω) space where &tgr; is most energetic: i.e., for 0≤l≤1.1¿10-3 cpkm in summer and -1.1¿10-3≤l≤0 cpkm in winter. Results are compared with predictions from theoretical models for wind-driven coastally trapped waves. The observed transfer functions show evidence for near-resonant &xgr; response in both seasons, with the largest gain found along a single ridge of approximately constant poleward phase speed, along which the response is approximately in phase with &tgr;.

This behavior is consistent with a response dominated by one coastally trapped wave mode governed by a forced, first-order wave equation with a linear friction term. The maximum gain tends to decrease along this ridge with increasing l and ω in summer, and at least with increasing ω in winter, during which we could not resolve the l dependence. This observed decrease in maximum gain is not predicted by a single wave equation. The contribution of higher wave modes to the total &xgr; response may explain a part of this observed decrease. ¿ American Geophysical Union 1987