Large‐scale response to atmospheric forcing of shelf currents and coastal sea level off the west coast of North America: May‐July 1981 and 1982

Abstract

The response of alongshore currents V over the continental shelf and of coastal sea level ζ along the Pacific coast of North America to atmospheric forcing on large alongshore scales (100–2000 km) during May‐July 1981 (Coastal Ocean Dynamics Experiment (CODE) 1) and May‐July 1982 (CODE 2) has been studied using sea level and wind measurements from coastal stations and buoys between 33°N and 48°N and using current measurements from five locations between 35°N and 48°N. Characteristics of the large scale response of V and ζ to fluctuations in the alongshore component of the wind stress τ varied with latitude and with year in relation to variations in τ. In 1981, large‐scale ζ fluctuations were coherent within, but not between, separate domains north and south of 42°N. In 1982, large‐scale coherent fluctuations were found only in a northern domain that extended much farther south to near 37°N. Accordingly, in 1981, alongshore current V fluctuations from 39°N (CODE) were coherent with V from 35°N, while in 1982, V fluctuations from 39°N were coherent with those north to 48°N. In 1981 the northern domain fluctuations were forced by τ between 42°N and 45°N with southern domain forcing at 35°–36°N. In 1982, northern domain fluctuations were forced by τ over a region of much larger along‐shore extent (37°–48°N). High values of space‐lagged coherences between V and τ(y) (y is the alongshore coordinate) indicate that V at 39°N was strongly wind driven. Maximum squared coherences of V versus τ(y) are found with τ to the south by 100–300 km for low frequencies (ω ≤ 0.23 cpd in 1981, ω ≤ 0.14 cpd in 1982) and with τ from near‐local stations at higher ω, indicating the importance of remote forcing at low frequencies and local forcing at high frequencies. The response of V to τ(y) at 35°N and 43°N was more complicated, but evidence for strong wind forcing is found in several frequency bands for V at 43°N during 1981 and 1982 and at 35°N during 1981. At 48°N, V was forced in 1982 for ω ≤ 0.18 cpd by τ from locations 300–1400 km to the south.