Long Coastal-Trapped Waves off the West Coast of the United States, Summer 1973

Abstract

Evidence for long coastal-trapped waves off the west coast of the United States is obtained from sea level, surface atmospheric pressure and wind records over a 1500 km alongshore separation for two months in the summer of 1973. Corresponding evidence is obtained from current measurements off the northern Oregon coast. The dominant low-frequency motion occurred at a period of 10 days. Consistent with the theory of coastal-trapped waves, the observations indicate that 1) the alongshore current fluctuations were mainly barotropic, coastally trapped, and were related geostrophically to the adjusted sea level fluctuations; and 2) the adjusted sea level fluctuations propagated northward with a phase speed which depended upon the local shelf geometry. The 10-day fluctuations in the coastal ocean were driven by the northward traveling, large-scale winds; the response was nonlocal. There were also 4-day fluctuations for which the coastal water response was essentially local. The difference in response characteristics for the two periods can be explained with simple forced wave theory.