Large‐scale structure of the spring transition in the coastal ocean off western North America

Abstract

Past measurements off the coast of central Oregon and Washington have shown that the rapid change from northward monthly mean winter winds to southward summer winds forces a “spring transition” of the coastal ocean: sea levels and temperatures drop, and mean surface currents shift from northward to southward. Current and water temperature data from 35°N to 48°N from 1981 and 1982, and sea level and wind stress data from 1971–1975 and 1980–1983, show the transition to have a large alongshore scale, typically 500 to 2000 km; the large‐scale wind stress appears to be the forcing mechanism at latitudes north of approximately 37°N. South of 37°N, sea level usually falls more gradually before the northern transition event. Both wind and sea level events generally progress from south to north over a 3‐ to 10‐day period, but this is not always true. Several aspects of the spring transition reflect coastal trapped wave dynamics. Previous studies at 45°N found persistent vertical shear of the southward summer current, associated with a cross‐shelf density gradient. During 1982 the shear and the density front develop over the shelf break immediately after the transition at 43°N and to the south, but they are much less persistent than at 48°N. The stronger winds between 38°N and 42°N and the narrower shelf result in an offshore displacement of the density front and vertical shear past the shelf break, leaving the water over the shelf less stratified and more subject to barotropic reversals of the current than that farther north, where the front stays closer to the coast.