Surface Currents off Oregon as Determined from Drift Bottle Returns

Abstract

It is generally known that surface currents off Oregon flow equatorward in summer when winds are from the northwest and poleward in the winter when winds are from the south. A detailed pattern of surface currents has been derived from drift bottle returns and releases observed during a 10-year study from 1961?71. A total of 21,285 drift bottles were released within 165 n mi of the Oregon coast, of which 2937 bottles were eventually recovered. The annual cycle of surface currents indicated from these returns was as follows: Northward?October through February Southward?May through August Variable?March, April and September The Davidson current or (northward) winter current was considered to be any northward prevalence of surface flow. The Davidson current commonly reached 48?50N. Every year of the study, some surface blow off Oregon reached as far north as southeast Alaska to about 55N. Although drift bottle observations often indicate northward surface flow all the way to at least the edge of the study area, 165 mi offshore, it is believed the main flow of the Davidson current occurs within 100 mi of the coast of Oregon and Washington. Maximum speeds between 0.5 and 2 kt occurred within 20 mi of the coast during the month of November. Southward surface currents near 0.30 kt were occasionally measured in summer, and they appeared to be consistent for a distance up to 200 mi along the coast. Mean surface currents in both summer and winter were 0.2 kt (150 n mi per month). Unseasonal winds during both winter and summer resulted in anomalous currents. The major direction changes of currents from northward to southward occurred in March and April. The shift in currents back to northward occurred in September, a time when onshore surface flow was also prominent.