Coastally trapped wind reversals along the United States west coast during the warm season. Part I: Climatology and temporal evolution

Abstract

The northerly winds that predominate along the U.S. west coast during April-September are interrupted periodically by abrupt reversals to southerly flow. The climatology and composite temporal evolution of these reversals from Point Conception to the Canadian border are documented using hourly data from moored coastal buoys and Coastal-Marine Automated Network stations for the period 1981-91. The reversals are divided into two categories: coastally trapped reversals, in which the southerly flow is highly ageostrophic and restricted to the coastal zone, and synoptic reversals, which are associated with landfalling troughs or fronts. Coastally trapped events occur on average about 1.5 times per month along the central and northern California coast, about twice a month near the California-Oregon border, and about once a month near the Oregon-Washington border. The ratio of coastally trapped reversals to synoptic reversals is higher during July-September and lower during April-June, particularly in the north. Roughly one-quarter of the coastally trapped reversals have a southerly wind component that exceeds 5 m s-1. Reversals along the California coast are gradual; the changes in the alongshore winds usually occur over a period of 6 h or longer, and the maximum southerlies are less than 8 m s-1. In contrast, roughly one-half of the reversals north of the California-Oregon border feature abrupt changes with southerly winds reaching approximately 10-12 m s-1 within 2-3 h of the wind shifts. These stronger northern events often include substantial decreases in air temperature and rises in pressure. The southerlies associated with coastally trapped reversals persist for an average of about 30 h at a particular location. There is a strong tendency for coastally trapped reversals to occur during the night or morning. North of Monterey Bay, the reversals typically advance poleward (but not necessarily in a smoothly continuous manner) at a mean speed of 7-8 m s-1 and maintain significant amplitude for an alongshore distance of 500-1000 km.