COASTALLY TRAPPED WIND REVERSALS ALONG THE UNITED-STATES WEST-COAST DURING THE WARM-SEASON .1. CLIMATOLOGY AND TEMPORAL EVOLUTION

The northerly winds that predominate along the U.S. west coast during April-September are interrupted periodically by abrupt reversals to so utherly flow. The climatology and composite temporal evolution of thes e reversals from Point Conception to the Canadian border are documente d using hourly data from moored coastal buoys and Coastal-Marine Autom ated Network stations for the period 1981-91. The reversals are divide d into two categories: coastally trapped reversals, in which the south erly flow is highly ageostrophic and restricted to the coastal zone, a nd synoptic reversals, which are associated with landfalling troughs o r fronts. Coastally trapped events occur on average about 1.5 times pe r 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 durin g April-June, particularly in the north. Roughly one-quarter of the co astally trapped reversals have a southerly wind component that exceeds 5 m s(-1). Reversals along the California coast are gradual; the chan ges in the alongshore winds usually occur over a period of 6 h or long er, and the maximum southerlies are less than 8 m s(-1). In contrast, roughly one-half of the reversals north of the California-Oregon borde r feature abrupt changes with southerly winds reaching approximately 1 0-12 m s(-1) within 2-3 h of the wind shifts. These stronger northern events often include substantial decreases in air temperature and rise s in pressure. The southerlies associated with coastally trapped rever sals persist for an average of about 30 h at a particular location. Th ere is a strong tendency for coastally trapped reversals to occur duri ng the night or morning. North of Monterey Bay, the reversals typicall y advance poleward (but not necessarily in a smoothly continuous manne r) at a mean speed of 7-8 m s(-1) and maintain significant amplitude f or an alongshore distance of 500-1000 km.