Coastal Southerlies and Alongshore Surges of the West Coast of North America: Evidence of Mesoscale Topographically Trapped Response to Synoptic Forcing

Abstract

One of the most important warm season weather phenomena of the west coast of North America is the temporal transition from northerly to southerly flow within a few hundred kilometers offshore from the coastal mountains. Such wind shifts are often accompanied by cooler temperatures, higher pressure, and a change from nearly cloud-free to low overcast conditions. The more vigorous coastal transitions, termed alongshore surges, are characterized by an abrupt change in wind direction, a sudden increase in wind speed to 15 m s?1 or more, a precipitous temperature drop exceeding 10°C, and a sharp rise in sea-level pressure. This paper presents two detailed case studies of topographically-trapped coastal southerlies: the strong surge event of 15?17 May 1985 and the far weaker case of 3?7 May 1982. It is shown that coastal southerlies and alongshore surges are controlled by the alongshore pressure gradients created by the synoptic scale flow. At low levels and within approximately one Rossby radius of the coastal topography, geostrophic balance with the alongshore pressure gradient is not possible so that air flows downgradient ageostrophically. Under the proper conditions, southerly flow in the coastal zone can propagate northward as a topographically trapped density current. It is shown that similar phenomena occur near topographic barriers throughout the world.