Coastally Trapped Wind Reversals along the United States West Coast during the Warm Season. Part II: Synoptic Evolution

Abstract

This paper describes the synoptic evolution associated with coastally trapped wind reversals along the west coast of the United States from April through September. Gridded National Centers for Environmental Prediction (formerly National Meteorological Center) synoptic analyses are composited relative to the initiation of both weak and strong coastal southerlies at four coastal buoys offshore of Oregon and California. A similar composite synoptic evolution accompanies the transition from northerlies to southerlies at each site. At 500 mb there is anomalous ridging over the eastern Pacific, while at 850 mb and the surface the east Pacific high extends into southwestern Canada and the Pacific Northwest. The result is offshore flow and subsidence along the mountainous West Coast that produces a strengthening and coastal extension of the trough normally resident over the interior of southern and central California. This anomalous coastal troughing weakens the offshore gradient and establishes an alongshore pressure gradient that is either flat or increases to the south, in sharp contrast to the climatological alongshore pressure gradient that increases to the north. Examination of the individual events revealed that virtually all of the trapped wind reversals are associated with the development of higher sea level pressure to the south along the coast, typically by 0.5?1 mb over about 200 km. The acceleration of air down this coastal pressure gradient produces the observed trapped southerly flow. These results suggest that coastally trapped southerly flow is controlled by the coastal pressure gradients associated with the synoptic-scale flow and its interaction with West Coast orography. The response of the marine layer can modulate and amplify the synoptic-scale signal.