Impact of U.S. west coastline inhomogeneity and synoptic forcing on winds, wind stress, and wind stress curl during upwelling season

Abstract

Although buoy and aircraft measurements, as well as numerical simulations, have shown intense over‐shelf and slope dynamics of the west coast of the United States in the summer upwelling season, satellite footprint limitations of approximately 25 km resolution have thus far precluded long term, spatially extended monitoring of the near‐coastline dynamics. However, recent advancements in satellite data processing have allowed a finer footprint, of approximately 12 km resolution, to investigate further the properties of coastal winds and consequent upwelling. This improved satellite data analysis has confirmed the intense coastal winds over the shelf and slope and revealed their spatial extensions and inhomogeneities on event and multiday scales. The inhomogeneities are dominated by the along‐coast pressure gradient modulated by the synoptic effects and topographical forcing of the five major capes, which also generate upwelling wind stress and curl pattern inhomogeneities. Synoptic forcing of the coastal flow was evidenced by high correlation coefficients, in excess of 0.8, between the buoy‐measured pressure differences and wind speeds; wind speeds greater than 11 m s−1 occurred only when the along‐coast pressure gradient was greater than 0.8 hPa/100 km. Based on Bernoulli flow principles, the observed upper limit of the wind speed on the downwind sides of the major capes is explained by using characteristic values of atmospheric marine layer parameters. Numerical simulations at a similar resolution (12 km) as the new satellite data footprint for June 2001, completed as part of multiyear regional climate modeling efforts, were able to reproduce the main characteristics of the flow.