Wednesday, 12 February 2003
Wind stress in the Oregon and California coastal zone from the QuikSCAT scatterometer and models
The availability of twice-daily QuikSCAT/SeaWinds scatterometer observations of global surface wind stress fields over the ocean, with nominal horizontal resolution of 25 km, provides unprecedented opportunities to determine the structure of lower troposphere winds and their influence on ocean circulation. In the present study, scatterometer measurements during two summer periods, June through September 2000 and 2001, are analyzed and compared with several different atmospheric model products, to study the structure and dynamics of the surface wind fields in the coastal region west of Oregon and California, extending offshore to 130 W. The mean summer wind stress has a southward component over this entire region in both years. Orographic intensifications of both the mean and variable wind stress occur near Cape Blanco, Cape Mendocino, and Point Arena. An estimate of the mean diurnal cycle is made possible because the daily scatterometer passes over this region occur approximately at extreme phases of the cycle. Peak-to-peak mean diurnal fluctuations in wind stress greater than 0.03 N m-2 over a broad region extending offshore up to 150 km, with maximum values exceeding 0.1 N m-2 near the coast. The sense of this cycle reverses near 41.5 N, with mean southward stress largest in the evening north of 41.5 N and largest in the morning south of this latitude, except near the regions of orographic intensification, where the southward stress adjacent to the coast is always largest in the evening. The model products are derived from an operational global model, an operational moderate-resolution mesoscale model, and two nested high-resolution mesoscale models centered on the Oregon coastal zone. Substantial differences between the model products are found for the mean, variable, and diurnal wind stress fields, which are partly due to differences in boundary-layer stability corrections. Temporal correlations with the QuikSCAT observations are highest for the operational models, and are not improved by either nested model. The highest resolution nested model degrades these temporal correlations, but improves moderately, relative to QuikSCAT, the spatial structure of the mean stress in the region of orographic intensification near Cape Blanco. Simulated diurnal cycles differ from the observed cycle, with the significant differences between the models.