Dye tracer studies of wind-driven upwelling on the Oregon shelf

A series of dye releases from a small boat during the summers of 2001/2002 has investigated cross-shelf circulation in a region dominated by intermittent wind-driven upwelling. The aim was to make direct, Lagrangian observations of the pathways of cross-shelf flow during active upwelling. Lagrangian techniques provide insight that is not available from Eulerian measurements because of the dominance of alongshelf flow. Preliminary results will be reported, with emphasis on a release from August 2002, made as winds transitioned from weakly downwelling-favorable to weakly upwelling-favorable. Dye was seeded in an alongshore streak at 20m depth in 55m of water, a little beneath the pycnocline. Of note was the rapid onshore spreading of the dye patch, having a cross-shelf extent of 5km after 48 hours, and extending into shallow water (less than 20m bottom depth). The dye remained largely isopycnal, following the upper edge of the bottom boundary layer onshore. The offshore edge of the patch became associated with a strong temperature inversion - a warm layer, apparently composed of subducted near-surface water. This dataset will provide an opportunity to determine the source of water in such inversions and to evaluate potential mechanisms for their formation.