Lagrangian trajectories on the Oregon shelf during upwelling

Abstract

Neutrally buoyant, isopycnal-following floats were deployed on the Oregon continental shelf during the upwelling seasons of 2000 and 2001 and were carried southward by the mean current. The floats made CTD profiles and obtained GPS fixes twice daily, thus providing a hydrographic section along a known track. The floats followed the water accurately while at depth, but were displaced from the trajectories of the deep water during semidiurnal surfacings. These effects were large for water depths shallower than 100m, but small on the rest of the shelf. Float trajectories, corrected for advection while on the surface, showed significant error when near the shore, but little net effect offshore. Some floats moved onshore and upward along the sloping isopycnals as expected during upwelling. Although the position of the isopycnal could be predicted accurately from the wind, the motion along the isopycnal showed significant fluctuations unrelated to the wind. These may be due to barotropic shelf waves. Some floats moved southward, roughly following the isobaths around Heceta Bank to Cape Blanco. Here they underwent large vertical and cross-shelf excursions and eventually moved offshore. Two floats passed through this region 25 days apart following different trajectories, indicating an unsteady flow. Overall, these data show the expected mix of a classical upwelling circulation in the north, an offshore jet with eddies in the south, and a strong influence of topography on both the mean flow and its fluctuations.