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Abstract A three-dimensional inertial model that conserves quasigeostrophic potential vorticity is proposed for wind-driven coastal upwelling along western boundaries. The dominant response to upwelling favorable winds is a surface-intensified baroclinic meridional boundary current with a subsurface countercurrent. The width of the current is not the baroclinic deformation radius but instead scales with the inertial boundary layer thickness while the depth scales as the ratio of the inertial boundary layer thickness to the baroclinic deformation radius. Thus, the boundary current scales depend on the stratification, wind stress, Coriolis parameter, and its meridional variation. In contrast to two-dimensional wind-driven coastal upwelling, the source waters that feed the Ekman upwelling are provided over the depth scale of this baroclinic current through a combination of onshore barotropic flow and from alongshore in the narrow boundary current. Topography forces an additional current whose characteristics depend on the topographic slope and width. For topography wider than the inertial boundary layer thickness the current is bottom intensified, while for narrow topography the current is wave-like in the vertical and trapped over the topography within the inertial boundary layer. An idealized primitive equation numerical model produces a similar baroclinic boundary current whose vertical length scale agrees with the theoretical scaling for both upwelling and downwelling favorable winds.
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This content will become publicly available on June 1, 2026
Nonlinear Dynamics over the Inner Shelf and in the Surface Boundary Layer during Coastal Upwelling
Abstract Theoretical understanding of the upward vertical motion into the surface layer during coastal upwelling is often based on steady linear Ekman dynamics. In steady linear theory, the divergence of surface transport that leads to upwelling is associated with either overlap of the frictional boundary layers over the inner shelf or wind stress curl farther offshore. However, the alongshore current associated with a coastal upwelling front is associated with relative vorticity which modifies surface transport. A new nonlinear theory shows that, under spatially uniform wind forcing, the fraction of Ekman transport upwelled over the inner shelf tends to decrease with increasing slope Burger numberSas the baroclinic alongshore jet strengthens and cyclonic vorticity increases. Similar patterns are shown in a set of idealized numerical experiments. Unsteadiness in the alongshore flow, neglected in the theory, strongly influences the cross-shelf distribution of upwelling in the numerical model at locations offshore of the inner shelf and near the core of the upwelling jet. The theory and numerical modeling are extended to explore the effect of a large-scale alongshore pressure gradient force (PGF) that forms in response to alongshore variations in wind stress. At highS, a baroclinic PGF is associated with a shallow onshore return flow, but the fraction of modeled upwelling that occurs over the inner shelf is not strongly affected. The results emphasize that the strength and location of the alongshore jet strongly influence the cross-shelf distribution of coastal upwelling in the presence of stratification and a sloping bottom. Significance StatementWind-driven coastal upwelling is important for supplying nutrients to phytoplankton at the base of marine ecosystems. This study uses simple models to investigate factors that determine where upwelling of water into the surface layer occurs when wind blows along the coastline. With a larger difference in density between the surface and bottom layers, a steeply sloping seafloor, and at latitudes closer to the equator, the upwelling region shifts farther offshore because of the strength and location of faster ocean currents that flow along the coastline.
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- Award ID(s):
- 2242163
- PAR ID:
- 10631146
- Publisher / Repository:
- American Meteorological Society
- Date Published:
- Journal Name:
- Journal of Physical Oceanography
- Volume:
- 55
- Issue:
- 6
- ISSN:
- 0022-3670
- Page Range / eLocation ID:
- 695 to 715
- Format(s):
- Medium: X
- Sponsoring Org:
- National Science Foundation
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