Mean Coastal Upwelling Circulation
The canonical description of the coastal upwelling circulation has existed since the early days of oceanography (Sverdrup et al., 1942). In an eastern boundary upwelling system like off California, the equatorward wind drives a surface layer offshore, with the transport at 90° to the right of the wind as originally quantified by Ekman (1905). This offshore transport requires upwelling in a region close to shore to conserve mass. Finally, there must be an onshore flow at depth to complete the upwelling cell.
This canonical description includes a momentum balance for the offshore flow, a mass balance that requires the upwelling and onshore flow, but has no comment on the momentum balance of the deeper onshore flow.
Our work offers an update to the canon that includes a momentum balance for the onshore flow (Johnston and Rudnick, 2026). On every eastern boundary the density increases poleward. In accordance with the thermal wind balance, this density gradient causes a geostrophic onshore flow that increases towards the surface. Wind-driven offshore flow replaced by a source flow below is consistent with canon. The competition between the two flows, with wind-driven stronger at the surface, and geostrophic extending deeper, produces the overturning cell, as in the figure below.
Glider Observations
This work has been made possible with sustained glider observations on an alongshore line in the California Current System. Access and view data from the Along-shore line on our California Underwater Glider Network Climatologies page via the link below.
To view figures of the along-shore climatologies, use the menus to select the short-term baseline and then select the Along-shore Line location.
References
Ekman, V. W., 1905: On the influence of the earth's rotation on ocean currents. Ark. Mat. Astron. Fys., 2, 1–52.
Johnston, T. M. S. and D. L. Rudnick, 2026: The mean across-shore coastal upwelling cell in the Southern California Current System. Journal of Geophysical Research: Oceans, 131, doi: 10.1029/2025JC023651.
Sverdrup, H. U., M. W. Johnson, and R. H. Fleming, 1942: The Oceans: Their Physics, Chemistry, and General Biology. Prentice-Hall, 1087 pp.