Dynamics of wind‐driven upwelling and relaxation between Monterey Bay and Point Arena: Local‐, regional‐, and gyre‐scale controls

In north and central California, equatorward winds drive equatorward flows and the upwelling of cold dense water over the shelf during the midspring and summer upwelling season. When the winds temporarily weaken, the upwelling flows between Point Reyes and Point Arena “relax,” becoming strongly poleward over the shelf. Analytical and numerical models are used to describe the effect of alongshore variability of winds, bathymetry, and basin‐scale pressure gradients on the strength of upwelling and its relaxation. Alongshore winds weaken to the south of Point Reyes, and the shelf becomes narrower from Point Reyes to Monterey Bay. Both of these lead to reduced upwelling at and to the north of Point Reyes, causing an alongshore gradient of temperature and density on the shelf. These alongshore gradients lead to an along‐isobath pressure gradient over the shelf that drive the relaxation flows. A simple analytical model is used to explain the dynamics, magnitude, and structure of the relaxation flows. The modeling also suggests that the depth of origin of the upwelled waters, and thus their temperature, is controlled by the along‐isobath pressure gradient that exists over the continental slope. This along‐slope pressure gradient is also responsible for the California undercurrent in this region. This pressure gradient is not generated in a model of the Californian coast extending from 32°N to 42°N and integrated for several months, suggesting it is caused by dynamics whose spatial or temporal scales are larger than the Californian coast and/or longer than several months.