Abstract
The California Current System is characterized by summertime wind-driven upwelling, high biological productivity, and an intense equatorward upwelling jet. The upwelling jet is generally located close to shore to the north of Cape Blanco (43°N), but it separates from the coast at the cape during summer extending farther offshore downstream of the separation point. Jet separation results in a wider region influenced by cold, nutrient-rich upwelled waters, strongly affecting biological productivity, mesoscale activity, and air-sea interactions. Flow-topography interactions are thought to play a dominant role in jet separation. Here, we use a high-resolution ocean model to show that the wind stress curl is a dominant forcing controlling jet separation, and that separation can occur independently of flow-topography interactions. While jet separation occurs in simulations with realistic wind stress curl and modified topography with no submarine banks or capes, jet separation is substantially reduced when the wind stress curl is removed, even in the presence of realistic topography. This novel insight indicates that future changes in winds, as the predicted delay in the seasonal development of wind stress curl intensifications, may result in substantial changes in ocean circulation in the California Current System.
Highlights
Eastern Boundary Current Systems have long been recognized as highly productive zones
We examine the evolution of sea surface temperature (SST) gradients away from the coast because SST fronts have been shown to be good proxies for the location of the upwelling jet in the California Current System and in other Eastern Boundary Current Systems[5,21,22,23,24]
The widening of the region of strong SST gradients to the south of Cape Blanco during summer is tightly coupled to the offshore movement of the upwelling jet (Fig. 3a) as it separates from the coast at the cape[5,26]
Summary
Eastern Boundary Current Systems have long been recognized as highly productive zones. The offshore displacement of the upwelling jet results in strong cross-shelf transport and provides an important mechanism for exporting material from the highly productive continental shelf[6] This results in a broader area influenced by the upwelling circulation to the south of Cape Blanco compared to regions to the north[5,7,8], with important consequences for both the coastal and adjacent deep-ocean ecosystems. We use high-resolution ocean model simulations to identify the relative contributions of interactions of the flow with Cape Blanco and of intensifications in wind stress and in wind stress curl that are observed in the lee of the cape on jet separation in the California Current System. We show that the wind stress curl intensification in the lee of the cape is more important than the interactions of the flow with the cape itself to drive jet separation and to enhance the offshore export of recently upwelled, nutrient-rich waters
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