Ekman transport of surface water offshore in the California Current System (CCS)
(Fig. 11.38) leads to a cross-shelf pressure gradient producing a coastal upwelling jet
5–30 km from shore (Fig. 11.39) and leads to rise of the deeper, nutrient-rich,
halocline water to the surface inshore of the coastal jet. In addition to this fast vertical
coastal upwelling, offshore wind-stress curl leads to slower vertical transport (Ekman
pumping) farther offshore (Fig. 11.39). Chavez & Messié (2009) estimated that
Ekman transport provides an average of 69–79% of the upwelling across the four
EBCS. However, the rate of Ekman pumping is likely to be more significant in some
subregions, e.g. wind-stress curl contributes 33% of the upward flux in the central
CCS (Dever et al. 2006) but 60–80% in its southern subregions (Rykaczewski &
Checkley 2008). Turbulent fluxes provide a third mechanism for nutrient input. By
combining high-resolution measurements of turbulence and nutrient gradients, Hales
et al. (2005) showed that, offshore of the 30 m isobath in the northern CCS, turbulent
fluxes lead to cross-isopycnal mixing of nutrients and provide nutrients at 25% of the
rate from coastal upwelling. Turbulent fluxes may be even higher inshore. The
importance of this nutrient-supply mechanism in other EBCS is not known.
Fig. 11.38 Map of the California Current System. Major regions, currents, and
geographical features are shown. The California Current derives from both the North
Pacific Current to the north, and the coastal jet to the east.
(^) (After Checkley & Barth 2009.)