Physical and biogeochemical properties of California Current upwelled source waters

Coastal upwelling variability in the California Current region, one of the
four main eastern boundary current upwelling systems, is controlled by
processes acting over a wide range of spatial and temporal scales. While
the ensuing ecosystem response depends strongly on upwelled water
properties, determining their exact physical and biogeochemical
characteristics is notoriously difficult as it requires tracking water
masses backward in space and time from the moment they upwell near the
coast to their subsurface origin. Adjoint model simulations have been used
successfully to track water masses in coastal upwelling systems and the
work presented here extends these applications to determining the
co-variability of physical and biogeochemical properties of source waters
at spatial scales that resolve the known alongshore variability of coastal
upwelling dynamics in the region. Notably, the results identify that the
modulation of coastal upwelling efficiency by onshore/offshore geostrophic
meanders is the dominant mechanism explaining alongshore variability in
source depth and properties of upwelled waters. The simulations also
reveal that source water properties vary seasonally in response to
different balances between coastal upwelling intensity and biogeochemical
processes. During spring, interannual variability of physical and
biogeochemical properties is directly tied to the intensity of
upwelling-favorable alongshore winds, whereas, during summer,
biogeochemical properties respond more strongly to biological activity and
subsequent organic matter remineralization at depth. Overall, the present
work provides important insight into the mechanisms responsible for the
alongshore mosaic and seasonal variation upwelled source water properties
in the central California Current region.