Di Lorenzo, E., A. J. Miller, N. Schneider and J. C. McWilliams,
The warming of the California Current:
Dynamics and ecosystem implications.
Journal of Physical Oceanography, 35, 336-362.
The long-term changes in the observed temperature and salinity
along the Southern California coast are studied using a four dimensional
space-time analysis of the 52 year (1949-2000) California Cooperative
Oceanic Fisheries Investigations (CalCOFI) hydrography combined with a
sensitivity analysis of an eddy permitting primitive equation ocean model
under various forcing scenarios. A warming trend in temperature and a
deepening trend in the depth of the mean thermocline between 1950 and 1998
are found to be primarily forced by large-scale decadal fluctuations in
surface heat fluxes combined with horizontal advection by the mean
currents. After 1998 the surface heat fluxes suggest the beginning of a
period of cooling, which is consistent with colder observed ocean
temperatures. Salinity changes are decoupled from temperature and are
primarily controlled by horizontal advection by anomalous currents.
A cooling trend in SST is driven in the ocean model by the 50 year
NCEP wind reanalysis, which contains a positive trend in upwelling
favorable winds along the Southern California Coast. The magnitude of this
cooling (0.2 degrees Celsius), however, is small compared to the observed
warming trend (1 degree Celsius) and is not detectable in the CalCOFI
hydrography. The signature of the increased winds also is evident in both
model and observations as an intensification of the mean currents of the
Southern California Current System (SCCS). Model mesoscale eddy variance
significantly increases in recent decades in response to both the stronger
upwelling winds and the deepened isopycnals, suggesting that the stability
properties of the SCCS have also changed.
Within 50 to 100 km of the coast, the ocean model simulations show
strong evidence that the isopycnal deepening reduces the nutrient flux to
the ocean surface, counteracting any effects of the increased upwelling
winds. The long-term trend of the model proxy for surface nutrients is
consistent with the observed decline in zooplankton concentration.