Song, H., A. J. Miller, B. D. Cornuelle and E. Di Lorenzo, 2011:
Changes in upwelling and its water sources in the
California Current System driven by different wind forcing
Dynamics of Atmospheres and Oceans, 52, 170-191.
Abstract.
In the California Current System (CCS), upwelling is one of the most important
features that enrich the coastal ecosystem. It is highly dependent
on both wind stress and wind stress curl, because they contribute to the
upwelling system through Ekman transport away from the coast and Ekman
pumping as a result of the surface divergence, respectively. Various wind
stress products are known to contain sharply different patterns of wind stress,
and well-resolved wind forcing products have been shown to drive stronger
upwelling due to their better-resolved wind stress curl in previous studies.
However, sensitivities of upwelling to changes in wind stress patterns, and
each of their control to the source waters and paths of the upwelling cells,
are not yet well known for the CCS. Here we study these ects using the
Regional Ocean Modeling System (ROMS) and its adjoint model under
idealized wind stress forcing patterns representing three widely-used products in
addition to a constant wind stress field (no curl): the NCEP/NCAR Reanalysis,
the QuikSCAT satellite observations, and the Regional Spectral Model
(RSM) downscaling.
Changes in currents and isopycnal patterns during the upwelling season
are first studied in ROMS under the four different wind stress fields. The
model simulations show that the locations of the core of the equatorward flow
and the gradient of the cross-shore isopycnals are controlled by the wind stress
curl field. The core of the equatorward
flow is found under negative wind
stress curl, and a deeper upwelling cell is found as the gradient from positive
and negative wind stress curl increases. Source waters for the upwelling in
each of the four wind stress patterns are investigated using the ROMS adjoint
model. The simulations follow a passive tracer backward in time and track
the source waters for upwelling in two key areas of interest: inshore and
offshore of the Point Sur region of California. The upwelling source waters
depend strongly on the depth of the upwelling cell and the alongshore current
location. We further relate these results to recent studies of the observed
trends in upwelling favorable winds and consequent wind stress curl changes
in the CCS.
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