Moore, A. M., H. G. Arango, E. Di Lorenzo,
A. J. Miller
and B. D. Cornuelle, 2008:
Journal of Physical Oceanography, sub judice.
An adjoint sensitivity analysis of the
Southern California Current circulation and
ecosystem. Part I: The physical circulation.
Journal of Physical Oceanography, sub judice.
Abstract.
The adjoint method of sensitivity analysis has been applied in the Regional Ocean Modeling
Systems (ROMS) configured for the southern portion of the California Current system, with
the aim of exploring the sensitivity of the physical and biological circulations to variations
in the physical and biological attributes of the system, in particular on the sensitivity to
surface forcing. The sensitivities of the physical circulation will be explored in this first part.
Indices of upwelling, kinetic energy and the potential for baroclinic instability were identified,
and the sensitivities of these indices to variations in the circulation and forcing computed
using the adjoint of ROMS linearized about time evolving circulations of the nonlinear ROMS.
In general, it was found that all three indices are most sensitive to variations in the model
prognostic variables, a result that is most relevant to the prediction problem. In nature,
however, variations in the prognostic variables must, necessarily, be driven by changes in the
surface forcing.
All of the indices considered exhibit significant seasonal variations in their sensitivity to
surface forcing. Upwelling temperatures along the California central coast are equally sensitive
to variations in wind stress and surface heat flux, and are less sensitive to variations in the wind
stress curl. The upwelling circulation typically displays greatest sensitivity to surface forcing
variations during Summer and Fall when the upwelling favorable alongshore winds are relaxing,
and least sensitivity during the Spring at the peak of the upwelling season. The sensitivity in
Summer is typically some 2-4 times larger than in the Spring, but larger variations (~5-10)
during any particular year are possible.
The sensitivity of eddy kinetic energy to surface forcing off the central coast exhibits little
i
in the way of seasonal trends and there is much interannual variability. The eddy kinetic energy
forcing sensitivity varies by about a factor of 3 over the course of a year.
The potential for baroclinic instability also displays seasonal trends and variations in sensitivity,
being greatest during the Spring months when the horizontal temperature gradients
near the coast are largest which, by virtue of the thermal wind relation, implies larger vertical
shear and hence a greater tendency for the alongshore geostrophic flow to be baroclinically unstable.
The greatest sensitivity is associated with variations in wind stress, and the sensitivity
varies by about a factor of ~2 throughout the year. The sensitivity of baroclinic instability
to variations in wind stress curl is also significant.
Preprint (pdf)