Surface fluxes and Ocean state estimates in the eastern
Subtropical North Atlantic.

Olwijn Leeuwenburgh, Patrick Heimbach and Detlef Stammer

A coarse resolution regional ocean model of the eastern
subtropical North Atlantic is used in a 5-year run to  
investigate the effects of two different surface forcing
formulations and of several strategies to minimize a cost
function in the context of ocean state estimation. 
The model is a 2 degree resolution application of the
MITgcm which is forced at the open boundaries by
temperature, salinity and current velocities obtained
from a global implementation of the same OGCM. The assimilated
data consist of T/P and ERS altimetry, a mean sea surface
estimate, and Reynolds SST, and the results are compared 
with independent buoy data obtained from the Subduction  
Experiment.  Different strategies to bring the model
into consistency with ocean observations are being discussed.
All of them use the models adjoint to adjust control variables,
such as the initial state, lateral boundary conditions  and surface
forcing.  Control terms include either the surface fluxes of momentum,
net heat and freshwater fluxes, or the atmospheric state as control
variables. Early results indicate that there are distinct differences
in magnitude and behavior of the cost function depending on
the type of forcing and the manner in which control
variables are included and adjusted. While a bulk-formulae
type forcing leads to better initial agreement between
model and data, the optimization is more effective (faster
decay) with the traditional type of flux forcing.