Dataset: Model Results of Si- and Fe-regulated Production in Equatorial Pacific
Deployment: USJGOFS_SMP

To identify and quantify the principal processes
that control the partitionin g of carbon among oceanic reservoirs and between
the ocean and atmosphere on local and regional scales, with a view towards
synthesis and prediction on a global scale, is a specific goal of the U.S.
JGOFS Synthesis and Modeling Project.  As a contribution towards achieving
this goal, Drs. Barber, Peng, Chai, Dugdale and Wilkerson will develop
an ecosystem model for the equatorial Pacific Ocean, with a focus on how
silicate and iron affect new and export productivity and the partitioning
of carbon between the atmosphere, surface ocean and deep ocean.  The
study will use an ecosystem model embedded in a state-of-the-art general
circulation model for the equatorial Pacific Ocean to investigate how new
and export productivity responds to changing physical and chemical forcing. 
The domain of the model is between 30S and 30N, 120E and 70W, with real
geometry and topography, but analysis will focus on the equatorial region
from 5N to 5S.  The recent upgrade of supercomputers at North Carolina
Supercomputing Center (NCSC) (CrayT90) and Arctic Region Supercomputing
Center (ARSC) (Cray-YMP) and the award of several hundred hours of CPU
time to Peng, Chai and Barber make it possible to embed an ecosystem model
with modest complexity in a high resolution, three dimensional prognostic
ocean model,and to conduct numerous experiments on the ecosystem model
structure and parameters in a timely and efficient manner.

Phase 1 of the project will modify an existing five-compartment ecosystem
model by adding three more compartments (silicate, diatoms and mesozooplanktonic
grazers) following the approach of Dugdale et al.  The preliminary
objective of this three-dimensional Si/N/light model is to reproduce High 
Nitrate-Low Silicate-Low Chlorophyll (HNLSLC) conditions.  With size-depend
ent growth rate responses in small phytoplankton and diatoms and varying
grazing vulnerability, the role of new diatom production regulating on
Si and Fe can be thoroughly investigated.  Also in Phase 1, TCO₂

and total alkalinity (ALK) will be added in order to calculate pCO₂.
The pre-industrial atmospheric CO₂ (280 ppm) will be used to
hindcast air-sea flux of CO₂ in the equatorial Pacific. 
New production regulating on silicate should provide a more accurate calculation
of CO₂ compared to using nitrate as a regulating nutrient.

In Phase 2 the effect of iron is added to the model making a, the initial
slope of the photosynthesis vs. irradiance curve, a function of iron. The
values of a are based on equatorial observations of natural and experimental
iron additions. Independently, Ks for Si(OH₄) is made a function
of iron, an effect that involves only diatoms.  The `balance to bloom`
transition will be simulated with the two iron effects to reproduce the
IronEx 1 and 2 phytoplankton responses to a transient iron addition. 

This modeling study will provide estimates of new and export productivity,
and a formal description of Si and Fe as regulating mechanisms in the equatorial
Pacific Ocean.  When new and export productivity is modeled accurately
and validated with JGOFS studies, it will possible to predict with increased
confidence how climate change may alter, via biogenic export, maintenance
of the air-sea dpCO₂ and hence the ocean's uptake and release
of CO₂.