Modeling the Carbon Cycle in the Equatorial Pacific with New and
Export Productivity Regulation by Si and Fe.
F. Chai, University of Maine
R. Barber, Duke University
R. C. Dugdale and F. P. Wilkerson, San Francisco State University
To identify and quantify the physical, chemical, and biological
processes that control the carbon cycle in the Equatorial Pacific Ocean,
with a focus on how silicate and iron affect new and export productivity
and partitioning of carbon between the atmosphere, surface ocean and
The eastern and central equatorial Pacific Ocean is a vast area
representing a large portion of the ocean-atmosphere carbon system. The
most prominent feature of this area is the upwelling of CO2-rich
subsurface water, which creates the largest regional source for the net
CO2 flux from the ocean to the atmosphere. The rich nutrients brought
up by the upwelling water also promote biological activities. New
production, equivalent to carbon export production under steady state
condition, in this region could reach as high as 25 to 50% of the global
ocean values. Extensive data sets have been obtained by the U.S. JGOFS
EqPac (Equatorial Pacific) process study program and NOAA/OACES CO2
survey program. To analyze and to synthesize these field data for a
better understanding of the carbon cycle dynamics in the Equatorial
Pacific should help to reach the goal of this project.
The ecosystem model to be used in this study is based on the Modular
Ocean Model (MOM) of the NOAA Geophysical Fluid Dynamics Laboratory
(GFDL) Ocean General Circulation Model (OGCM). It consists of five
components describing phytoplankton (P), zooplankton (Z), detritus (D),
and two forms of dissolved inorganic nitrogen: nitrate (NO3) and
ammonium (NH4). The carbonate chemistry
including dissolved CO2 gas,
bicarbonate and carbonate ions will be parameterized in the model. This
model will be calibrated with the historical processed field data
(Levitus, 1982; Levitus et al., 1993), and the data collected from US
JGOFS EqPac process study and NOAA/OACES field programs. This study
will investigate how new and export productivity responds to changing
physical and chemical forcing. The domain of the model is between 30o S
and 30o N, 120o E and 70o W, with real geometry and topography, but
analysis will focus on the equatorial region between 5o S and 5o N. In
1998, we will modify an existing five-box ecosystem model (Chai et al.,
1996) by adding three more boxes (silicate, diatoms and
mesozooplanktonic grazers) following the approach of Dugdale et al.
(1995) and Dugdale and Wilkerson (1997). We will add TCO2 and
alkalinity for studying carbon cycle. The pre-industrial atmospheric
CO2(=280 uatm) will be used to hind cast air-sea flux of CO2 in the
equatorial Pacific. New production regulating on silicate should
provide a more accurate computation of CO2 compared to using nitrate as
a regulating nutrient.
To analyze and to synthesize observed field data for a better
understanding of the carbon cycle dynamics in the Equatorial Pacific is
a specific goal of the U.S. JGOFS Synthesis and Modeling Project. As a
contribution towards achieving this goal, a joint proposal with Barber
of Duke University, Chai of University of Maine, and Dugdale and
Wilkerson of San Francisco State University has been submitted to NSF,
and it has subsequently been funded to develop an ecosystem model (based
on marine ecosystem model of Chai et al., 1996) with a focus on how
silicate and iron affect new and export productivity and the
partitioning of carbon between atmosphere, surface ocean and deep ocean.
The award of 100 hours CPU time of ARSC Cray-YMP to Peng for 1997 has
been used as a necessary resource to accomplish the proposed NSF
objective. A proposal requesting an additional 50 hours of CPU times in
1998 has been submitted to NOAA/ERL. The modeling work under NSF
funding has just begun since December of 1997.
Chai, F., S.T. Lindley, and R.T. Barber (1996): Origin and maintenance of
high nutrient condition in the equatorial Pacific. Deep-Sea Res. II, Vol.
42, No. 4-6, 1031-1064.
Dugdale, R. C., F. P. Wilkerson, and H. J. Minas (1995). The role of a
silicate pump in driving new production, Deep-Sea Res. 42: 697-719.
Dugdale, R. C. and F. P. Wilkerson (1997). Silicate regulation of new
production in the eastern equatorial Pacific, Nature.
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