Modeling the Carbon Cycle in the Equatorial Pacific with New and Export Productivity Regulation by Si and Fe.

Principal Investigator: Tsung-Hung Peng
Collarborating scientists:
F. Chai, University of Maine
R. Barber, Duke University
R. C. Dugdale and F. P. Wilkerson, San Francisco State University
Objective: 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 deep ocean.
Rationale: 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.
Method: 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.
Accomplishment: 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.
Key reference:
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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