The one-dimensional upper ocean ecosystem model includes several important physical, biological, and chemical processes in the central and easten equatorial Pacific (5S to 5N and 90W to 180, the Wyrtki Box). The model is uesd to simulate the Low- Silicate, High-Nitrate, Low-Chlorophyll (LSHNLC) conditions, to provide an estimate of ocean-to-atmosphere CO2 flux from this upwelling-dominated region (see Diagram for the model details). Also, the set of Equations provides the formulation of the model. The vertical domain of the model is from the ocean surface down to 200m. The inputs of the physical forcing are the area-averaged (over the Wyrtki Box) annual mean upwelling velocity and vertical diffusivity, which are obtained from the simulations of the three-dimensiaonl ocean circulation model in the region (see Chai et al. 1997 for details). The model also includes both temperature and salinity as two state variables. By doing so, the modeled sea surface temperature (SST) and the air-sea heat flux are compared with observations, and such comparisons are served as the criteria on how to adjust the upwelling velocity and vertical diffusivity. For example, if the modeled SST is too low and the heat flux from the atmosphere to the ocean is too high, which indicate the upwelling velocity used in this simulation are too strong, then the upwelling rates need to be reduced. In general, the SST is between 25C to 26C and the heat flux is about 100 W/m2 in the Wyrtki Box.
There are 33 levels in the vertical which provides a resolution of 5 m within the upper 120m and 10m resolution between 120m to 200m. The time interval is one hour. The model resolves the day-and-night cycle, i.e., no primary production takes place during the night. The initial conditions are the area-averaged observed temperature, salinity,NO3, and Si(OH)4 (Levitus et al., 1994), and observed TCO2 (JGOFS EqPac). The initial conditions for all other variables are assigned with a value of 0.01 mmol/m3. Normally, the model needs to run up to 1500 days to reach the steady state for all the variables. The biological variables (P1, P2, Z1, Z2, NH4, Detritus) reach the steady state very fast, 30 to 60 days, whereas the temperature (T), NO3 and Si(OH)4 take much longer time to adjust.
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