Upper Ocean 1-D Seasonal Models
File last modified 16 November 1998
19.4b Putting Biological Productivity in the Model
The vertical distribution of production is clearly a critical factor in this modeling exercise. The problem is not completely unconstrained, for we do have observations of the distribution of oxygen excess, and we do have an idea about the vertical distribution of chlorophyll and photosynthetically available radiation, so some degree of common sense can prevail in restricting the scope of our choices. However, we will suggest two candidate profiles of production which encompass two extremes in possible scenarios. The first is to ask the question "what if all of the production took place in the mixed layer?" Now nobody seriously believes this to be the case, but it does have one interesting consequence: this scenario constitutes the worst case maximum oxygen productivity because this is the situation where we will get the most leakage of photosynthetically produced oxygen from the ocean. Thus whatever oxygen production level we obtain from this scenario is the upper bound on production.
The second scenario is to produce the oxygen as far down in the water column as is consistent with reasonable biological activity. Given the exponential decrease of PAR with depth, we would expect that an insignificant fraction of production (less than 10%) occurs below 75m depth. Putting the level deeper appears inconsistent with the observed oxygen profiles. (If you don't agree with this assertion, then it is a simple matter to run a model calculation with a different profile.) So our candidate "conservative" production profile would be a half-sine function (0 to pi) between 25 and 75m depth. The two profiles might look like the following:
One final addition which is necessary, is that below the euphotic zone, oxygen is consumed by oxidation/remineralization processes. Because the aphotic zone is undersaturated in oxygen due to this process, we need to include oxygen consumption below 100m depth. We use simple linearly decreasing oxygen consumption rate (maximum at 100m, zero at 300m) and set its magnitude at observed oxygen consumption rates based on tritium-helium dating. This is not a big deal, it just keeps the deeper part of the water column undersaturated, like the observations, and that tends to reshape the oxygen profiles a little.
Incorporating production into the model is in fact rather trivial. You just add a step in the time loop, which we call oxyprod.m, which produces/consumes oxygen as a function of time and depth. Since oxygen is already carried along in the calculation for mixing/diffusion/advection, and gas exchange/air injection, then the rest is taken care of. Well, almost… you need to slightly modify inifctr.m, inigas.m, and of course a new driver program pwpb.m as listed below:
oxyamp=5; % water column production mol/m2/y mloxyamp=0; % mixed layer production mol/m2/y inifctr; % Initialize useful factors inihydro; % initialize water column profile inigas; % initialize water column gases % % Main time step loop % for it=1:nt T(1)=T(1)+thf(it); % add sensible + latent heat flux S(1)=S(1)-FWFlux(it); % flux fresh water (latent/precip) T=T+rhf(it)*dRdz; % add radiant heat to profile dostins; % do static instability adjustment addmom; % add wind stress induced momentum dobrino; % do bulk Ri No Adjustment gasexch; % exchange gases oxyprod; % do oxygen productivity dogrino; % do gradient Ri No Adjustment advdif; % advect and diffuse dooutg; % if time, save data end
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