ASSIMILATION OF SURFACE DATA IN A ONE-DIMENSIONAL PHYSICAL-BIOGEOCHEMICAL MODEL OF THE SURFACE OCEAN .2. ADJUSTING A SIMPLE TROPHIC MODEL TO CHLOROPHYLL, TEMPERATURE, NITRATE, AND PCO(2) DATA

A one-dimensional coupled physical bio-geochemical model of the surfac e ocean was constrained to reproduce the seasonal evolution of surface chlorophyll, temperature and nitrate concentration by adjusting the p arameters of a simple trophic system using a variational technique. It is found that assimilation of surface chlorophyll data only not allow a robust constraint of the simulated trophic cycle. When surface chlo rophyll, temperature and nitrate data are used simultaneously, seven l inear combinations of the model parameters can be adjusted. While temp erature values are useful to constrain vertical diffusion below the mi xed layer, the seasonal nitrate evolution (2 values/year) is a strong constraint on the balance between vertical diffusion and particle sink ing rate. The three types of surface information provide robust constr aints on the model parameters and on the main carbon fluxes within the euphotic layer. For the model to reproduce observed surface pCO(2) co ncentrations, it is found as necessary to adjust the C/N ratios. An ex ample is given, where the C/N ratio for net organic matter export from euphotic zone is increased to 12 +/- 1 and the C/N ratio for detritus remineralization is decreased to 3.3 +/- 2, rather than the canonical value of 7.