Oceanic vertical exchange and new production: a comparison between models and observations

This paper explores the relationship between large-scale vertical exchange and the cycling of biologically active nutrients within the ocean. It considers how the parameterization of vertical and lateral mixing effects estimates of new production (defined as the net uptake of phosphate). A baseline case is run with low vertical mixing in the pycnocline and a relatively low lateral diffusion coefficient. The magnitude of the diapycnal diffusion coefficient is then increased within the pycnocline, within the pycnocline of the Southern Ocean, and in the top 50 m; while the lateral diffusion coefficient is increased throughout the ocean. It is shown that it is possible to change lateral and vertical diffusion coefficients so as to preserve the structure of the pycnocline while changing the pathways of vertical exchange and hence the cycling of nutrients. Comparisons between the different models reveal that new production is very sensitive to the level of vertical mixing within the pycnocline, but only weakly sensitive to the level of lateral and upper ocean diffusion. The results are compared with two estimates of new production based on ocean color and the annual cycle of nutrients. On a global scale, the observational estimates are most consistent with the circulation produced with a low diffusion coefficient within the pycnocline, resulting in a new production of around 10 GtC yr : On a regional level, however, large differences appear between observational and model based estimates. In the tropics, the models yield systematically higher levels of new production than the observational estimates. Evidence from the Eastern Equatorial Pacific suggests that this is due to both biases in the data used to generate the observational estimates and problems with the models. In the North Atlantic, the observational estimates vary more than the models, due in part to the methodology by which the nutrient-based climatology is constructed. In the North Pacific, the modelled values of new production are all much lower than the observational estimates, probably as a result of the failure to form intermediate *Corresponding author. E-mail addresses: [email protected] (A. Gnanadesikan), [email protected] (R.D. Slater), [email protected] (J.L. Sarmiento). 0967-0645/01/$ see front matter r 2001 Published by Elsevier Science Ltd. PII: S 0 9 6 7 0 6 4 5 ( 0 1 ) 0 0 1 0 7 2 water with the right properties. The results demonstrate the potential usefulness of new production for evaluating circulation models. r 2001 Published by Elsevier Science Ltd.