OCEANIC VERTICAL EXCHANGE AND NEW PRODUCTION: A COMPARISON BETWEEN MODELS AND OBSERVATIONS.

Reference
Gnanadesikan, A., R. D. Slater, N. Gruber, and J.L. Sarmiento. (2002), Oceanic vertical exchange and new production: A comparison between models and observations. Deep-Sea Research II, 49, 363-401.

Abstract

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 newproducti on (defined as the net uptake of phosphate). A baseline case is run with low
vertical mixing in the pycnocline and a relatively lowlate ral diffusion coeffcient. The magnitude of the
diapycnal diffusion coeffcient is then increased within the pycnocline, within the pycnocline of the Southern
Ocean, and in the top 50 m; while the lateral diffusion coeffcient is increased throughout the ocean. It is
shown that it is possible to change lateral and vertical diffusion coeffcients 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 coeffcient within the pycnocline, resulting in a new production of
around 10 GtC yr-1: On a regional level, however, large differences appear between observational and
model based estimates. In the tropics, the models yield systematically higher levels of newprodu ction 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 newproducti on
are all much lower than the observational estimates, probably as a result of the failure to form intermediate
water with the right properties. The results demonstrate the potential usefulness of new production for
evaluating circulation models.

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