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The formation rate of water masses and its relation to air-sea fluxes and interior mixing are examined in an isopycnic model of the North (and tropical) Atlantic that includes a mixed layer. The diagnostics follow Walin's formulation, linking volume and potential density budgets for an isopycnal layer. The authors consider the balance between water mass production, mixing, and air-sea fluxes in the model in the context of two limit cases: (i) with no mixing, where air-sea fluxes drive water mass formation directly, and (ii) a steady state in a closed basin, where air-sea fluxes are balanced by diffusion. In such a steady state, since mixing always acts to reduce density contrast, surface forcing must act to increase it. Considered over the whole basin, including the Tropics, the model is in steady state apart from the densest layers. Most of the mixing is achieved by diapycnal diffusion in the strong density gradients within upwelling regions in the Tropics, and by entrainment into the tropical mixed layer. Mixing from entrainment associated with the seasonal cycle of mixed layer depth in mid and high latitudes and lateral mixing of density within the mixed layer are less important than this tropical mixing. These model results as to the relative importance of the different mixing processes are consistent with a simple scaling analysis. Outside the Tropics, the upwelling-linked mixing is no longer important, and a first-order estimate of water mass formation rates may be made from the surface fluxes. Lateral mixing of density within the mixed layer and seasonal entrainment mixing are as important as the remaining thermocline mixing within this domain. An apparent vertical diffusivity is diagnosed over both the full and extratropical domain. It reaches 10 Ϫ4 m 2 s Ϫ1 for the denser waters, about four times as large as the explicit diapycnal diffusion within the thermocline.

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A model of biological production in the euphotic zone of the North Atlantic has been developed by coupling a seven‐compartment nitrogen‐based ecosystem model with a three‐dimensional seasonal general circulation model. The predicted seasonal cycles of phytoplankton, Zooplankton, bacteria, nitrate, ammonium, primary production, and particle flux have been compared to data from Bermuda Station “S” and Ocean Weather Station “India”. Bearing in mind the simplicity of the model and the paucity of data, the results are encouraging. However, deficiencies in the physical model lead to winter nitrate values at Bermuda being overestimated, and at both positions the predicted magnitude of the spring phytoplankton bloom was too high. Simulations were carried out with different detrital sinking rates and and it was found that a sinking rate of 10 m d−1 gave the best agreement with observations. The model was used to investigate the factors affecting the population growth of phytoplankton and it was found that the model supported the generally held theory that the spring bloom is initiated by the cessation of physical mixing. After the bloom, phytoplankton are controlled by Zooplankton grazing. At Ocean Weather Station “India” the model reproduced the observed high summer nitrate levels and suggested that these high values are caused by a combination of high vertical nitrate transport, ammonium inhibition of nitrate uptake, and Zooplankton grazing control. The model demonstrated the critical importance of Zooplankton in understanding ecosystem dynamics and highlights the need for more observational data on the seasonal cycles of Zooplankton biomass and growth rates.