Diagnosing Water Mass Formation from Air–Sea Fluxes and Surface Mixing

Abstract

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?4 m2 s?1 for the denser waters, about four times as large as the explicit diapycnal diffusion within the thermocline.