Middepth spreading in the subpolar North Atlantic Ocean: Reconciling CFC-11 and float observations

Abstract

Chlorofluorocarbons (CFCs) enter the middepth layers of the North Atlantic Ocean from the atmosphere during the formation of dense mode water in the Labrador and Irminger Seas of the subpolar gyre. The CFC-bearing waters then spread from the convection regions through advection and diffusion of the water masses. Using recent estimates of the circulation at 1500 m depth from subsurface profiling floats the spreading of CFC-11 is simulated in the subpolar North Atlantic with an advective-diffusive model. Several numerical experiments are performed with different stream functions, lateral diffusivities and variations in the CFC-11 sources. The results are then compared to the observed CFC-11 field during 1996-1998. Poor fits are found for diffusivities less than about 500 m2 s-1. Better fits are found for diffusivities ranging from 500-12,000 m2 s-1, although unrealistically smooth model solutions are produced if the diffusivity exceeds about 3000 m2 s-1. Simulations that include both Labrador and Irminger Sea CFC-11 sources fit the data better than with Labrador sources alone. None of the model CFC solutions fit the data within the CFC uncertainty over the whole domain; the model performs well in the western part of the subpolar gyre, but CFC-11 concentrations are consistently too low in the West European Basin. It is possible that uncertainty in the float-based circulation can account for these misfits, and a more accurate circulation estimate might be able to fit the observed CFC-11 field. Alternatively, time variations in the flow or deep water formation processes, which clearly exist in the real ocean, may need to be included. Copyright 2008 by the American Geophysical Union.