Is bottom boundary layer mixing slowly ventilating Greenland Sea deep water?

Abstract

Bottom water temperatures in the central Greenland Sea have been increasing for the last two decades. The warming is most likely related to the absence of deep convective mixing, which cools and freshens the deep water. However, recent observations confirm a slow and steady increase of anthropogenic tracers such as chlorofluorocarbons (CFCs). This points to some amount of bottom water 'ventilation' in the absence of deep convective mixing and poses a challenge to our understanding of deep water renewal. One explanation for the observed trends in both temperature and CFCs is significant vertical mixing. The basin-averaged diapycnal diffusivity, required to explain both trends, k(u.av) ~ 2-3 (x10-3 m2 s-1), is very unlikely to occur in the interior of the ocean. However, a diffusivity of k(u.bbl) ~ 10-2 m2 s-1 within a 150-m thick bottom boundary layer would be sufficient to explain the deep tracer increase. The implications of a secondary circulation driven by such large boundary layer mixing are discussed.