CLIMATIC VARIABILITY IN UPPER OCEAN VENTILATION RATES DIAGNOSED USINGCHLOROFLUOROCARBONS

The chlorofluorocarbon CFC-12 (CCl2F2) distributions from two occupati ons of a meridional hydrographic section in the eastern North Atlantic are used to describe the oceanic penetration of CFCs and change in th e integrated ventilation patterns over the five years from 1988 to 199 3. The CFC-12 water-column inventories increased by 30-40%, despite a slowing atmospheric growth rate (14%), because of continuing uptake by undersaturated subsurface water masses whose response is lagged by th e ventilation time-scales. After removing the long-term CFC temporal t rend using a tracer age based normalization technique, we observe a di stinct dipole pattern in upper ocean ventilation, with reduced convect ion in the subpolar gyre and enhanced production of saline subtropical underwater in 1993. These differences are discussed in relation to in terannual variability in atmospheric surface forcing, upper ocean anom alies, and convection patterns associated with the North Atlantic Osci llation.