CFC time series in the deep water masses of the western tropical Atlantic, 1990–1999

A unique CFC time series was collected along the 35°W meridian in the tropical Atlantic during eight cruises from 1990 to 1999, in order to investigate the large-scale variability of the circulation of the North Atlantic Deep Water (NADW) through transient tracer distributions. Within the upper NADW (UNADW), it appears that CFC distributions, are highly variable; cores of maximal CFC concentrations are closely associated with salinity and are principally caused by the local dynamics of equatorial and extra-equatorial deep jets within the three particular regimes of the 3–1°S, 1°S–1°N and 1–3°N bands. CFC distribution within the lower NADW (LNADW) is less heterogeneous and linked to oxygen. A double core has been observed for 1999, possibly due to the recirculation of the deep flow constrained by the circulation of the underlying Antarctic Bottom Water (AABW). The variability of the maximal concentrations presumably results from the seasonal or semi-annual variability of the regional deep circulation. For both layers, the temporal evolution of the mean concentrations has been traced throughout the 1990s. At the UNADW level, the temporal increase of the CFC concentration within the 3°S–3°N band is mainly due to the transient signal from the atmosphere. This increase is clearly dominant compared with the local dynamics. Similar behavior has been observed for the LNADW in the 3°S–1°N band, corresponding to the equatorial channel. An attempt has been made to access the transit time for the NADW pathway from the Labrador and Nordic Seas to the tropics. The mean transit time around 25 yr for the UNADW has been compared to other evaluations from hydrographic measurements.