Tracing the North Atlantic Deep Water through the Romanche and Chain fracture zones with chlorofluoromethanes

Chlorofluoromethanes (CFMs) F-11 and F-12 were measured during August 1991 and November 1992 in the Romanche and Chain Fracture Zones in the equatoria l Atlantic. The CFM distributions showed the two familiar signatures of the more recently ventilated North Atlantic Deep Water (NADW) seen in the Deep Western Boundary Current (DWBC). The upper maximum is centered around 1600 m at the level of the Upper North Atlantic Deep water (UNADW) and the deep er maximum around 3800 m at level of the Lower North Atlantic Deep Water (L NADW). These observations suggest a bifurcation at the western boundary, so me of the NADW spreading eastward with the LNADW entering the Romanche and the Chain Fracture Zones. The upper core (sigma(1.5) = 34.70 kg m(-3)) was observed eastward as far as 5 degrees W. The deep CFM maximum (sigma(4) = 4 5.87 kg m(-3)), associated with an oxygen maximum, decreased dramatically a t the sills of the Romanche Fracture Zone: east of the sills, the shape of the CFM profiles reflects mixing and deepening of isopycnals. Mean apparent water "ages" computed from the F-11/F-12 ratio are estimated. Near the bot tom, no enrichment in CFMs is detected at the entrance of the fracture zone s in the cold water mass originating from the Antarctic Bottom Water flow. (C) 1999 Elsevier Science Ltd. All rights reserved.