Chlorofluorocarbons (CFCs) in the North Pacific Central Mode Water: Possibility of under-saturation of CFCs in the wintertime mixed layer

Abstract

Analysis of chlorofluorocarbons (CFCs) data in seawater in the Kuroshio-Oyashio Transition Area obtained during the WOCE Campaign P-13 along 165°E in August to October 1992 and P-14 along 179°E in July to September 1993 revealed that CFC-12 and CFC-11 in the North Pacific Central Mode Water (NPCMW) were significantly under-saturated with respect to the atmospheric CFC-12 (75.5 ± 12.3% for P-13 and 85.9 ± 6.3% for P-14) and CFC-11 (72.9 ± 10.6% for P-13 and 82.6 ± 5.3% for P-14). Since the mode water is derived from the vertical convection in the surface layer in winter and is considered not greatly influenced by mixing with ambient water during the subsequent advection, the observed under-saturation of CFC-12 and CFC-11 in the mode water suggests that CFCs in the mixed layer in winter has already been under-saturated with respect to the atmospheric CFC-12 and CFC-11. A result from a simple one-dimensional model for the mixed layer also supports the entrainment of the water with lower degree of CFC-12 and CFC-11 saturation and the under-saturation in the wintertime mixed layer.