Chlorofluorocarbons in the central North Pacific and southward spreading time of North Pacific intermediate water

Abstract

From August to October 1992, cross sections of Chlorofluorocarbons (CFCs) trichlorofluoromethane (CFC‐11) and dichlorofluoromethane (CFC‐12) were obtained between 48°N and 15°S along 175°E in the central North Pacific. On the basis of the distribution of CFCs, apparent ages of the North Pacific Intermediate Water (NPIW) along 175°E were determined for the isopycnal horizons of 26.40 to 27.40 σθ. The apparent age of NPIW was usually older toward the south and with increase of σθ along the section. The oldest NPIW on all isopycnal horizons in the North Pacific was located near 10°N. North of 20°N, NPIW on isopycnal horizons less than 26.80 σθ outcrops in winter in subpolar regions and appears to have formed after 1970. It is suggested that NPIW on isopycnal horizons less than 26.80 σθ is laterally well mixed between subpolar and subtropical regions on timescales of a few decades. NPIW on isopycnal horizons greater than 26.80 σθ near 37°N was older than its surroundings. It is suggested that new NPIW formed in the subpolar region is not transported directly southward and that the major current of NPIW is zonal in the central North Pacific, flowing eastward in the subpolar region and then turning more to the south, flowing toward 10°N near the eastern boundary. Thus we also estimated the zonal mean apparent ages of NPIW. The mean ages of NPIW suggested that the apparent southward spreading rates and the apparent southward spreading times of NPIW from the subpolar region to 10°N were approximately 80–170 km yr−1 and 20–45 years, respectively. It is concluded that the southward spreading time of NPIW is a few decades at most.