Constant ventilation age of thermocline water in the eastern subtropical North Pacific Ocean from chlorofluorocarbon measurements over a 12‐year period

Abstract

Northeastern Pacific chlorofluorocarbon (CFC) data collected between 1982 and 1994 near Geochemical Ocean Sections Study (GEOSECS) station 1 (28.5°N, 122.5°W) record decadal timescale ventilation processes of the subtropical thermocline in this region. The CFC‐12 concentration age field versus potential density has been remarkably constant over the 12‐year period, although CFC concentrations in the upper kilometer of the water column have increased with time. Results from a simple one‐dimensional advection‐diffusion model are consistent with an advection velocity of ca. 0.8–0.9 cm s−1 from the source area. The influence of the 1982–83 El Niño is noticeable in the 1983 observations. While the main stream of subarctic source water appears to spread southward at a constant rate, during El Niño years the influence of comparatively CFC‐free tropical thermocline waters is enhanced in this region, leading to reduced vertical inventories of CFCs, but without changing the apparent CFC ages as functions of potential density. Apparent oxygen utilization rates decrease with increasing CFC age, and also appear not to have changed significantly over the 12‐year measurement period.