Circulation rate changes in the eastern subtropical North Pacific based on chlorofluorocarbon ages

Abstract

A space‐time regression analysis of chlorofluorocarbon (CFC) data from the eastern North Pacific Ocean's subtropical thermocline (25.6–26.6 σθ) is used to extract age and velocity changes from a collection of 1980s and 1990s hydrographic sections. Results indicate substantial increases of CFC ages, from 0.2 yr yr−1 on 25.8 σθ increasing with depth up to 0.4 yr yr−1 on 26.6 σθ, and increases of meridional age gradients, implying a circulation slowdown. Using the regression‐derived spatial and temporal derivatives, an isopycnal balance of advection and diffusion, including the effects of mixing on CFC ages, was applied to estimate changes in flow. Southward velocities at 20°N, 145°W generally decreased by ∼ 0.03 ± 0.02 cm s−1 yr−1 on isopycnals 25.6 to 26.4 σθ during the 1980s and 1990s, consistent with overturning changes from historical hydrographic data and with concurrent increases in AOU. The deeper (26.6 σθ) age increase was consistent with mixing effects.