Influence of mixing on CFC uptake and CFC ages in the North Pacific thermocline

Abstract

[1] A diagnostic, isopycnal advection-diffusion model based on a climatological, geostrophic flow field is used to study the uptake of chlorofluorocarbons (CFCs) into the portion of the thermocline that outcrops in the open North Pacific (s q 26.6 kg m À3 ). In addition to advection, isopycnal diffusion is required to match the CFC data collected during the World Ocean Circulation Experiment (WOCE) in the early 1990s. Using reduced outcrop saturations of 80-95% for isopycnals outcropping in the northwestern North Pacific (s q ! 25.4 kg m À3 ), together with an isopcynal interior diffusivity of 2000 m 2 s À1 and enhanced diffusion (5000 m 2 s À1 ) in the Kuroshio Extension region, further improves the model-data agreement. Along-isopycnal diffusion is particularly important for isopycnals with shadow zones/pool regions in the western subtropical North Pacific that are isolated from direct advective ventilation. The isopycnal mixing causes an estimated increase in CFC-12 inventories on these isopycnals, compared to advection only, ranging from 10-20% (s q = 25.6 kg m À3 ) to 50-130% (s q = 26.6 kg m À3) over the subtropics in 1993. This contribution has important consequences for subduction rate estimates derived from CFC inventories and for the location of the subsurface CFC maxima. When tracer ages are derived from the modeled CFC distributions, time-evolving mixing biases become apparent that reflect the nonlinearities in the atmospheric CFC time histories. Comparison with model-calculated ideal ages suggests that during the time of WOCE ($1993), ventilation ages based on CFC-12 were biased young by as much as 16-24 years for pCFC-12 ages of 25 years, underestimating ideal ages by as much as 40-50%.

Water Masses in the Pacific in CCSM3

“…In CPL, the subtropical waters in the Northern Hemisphere, which are significantly saltier than in the observations, are the dominant source of water for the tropical thermocline. This is consistent with the results of Mecking et al (2004), who found that, in a diagnostic model of CFCs in the subtropical North Pacific where currents are derived from the observed density distribution, isopycnal mixing, owing to mesoscale eddies, has to be included to accurately represent the spreading of CFCs into the interior of the subtropical gyre. It is important to keep in mind that there is restoration to observed surface salinity in OCN that forces modeled surface salinity to be close to observations.…”

Mixing and remineralization in waters detrained from the surface into Subantarctic Mode Water and Antarctic Intermediate Water in the southeastern Pacific

“…Mecking et al . [] show that age estimated from tracers with the growth histories of CFC‐11 and CFC‐12 would be biased too old for recently ventilated waters and too young for less recently ventilated waters, so our component‐weighted‐mean ages do adjust for this bias to a degree. However, a stronger statement is impossible without mixing bias estimates specific to this region for comparison.…”

“…In particular, we examine the distribution of water masses in the frontal region of the Antarctic Circumpolar Current (ACC) where SAMW and AAIW form (as did de Brauwere et al . []); we use our water mass distribution to estimate the role of cabbeling in the formation of our target water masses (similar to Yun and Talley [] for North Pacific Intermediate Water); we estimate remineralization ratios for biogeochemical cycling using nutrient measurement distributions [ Anderson and Sarmiento , ]; we include parameterizations allowing for uptake of anthropogenic carbon [ Sabine et al ., ]; we parameterize for the influence of mixing on CFC distributions [ Mecking et al ., ; Peacock and Maltrud , ]; and we estimate rates of in situ carbonate dissolution [ Feely et al ., ].…”

Use of SF6 to estimate anthropogenic CO2 in the upper ocean

“…Similarly, DeGrandpre et al [2006] found under‐saturated conditions in the Labrador Sea through the whole year with respect to CO 2 . Comparing CFC measurements from the ocean interior with data simulated in an advection‐diffusion model, Mecking et al [2004] estimated initial saturations of CFC‐12 ranging from 80–100%, with the initial saturation being correlated with density. Similarly, by comparing the saturation of all surface water samples from a cruise in the Southern Ocean, Tanhua et al [2004] found uncertainties in saturation of ∼5%.…”