Modeling CFC and SF<sub>6</sub> Mixed Layer Boundary Conditions

Chlorofluorocarbons (CFCs) and sulfur hexafluoride (SF₆) are man-made, transient tracers that enter the ocean surface mixed layer through air–sea gas exchange and that then are transported into the ocean interior as part of the general ocean circulation (including subduction and water mass convection processes). Because of their conservative and time-evolving nature, CFC-11 and CFC-12, and more recently also SF₆, are widely used to estimate ocean interior ventilation time scales (ages) as well as anthropogenic carbon uptake by the ocean.

The accuracy of these tracer-derived age and carbon estimates depends on the knowledge of the tracers' mixed layer boundary function. While it is often assumed that mixed layer concentrations of CFCs and SF₆ are in equilibrium with the atmosphere, it also known that particularly the combined effect of mixed layer cooling and deepening throughout winter may cause CFCs/SF₆ in the mixed layer to be undersaturated (= negative deviation from equilibrium state). Because oceanic tracer measurements during this time of year are sparse, this project aims at providing simulations of the evolution of the CFC and SF₆ mixed layer boundary conditions over the past ~70 years using a global ocean model.

Goals		Hypotheses
The overall objective is to produce realistic simulations of CFC-11, CFC-12, and SF₆ using a global ocean model with focus on mixed layer processes and deviations of CFC/SF₆ concentrations from air–sea equilibrium. Through a variety of model test cases, we will be able to quantify the role that different processes, such as air–sea gas exchange, mixed layer warming/cooling and entrainment, may have in causing these disequilibria and to present the time-evolution of the signal for each tracer in each case. The model results are intended to aid the interpretation of data from large, global surveys such as the WOCE and CLIVAR/CO₂ Repeat Hydrography programs as well as other observational programs.		Regions where the winter mixed layer become very deep are correlated with large CFC and SF₆ undersaturations because of the associated cooling, entrainment, and increase in volume of mixed layer waters that need to equilibrate. The patterns of gas disequilibria should be similar for CFC-11, CFC-12, and SF₆. The magnitude of under- and also supersaturations may vary depending on each tracer's Schmidt number (affecting gas exchange velocity), temperature-dependent solubility function (affecting equilibrium concentration values), and atmospheric time history (affecting growth rate).

Approach		Impact
The ocean model used in this study to derive physical fields is the Hallberg Isopycnal Model (HIM). Three-dimensional mass transports, isopycnal thickness, surface winds, temperature, and salinity from this model are then used to drive an offline model that simulates advection and diffusion of passive tracers, including CFC-11, CFC-12, and SF₆. Offline modeling of tracers allows for a greater time step, less computational cost, and hence faster execution of a variety of numerical experiments. The offline model contains the same number of layers (49) and grid cells (210 x 360) as the physical model and is run from ~1940 when detectable amounts of CFCs first occurred in the atmosphere until the present.		This research helps refine tracer-based estimations of anthropogenic carbon and ventilation time scales in the ocean, including decadal variability therein. Errors in inventories, ages, and anthropogenic carbon that occur when neglecting air–sea disequilibria of CFCs and SF₆ in the mixed layer are time-evolving and range from a few percent in recent years to order 10–20% earlier on. Knowledge of the pathways of mixed layer water into the ocean interior is required for the direct application of the modeled air–sea gas disequilibria to individual data points in the ocean. Implementation of the model adjoint is planned to help achieve this in a next step.


Maximum annual mixed layer depth — usually occurs in late winter.		Maximum CFC-11 undersaturation (% neg. deviation from equilibrium) in 1980 — usually occurs 0–2 month before maximum mixed layer depth.

Results

Mixed layer saturations of CFC-11, CFC-12, and SF₆ in a global isopycnal model

Shao, A.E., S. Mecking, L. Thompson, and R.E. Sonnerup, "Mixed layer saturations of CFC-11, CFC-12, and SF₆ in a global isopycnal model," J. Geophys. Res., 118, 4978-4988, doi:10.1002/jgrc.20370, 2013.

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4 Oct 2013

The use of CFC-11, CFC-12, and SF₆ to quantify oceanic ventilation rates, interior water age, and formation rates requires knowledge of the saturation levels at the sea surface. While their atmospheric histories are relatively well known, physical processes in the mixed layer in conjunction with limited air-sea gas exchange can cause surface concentrations to be in disequilibrium with the atmosphere. We use an offline tracer advection-diffusion code that evolves tracers using along-isopycnal and cross-isopycnal mass fluxes from a global, climatological run of the Hallberg Isopycnal Model to reconstruct the saturation level of all three tracers over the entirety of their atmospheric histories. Disequilibria on a global scale occur in regions associated with deep winter mixed layers and are found throughout the time period of the release of these chemicals into the atmosphere. Sensitivity studies using targeted model simulations, focusing on the North Pacific, show that seasonal cycles in temperature and salinity that affect gas solubility as well as entrainment of water containing low concentration of tracers during mixed layer deepening are the dominant causes of undersaturation. When using the transit time distribution method, our results show that these undersaturations introduce a significant bias toward older ages for North Pacific Central Mode Water but do not significantly affect estimates of anthropogenic carbon inventory.

Transit time distributions and oxygen utilization rates in the Northeast Pacific Ocean from chlorofluorocarbons and sulfur hexafluoride

Sonnerup, R.E., S. Mecking, and J.L. Bullister, "Transit time distributions and oxygen utilization rates in the Northeast Pacific Ocean from chlorofluorocarbons and sulfur hexafluoride," Deep-Sea Res. I, 72, 61-71, doi:10.1016/j.dsr.2012.10.013, 2013.

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1 Feb 2013

Depth profiles of dissolved chlorofluorocarbon-11 (CFC-11) and sulfur hexafluoride (SF6) were measured during a September 2008 cruise in the Northeast Pacific Ocean. For each water sample, the two tracers were used in concert to estimate likely mean ages and widths of parameterized 1-D transit time distributions (TTDs). In shallow waters (<250 m), the TTDs' mean ages were relatively loosely constrained due to the slow decrease of atmospheric CFC-11 since 1994. In the main thermocline (25.0–26.6 σ_θ, ~300–550 m), the CFC-11/SF₆ tracer pair constrained TTDs' mean ages to within ±10%. Deeper than 26.8 σ_θ (~600 m), SF₆ levels in 2008 were too low for the CFC-11/SF₆ tracer pair to constrain the TTDs' mean ages. Within the main thermocline of the subtropical North Pacific Ocean (20°–37°N along 152°W), the TTDs'9 mean ages were used to estimate Oxygen Utilization Rates (OURs) of ~11 μmol kg^-1 yr^-1 on 25.0–25.5 σ_θ (~160 m), attenuating to very low rates (0.12 μmol kg^-1 yr^-1) by 26.8–27.0 σ_θ (~600 m). Depth integration of the in-situ OURs implied an average carbon remineralization rate of 1.7±0.3 mol C m^-2 yr^-1 in this region and depth range, somewhat lower than other independent estimates. Along the 152°W section, depth integrating the apparent OURs implied carbon remineralization rates of 2.5–3.5 mol C m^-2 yr^-1 from 20°N to 30°N, 3.5–4.0 mol C m^-2 yr^-1 from 30°N to 40°N, and 2–2.7 mol C m^-2 yr^-1 north of 45°N.