Distribution Of Chlorofluorocarbons Research Articles

Isolating the role of mesoscale eddies in mixing of a passive tracer in an eddy resolving model

This study examines the role of mesoscale eddies in distribution of chlorofluorocarbon (CFC) in a numerical model of the North Atlantic. Our main conclusions are based on a comparison of two eddy‐resolving experiments, one with and one without eddy advection of CFC. The CFC distributions in these two experiments exhibit substantial differences within the subpolar gyre. In particular, in the presence of eddies the horizontal distribution of CFC in the upper 1000 m is more uniform and the CFC concentration is lower in the lower 1500 m. Our analysis demonstrates that these basin‐wide differences are caused by the interactions of eddy advection with convection and mean advection in several isolated locations. The corresponding eddy flux divergence terms are spatially non‐uniform in magnitude; their effects are often nonlocal and sometimes work against the spreading of CFC being carried out by the mean advection. These conclusions are further confirmed by a third experiment, in which the eddy advection was replaced by a down‐gradient diffusive parameterization. Our results demonstrate that advection by mesoscale eddies is crucial to the mixing of atmospheric gasses into the ocean, and outline difficulties in representing eddies by the down‐gradient diffusion.

Chlorofluorocarbons in the Sea of Okhotsk: Ventilation of the intermediate water

Ventilation of the intermediate layer in the Sea of Okhotsk is studied with regard to the ventilation of North Pacific Intermediate Water (NPIW). Measurements of chlorofluorocarbons (CFCs) from 1998 to 2000 reveal that the Okhotsk intermediate water is ventilated in two ways. The first consists of dense water formation in the polynyas that form on the northern shelves during winter. Measurements show that on the northwestern shelf, because of atmospheric cooling and brine rejection, the cold water that forms is dense enough to enter the intermediate layer. The CFC concentration in this dense shelf water (DSW) is high and almost saturated with respect to atmospheric CFCs, indicating that the DSW experiences active air‐water gas exchange during its formation. Away from the shelves, the CFC distribution shows that the DSW directly ventilates the upper level of the Okhotsk Sea Intermediate Water (OSIW). The second ventilation process is driven by the strong tidal mixing around the Bussol' Strait. The observed distribution of CFCs at the 27.4 σθ level suggest that diapycnal mixing around the strait transports CFCs from the surface to this deeper layer, ventilating the OSIW. Combination of both processes means that the OSIW is more ventilated than Pacific water at the same density levels.

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

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 (σθ ≤ 26.6 kg m−3). 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 (σθ ≥ 25.4 kg m−3), together with an isopcynal interior diffusivity of 2000 m2 s−1 and enhanced diffusion (5000 m2 s−1) 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% (σθ = 25.6 kg m−3) to 50–130% (σθ = 26.6 kg m−3) 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%.

Chlorofluorocarbons in a global ocean eddy‐resolving OGCM: Pathway and formation of Antarctic Bottom Water

We have investigated the pathway of Antarctic Bottom Water (AABW) using simulated chlorofluorocarbons (CFCs) distribution in a global eddy‐resolving OGCM. Our goal is to improve our understanding of the processes and pathways determining the distribution of CFCs in the Southern Ocean, where much of this tracer is entrained by formation of the deep and bottom water. The model exhibits considerable skill in reproducing the section along the Greenwich Meridian (AJAX) and 170°W (WOCE P15S). In the South Atlantic, the model indicates two pathways transporting CFC‐11 from the Weddell Sea to the AJAX section: an upper pathway through the Scotia Sea, and a lower pathway south along the South Scotia Ridge. The simulated CFC‐11 inventory along the two sections is much like that observed.

Impact of cosmic rays on stratospheric chlorine chemistry and ozone depletion.

Dissociation induced by cosmic rays of chlorofluorocarbons (CFC) and HCl on the surfaces of polar stratospheric clouds (PSC) has been suggested as playing a significant role in causing the ozone hole. However, observed stratospheric CFC distributions are inconsistent with a destruction of CFC on PSC surfaces and no significant correlation exists between ozone levels and cosmic-ray activity inside the polar regions. Model simulations indicate that this mechanism can have only a limited impact on chemical ozone loss and thus on the recovery of stratospheric ozone.

Anomalous chlorofluorocarbons in the Southern California Borderland Basins

During the past two decades, unexpectedly high concentrations of chlorofluorocarbons (CFCs) have been observed in the bottom waters of the Southern California Borderland Basins (SCBB), with relatively constant spatial distribution patterns. In contrast to offshore waters in this region, CFC concentrations below the oxygen minimum layer (OML) in the deep SCBB increase with depth. The uniformity of the bottom‐enhanced CFC signals and the near‐zero levels of tritium suggest that this feature is likely maintained by release of CFCs from sediments and vertical mixing, and not by dumped CFC‐bearing materials or an intrusion of recently ventilated waters. We hypothesize that CFC scavenging processes, either on particulate organic matter or hydrocarbon residues from the adjacent natural seeps, occur in these high‐productivity near‐surface coastal waters. The subsequent release of CFCs at the bottom boundary layer during the degradation of particulate material may cause the anomalous CFC distributions in the SCBB.

Annual anthropogenic carbon transport into the North Pacific intermediate Water through the Kuroshio/Oyashio Interfrontal zone : An estimation from CFCs distribution

Keywords: ⋅exDIC, ⋅CFCs, ⋅NPIW, ⋅transport, ⋅K/O zone. Vertical distribution of anthropogenic carbon content of the water (exDIC) in the Oyashio area just outside of the Kuroshio/Oyashio Interfrontal Zone (K/O Zone) was estimated by the simple 1-D advection-diffusion model calibrated by the distribution of chlorofluorocarbons (CFCs). The average concentration of exDIC for σ θ = 26.60- 27.00 is multiplied by the volume transport of Oyashio water into the North Pacific Intermediate Water (NPIW) to estimate the annual transport of exDIC into NPIW through K/O Zone. The estimated transport of exDIC was 0.018-0.020 GtC/y, which corresponds to 15% of the whole total exDIC accumulation in the temperate North Pacific. A simple assessment using the NPIW 1-box model indicates that the current study explains at least 70% of the total annual transport of exDIC into NPIW, and that small exDIC sources for NPIW still exists in addition to K/O Zone.

Thermocline ventilation and oxygen utilization rates in the subtropical North Pacific based on CFC distributions during WOCE

Thermocline ventilation rates for the subtropical North Pacific are determined using a 1-dimensional (meridional) along-isopycnal advective–diffusive model tuned to chlorofluorocarbon (CFC) concentrations measured along 152°W in 1991 during WOCE P16. Mean southward advection rates in the subtropics range from 1.03 to 0.56 cm s -1 between σ θ =25.5 and 26.6. Model-derived ventilation times for the subtropical gyre increase from about 10 to 27 years for that isopycnal range. Oxygen utilization rates (OURs) determined using the advective-diffusive model decrease with depth from 6.6 to 3.2 μmol kg -1 yr -1 between σ θ =25.5 and 26.6. Extrapolation of the OUR versus depth trend to the base of the euphotic zone with the 1/ Z power function of Martin et al. (1987) and integration from 500 to 100 m depth implies a carbon export rate from the overlying euphotic zone of 2.2±0.5 moles C m -2 yr -1 at 30°N, 152°W. Analysis of the WOCE radiocarbon and salinity distributions indicates that zonal and cross-isopycnal transport terms would have to be considered in modeling these tracers in the subtropical North Pacific.

Circulation, mixing, and production of Antarctic Bottom Water

The Antarctic source of bottom water to the abyssal layer of the World Ocean is examined, as well as its large-scale flow pattern and ultimate entrainment rate into the deep water above. We make use of the available high-quality station data in the Southern Ocean to construct bottom maps of neutral density and mean property maps, including Chlorofluorocarbon (CFC), for the abyssal layer underneath a selected neutral density surface. The maximum density at the sill depth of Drake Passage is used to distinguish between the voluminous deep water mass that is a continuous component of the Antarctic Circumpolar Current from the relatively denser bottom water originated along the Antarctic continental margins. Based on water density, Antarctic Bottom Water (AABW) is defined here generically to include all volumes of non-circumpolar water of Antarctic origin. Over the shelf regime multiple localized sources of specific AABW types contribute to the abyssal layer of the adjacent Antarctic basins. Characteristics of these dense bottom waters reflect closely those observed on the parent Shelf Water mass. Spreading paths of newly-formed deep and bottom waters over the slope regime, and their subsequent oceanic circulation patterns are analyzed on the basis of global property maps for the AABW layer. Interior mixing and interbasin exchanges of AABW are deduced from mean characteristic curves following the southern streamline of the Antarctic Circumpolar Current. Outflow and mixing of AABW from the Weddell Sea to the Argentine Basin is depicted using density and CFC distributions of two zonal hydrographic lines. Recirculation and mixing of deep and bottom waters within the Weddell Gyre are also detailed using a meridional section along the Greenwich Meridian. The strength of all localized sources of AABW combined is estimated by two independent approaches. An estimate of the total production rate of AABW is calculated based on the oceanic CFC budget for the AABW layer offshore of the 2500-m isobath. The sum of all downslope inputs of well-ventilated bottom water types underneath the top isopycnal must account for the measured CFC content in the bottom layer. The resulting total AABW production rate is about 8 Sv, which is a conservative figure that neglects the loss of CFC-bearing waters across the top isopycnal in recent years, whereas about 9.5 Sv is calculated assuming a well-mixed bottom layer. Making use of their transient nature, CFC distributions at the top of the AABW layer indicate that more direct and rapid entrainment of CFC-rich bottom waters below occurs over localized areas with relatively strong upwelling rates and enhanced vertical mixing. A second, more ad-hoc but independent oceanic mass budget of the bottom layer is also constructed. A typical basin-wide rate of deep upwelling of 3×10 −7 m s −1 requires 10 Sv (1 Sv=10 6 m 3 s −1) of newly-formed AABW to sink down the slope around Antarctica. We have also formulated a spatial distribution of deep upwelling on the isopycnal at the top of the AABW. It is expressed as a combination of wind, topographic, and turbulent components, which in turn are functions of the isopycnal depth, bottom depth, and bottom layer thickness. This non-uniform upwelling field yields about 12 Sv of AABW exported across the top isopycnal. Fortuitously, the overall average of upward speed at the top isopycnal (3.7×10 −7 m s −1) compares well with previous estimates of deep upwelling in the northern basins. A series of likely sites for strong vertical entrainment of AABW are clearly identified in the modeled distribution of deep upwelling, consonant with the observed CFC distributions on the top isopycnal. Altogether, regions with relatively high upwelling rates (>5×10 −7 m s −1) occupy only a quarter of the total areal extent of the top isopycnal, but they account for as much as 45% of the total vertical transport.

Variability in Atmospheric Chlorofluorocarbons (CCl3F and CCl2F2) near a Large Urban Area: Implications for Groundwater Dating

Chlorofluorocarbons (CFCs) are frequently used as tracers for age dating of young groundwaters. However, in urban environments with many CFC point sources, uncertainties in the delivery of CFCs to groundwater (input function) complicate quantitative interpretation of observed CFC distributions. To assess the potential impact of elevated atmospheric CFC mixing ratios on CFC-based groundwater ages near a large coastal urban center, we measured atmospheric mixing ratios of CFCs in Palisades, NY, 25 km north of New York City. We present and discuss a 16-month record of atmospheric CCl3F (CFC-11) and CCl2F2 (CFC-12) obtained from gas chromatographic analyses taken at intervals of approximately 10 min. Nearly all measured values are in excess of remote Northern Hemisphere (NH) atmospheric mixing ratios. The mean mixing ratios of CFC-11 and CFC-12 are 6 and 13% higher, respectively, than those measured at a remote NH location during the same time period. The temporal trends of CFC-11 and CFC-12 differ from those...

Modeling deep‐water renewal in Lake Baikal

Temperature, dissolved oxygen, nutrients, and chlorofluorocarbon‐12 data obtained from Lake Baikal are used to describe deep‐water renewal in a deep, temperate‐latitude lake. Observations are used to propose the physical mechanism governing convection and to formulate a model of deep ventilation. The key physical mechanism governing deep‐water renewal is the so‐called thermobaric instability. Because the temperature of maximum density decreases with depth, a lake can become conditionally unstable if the base of the cold mixed layer is displaced to a depth at which its temperature matches the local temperature of maximum density, thereby resulting in sinking plumes. An important consequence of this phenomenon is that deep temperate lakes such as Baikal do not completely mix twice yearly; instead, deep ventilation is episodic. A two‐dimensional model of a wind‐ and buoyancy‐driven lake shows many strong mixing events and a fairly realistic seasonal cycle, indicating that the hypothesis is physically realizable. A filling‐box model is used to deduce the annually averaged fluxes necessary to produce a steady vertical distribution of tracers as observed. Good fits are obtained to oxygen and chlorofluorocarbon distributions by this model.

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

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.

Chlorofluorocarbon uptake in a world ocean model: 1. Sensitivity to the surface gas forcing

The uptake and redistribution of chlorofluorocarbons (CFCs) CFC‐11 and CFC‐12 are studied in a series of world ocean model experiments. In part 1 of this study the sensitivity of the simulated CFC distributions to the model parameterization of air‐sea CFC fluxes is examined within a control experiment. The control experiment represents a low‐resolution ocean model with global coverage and a proper seasonal cycling in surface thermohaline and wind stress conditions. The specification of a surface ocean CFC concentration that is instantaneously in saturated equilibrium with the atmosphere is found to flux too much CFC into the model. Signatures of CFC‐11 are found to be grossly overestimated in regions of deep and bottom water formation, both in the surface mixed layer and at depth. The use of a classical air‐sea gas exchange formula (even one with a simplified gas transfer velocity that is independent of wind speed) is seen to greatly improve the CFC simulations at depth. In addition, the model reproduces many of the observed trends in surface CFC concentrations; namely, undersaturation in regions of deep convective overturn and near‐surface upwelling and supersaturation in the summer mixed layer. In further sensitivity experiments, we consider the effect of sea ice cover in limiting air‐sea gas exchange in polar waters. It is found that bottom water in the Arctic Ocean and around the Antarctic continent is significantly reduced in CFC content once regions covered with sea ice are limited to fractional air‐sea gas exchange. This more physically meaningful framework is found to further reduce the spurious uptake of CFC‐11 and CFC‐12 found under a “saturated surface” assumption. In a final sensitivity experiment the gas exchange rate is parameterized using a complete wind speed and Schmidt number dependence. The wind speed dependent gas forcing increases the surface CFC equilibration rate under the subpolar westerlies. On the other hand, the polar and tropical oceans witness reduced CFC uptake under a wind speed dependent flux regime. Simulated ocean CFC concentrations are compared directly with observational data in certain key areas for deep and bottom water formation. It is found that a reasonable representation of oceanic CFC is achieved in the convected water column in the Weddell and Labrador Seas. In contrast, deep waters that have left the convective area with the model ocean currents are found to be deficient in CFC‐11 in the North Atlantic Ocean. This is because the model advective timescale for North Atlantic Deep Water (NADW) outflow across the equator is too long compared with observed ocean estimates. The long timescale is not due to unrealistically sluggish deep currents. Rather, the path of NADW outflow includes a loop eastward from the Labrador Sea into the Northeastern Atlantic Basin, effectively increasing the required outflow journey by around 4000 km. This ages the water mass by almost 10 years, thereby yielding significantly lower CFC concentrations in the NADW extension. In addition, the outflow signature spreads too far into the eastern North Atlantic, presumably because the advective process is too broad and the horizontal diffusion too strong at depth. Contrasting the North Atlantic, bottom water CFC ventilation in the Southern Ocean is found to be too strong, even when significant levels of surface undersaturation are simulated in polar waters. CFC‐tagged waters flowing into the deep South Atlantic basin (from the Weddell Sea formation zone) are too enriched in CFC‐11, even when the deep signatures adjacent to the Antarctic shelf remain close to observations. This suggests that the advective timescale for bottom water ventilation is too rapid in the Southern Ocean. In addition, too much convective overturn persists in the Southern Ocean at 55°S–70°S, with unrealistically deep CFC‐11 penetration noted at particular longitudes. This is because not enough older (CFC‐deprived) water recirculates and upwells into the Southern Ocean. For example, more upwelled circumpolar deep water in the Southern Ocean would weaken the CFC‐11 concentrations by contributing to a lower CFC mixture and by suppressing the convective activity in the region. Bottom and deep level CFC signatures are broad and diffuse compared with the real ocean. The broadness of the CFC imprint is due, in part, to the model resolution, which gives any convective event a spatial extent of at least 3.75° longitude by 4.5° latitude and a bottom level CFC signal thickness in excess of 800 m. An important finding of our study is that the vertical convection of unstable waters acts as the efficient tracer ventilator of the ocean system. This has significant implications for numerical studies of the world's climate, since the meridional overturning has traditionally been considered the reason for the ocean's moderating influence during global warming scenarios. Our study suggests that the vertical convection would play a much greater role over the typical timescale for anthropogenic climate change.

A chlorofluorocarbon section in the eastern North Atlantic

We present the distributions of two chlorofluorocarbons (CFCs) CFC-11 and CFC-12, measured as part of a hydrographic section between Iceland and the equator during July and August 1988. CFC-tagged water has filled the entire subpolar water column and subtropical thermocline in the eastern North Atlantic. Measurable CFC concentrations are observed at the ocean bottom as far south as 35°N, and the CFC penetration depth shoals to ≈750 m in the tropics. Specific features in the CFC distributions include a clear signal of Labrador Sea mid-depth ventilation, a CFC-enriched overflow water boundary current along the Iceland slope, and a mid-depth, equatorial plume of upper North Atlantic Water. The CFC data are used, in conjuction with the hydrographic data from the cruise, to illustrate the ventilation time-scales and pathways for the water masses in the eastern basin. A subsurface CFC maximum at about 100–200 m depth in the subtropics is shown to be a by-product of the heating and degassing of the seasonal thermocline and of the temperature sensitivity of CFC solubility. The CFC concentrations in the subpolar mode water are undersaturated by 15–18% relative to the atmosphere, reflecting the age of the mode waters and the very deep winter mixed layers in the eastern subpolar gyre. The CFC concentrations in the oxygen minimum off tropical Africa are much lower than the concentrations in the subtropical gyre, supporting previous work that suggests that isolation and enhanced productivity both contribute to the formation of the tropical oxygen minimum. In addition, the CFC inventories at the tropical stations have increased between 1982 and 1983 (TTO/TAS) and the summer of 1988 at a slower rate relative to the subtropical inventories over the same period. Thermocline oxygen utilization rates calculated from the CFC concentration data range from 5 to 10 μmol kg −1y −1 and are in line with previous estimates for the eastern subtropical thermocline. The low CFC concentrations in Mediterranean Water, about one-quarter those in the Labrador Sea Water, are shown to result from entrainment near the Straits of Gibraltar of a large component of low CFC, lower Atlantic thermocline water. Based on the CFC and other transient tracer distributions, the deep eastern basin can be divided into two regions: the Iceland Basin and surrounding area influenced by Iceland-Scotland Overflow Water that is ventilated on a decadal time-scale, and the area south of ≈50°N that has little or no CFC and is ventilated from a southern source on a much longer time-scale. A northward flowing boundary current of low CFC, modified Eastern Basin Bottom Water is also found along the Rockall Plateau and in the Iceland Basin.

Gas exchange in a contaminated estuary inferred from chlorofluorocarbons

A new method of quantifying mean rates of gas exchange for natural water systems using the distribution of pollutant chlorofluorocarbons (CFCs) is presented. Concentrations of dissolved CCl3F (F‐11), CCl2F2 (F‐12), and CC12FCC1F2 (F‐113) in the Hudson estuary are supersaturated with respect to the remote troposphere by as much as an order of magnitude. The most plausible source appears to be discharges from waste water treatment facilities. Loss of these compounds from the estuary water to the atmosphere occurs both upstream and downstream of the zone of input. Using a multi‐box model, gas exchange coefficients were calculated to be between 2 and 4 cm hr−1 based on the observed concentrations. This rate of gas exchange is similar to values determined in lakes and coastal bays and substantially lower than the mean value for the open ocean.

Chemistry of the global troposphere: Fluorocarbons as tracers of air motion

Winds and convective mixing from a general circulation model of the atmosphere have been applied in a chemical tracer model (CTM) to simulate the global distribution and temporal variability of chlorofluorocarbons (CFCs). The seasonal cycle in moist convection, with maximum activity over continents in summer, leads to an annual cycle in the surface concentration of CFCs. Emissions are retained in the lowest levels of the atmosphere during winter, and surface concentrations peak near sources. In this season, CFCs from European sources are carried by low‐level winds into the Arctic. During summer, vertical exchange is more efficient, and pollutants are transported more rapidly to the middle atmosphere. Consequently, concentrations of CFCs during summer are relatively low near the surface and elevated in the middle troposphere. Time series analysis of data from Adrigole, Ireland, indicates that the model accurately simulates long‐range transport of air pollution. The model reproduces global distributions and trends for CFC‐11 and CFC‐12 observed by the ALE experiment; however, subgrid diffusion must be introduced into the model in order to reproduce the observed interhemispheric gradient. Interhemispheric exchange occurs mainly in the upper tropical troposphere, producing a profile which increases with altitude in the southern hemisphere, in agreement with observations. The distribution of CFCs is such that it is necessary to apply important corrections to observations at surface stations in order to derive global distributions.