North Atlantic Deep Water Components Research Articles

Annually Resolved Propagation of CFCs and SF6 in the Global Ocean Over Eight Decades

AbstractOceanic transient tracers, such as chlorofluorocarbons (CFCs) and sulfur‐hexafluoride (SF6), trace the propagation of intermediate‐to‐abyssal water masses in the ocean interior. Their temporal and spatial sparsity, however, has limited their utility in quantifying the global ocean circulation and its decadal variability. The Time‐Correction Method (TCM) presented here is a new approach to leverage the available CFCs and SF6 observations to solve for the Green's functions (GFs) describing the steady‐state transport from the surface to the ocean interior. From the GFs, we reconstruct global tracer concentrations (and associated uncertainties) in the ocean interior at annual resolution (1940–2021). The spatial resolution includes 50 neutral density levels that span the water column along World Ocean Circulation Experiment/Global Ocean Ship‐Based Hydrographic Investigations Program lines. The reconstructed tracer concentrations return a global view of CFCs and SF6 spreading into new regions of the interior ocean, such as the deep north‐western Pacific. For example, they capture the southward spreading and equatorial recirculation of distinct North Atlantic Deep Water components, and the spreading of CFC‐rich Antarctic Bottom Water out of the Southern Ocean and into the North Pacific, East Indian, and West Atlantic. The reconstructed tracer concentrations fit the data in most locations (∼75%), indicating that a steady‐state circulation holds for the most part. Discrepancies between the reconstructed and observed concentrations offer insight into ventilation rate changes on decadal timescales. As an example, we infer decadal changes in Subantartic Mode Water (SAMW) and find an increase in SAMW ventilation from 1992 to 2014, highlighting the skill of the TCM in leveraging the sparse tracer observations.

Studies of Labrador Sea Water formation and variability in the subpolar North Atlantic in the light of international partnership and collaboration

Labrador Sea Water (LSW), the lightest contribution to North Atlantic Deep Water (NADW) and one of the most prominent water masses of the subpolar North Atlantic, has seen remarkable changes over the past century. LSW originates in the Labrador Sea, where it is formed through wintertime ocean convection of varying intensity, depth and spatial extent. Formation of LSW, followed by its respective injection into the mid-depth circulation system, is mandatory for ventilating and renewing water layers of the interior ocean. Indispensably important for unraveling the history of variability in formation and properties of LSW as well as for mapping its large-scale spreading and export are sustained physical and chemical observations from the deep ocean. These observations started at the beginning of the 20th century from occasional mostly national surveys and today constitute large-scale multi-national collaborative efforts including a vast arsenal of sophisticated instrumentation. In a historical context, we revisit major milestones over the past 100years which have established and are constantly adding to shaping today’s knowledge on LSW, and present first details on the latest vintage of LSW generated during the strong winter of 2013/2014. Respective Argo data reveal mixed-layer depths greater than 1700m marking formation of a new cold and fresh anomaly that has spread since then over the subpolar North Atlantic. We further summarize the on-going observational efforts in the subpolar North Atlantic and present a compilation of hydrographic standard lines that serve to provide top-to-bottom information on NADW components.

The formation rate of North Atlantic Deep Water and Eighteen Degree Water calculated from CFC-11 inventories observed during WOCE

The accumulation of man-made chlorofluorocarbons (CFCs) in subsurface water masses is directly related to their formation rate, and the water mass formation rate can be calculated from its CFC inventory. CFC-11 inventories between 65°N and 10°S in the Atlantic Ocean have been calculated for Eighteen Degree Water (EDW) and the components of North Atlantic Deep Water (NADW) from data collected primarily between 1996 and 1998 as part of the World Ocean Circulation Experiment (WOCE). CFC-11 inventories for individual water masses are 5.4 million moles for EDW, 10.5 million moles for Upper Labrador Sea Water (ULSW) (4.6 million moles south of 42°N), 23.4 million moles for Classical Labrador Sea Water (CLSW), 10.4 million moles for Iceland-Scotland Overflow Water (ISOW), and 8.3 million moles for Denmark Strait Overflow Water (DSOW). The estimated error for these inventories is about ±10%. The sum of the NADW components (ULSW, CLSW, ISOW, DSOW) is 53.2 million moles which is about half of the total CFC-11 inventory, 103.8 million moles, in the North Atlantic Ocean. Maps of water column inventories illustrate the formation mechanisms and spreading pathways within these water masses. The inventories directly reflect the input of newly formed water in the North Atlantic over the time scale of the CFC transient, about 3 decades. The interior regions of the North Atlantic contain most (75–80%) of the CFC-11 inventory in NADW indicating strong recirculation and mixing of newly formed NADW from the DWBC into the interior with a time scale of 2–3 decades. Average water mass formation rates between 1970 and 1997 are: 3.3 Sv for EDW, 3.5 Sv for ULSW (2.0 Sv from the central Labrador Sea and 1.5 Sv from the southern Labrador Sea), 8.2 Sv for CLSW, 5.7 Sv for ISOW, and 2.2 Sv for DSOW. Estimated errors are ±20% for CLSW and ±16% for the other water masses. The total for NADW, which forms the deep limb of the North Atlantic Meridional Overturning Circulation, is 19.6 Sv. An extensive test of the effects of temporal variability on the average formation rate calculated from the CFC inventory indicates that the error introduced by the assumption of a constant water mass formation rate is no greater than 15% for CLSW and 10% for the other water masses.

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The variability of North Atlantic Deep Water (NADW) properties in the western tropical Atlantic between 66 ∘ W / 20 ∘ N and 60 ∘ W / 10 ∘ N is investigated based on repeated hydrographic sections. The water at the western boundary carries a CFC maximum, indicating the direct transport of newly formed NADW within the Deep Western Boundary Current towards the tropics. The offshore extension of this region with young water varies from 50 km at 16 ∘ N to 500 km at 10 ∘ N , depending on the width of the continental slope. The mean properties for the different NADW components reflect the changes in the water mass formation regions, especially the Labrador Sea. The greatest signal can be seen in the density range of the Labrador Sea Water (LSW), where a continuous freshening between 2000 and 2004 is observed. This is attributed to the arrival of low-salinity LSW formed in the Labrador Sea after 1988. In addition to the decreasing salinity in the LSW, the two lower NADW components also reveal a freshening trend, as has been observed in the subtropical and subpolar North Atlantic [Dickson. B., Yashayaev, I., Meincke, J., Turrell, B., Dye, S., Holfort, J., 2002. Rapid freshening of the deep North Atlantic Ocean over the past four decades. Nature 416, 832–837]. This tendency is not restricted to the boundary current region, but extends over the whole western tropical Atlantic. The spreading time of NADW from the Labrador Sea to the tropical Atlantic at 16 ∘ N is estimated by the salinity signal to be 10–13 years, corresponding to a mean velocity of 2.0–2.5 cm/s. The dilution factor of NADW determined from the CFC difference between the Labrador Sea and the tropics is 4–5 for all components, assuming an admixture of old, CFC-free waters. The NADW properties at 16 ∘ N on isopycnals show large spatial variability along the section. Profiles with high concentration of CFCs and low salinity in the LSW layer occur not only at the western boundary, but also in the interior, where they alternate with profiles influenced by older, CFC-poor water. At 10 ∘ N , the properties especially in the Upper NADW are horizontally homogenized. This is caused by the change of the shape of the continental slope, which is very steep at 16 ∘ N and gradually declining at 10 ∘ N , leading to a broadening of the upper portion of the boundary current between the two sections. The NADW at 10 ∘ N is a composite of the water from the western boundary at 16 ∘ N and about one third admixture of water from the interior. During the spreading from 16 ∘ N to 10 ∘ N , the uppermost portion of the Upper Labrador Sea Water becomes fresher and poor in CFCs by vertical mixing with the overlying Upper Circumpolar Deep Water. The resulting diffusivity K ρ is of the order of 10 - 4 m 2 / s .

Spreading velocities and dilution of North Atlantic Deep Water in the tropical Atlantic based on CFC time series

Repeated observations of CFC‐11 and CFC‐12 in the western tropical Atlantic over the period from 1990 to 2002 are used to fit parameters of the age spectra for several North Atlantic Deep Water components: the upper Labrador Sea water (ULSW), the Labrador Sea water (LSW), and the lower North Atlantic Deep Water (LNADW). The most robust results are the mean spreading velocities, u, from the formation region in the Labrador Sea and the Denmark Strait, of 1.5 ± 0.3 cm/s for the two LSW water masses and 2.0 ± 0.2 cm/s for LNADW, respectively. These values are equivalent to an age of about 15–22 years for all three components at 16°N and 21–31 years at the equator at 35°W. The results are compatible with simpler tracer methods, as CFC ratios or “effective” ages. The fractions, f, of the “young” component (i.e., the fraction of the water mass ventilated in the last 50 years), turned out to be quite small, ranging from 25% to less than 5%. The downstream decrease of f indicates mixing of young NADW with CFC‐poor older NADW vintages and/or mixing with other CFC‐free water masses from the Southern Hemisphere. The lowest fractions were found in the LNADW, with a sharp downstream decrease from the equator at 35°W to 10°S, probably caused by the bifurcation into a zonal flow along 2°S and a southward flow along the Brazilian coast. Unlike the small uncertainties estimated for the inferred velocities, isopycnal diffusivities, κ, range from 100 to 10000 m2/s. The temporal evolution of f in the LSW shows a maximum, propagating from 7°N in 1996, downstream to 5°S in the Deep Western Boundary Current, and along 2°S to 23°W. We interpret this signal as the arrival of the intensely ventilated LSW formed in 1972–1976. Decreasing fractions of the ULSW and LSW south of the equator between 2000 and 2002 are presumably caused by local dynamics.

Using a CFC effective age to estimate propagation and storage of climate anomalies in the deep western North Atlantic Ocean

1990s chlorofluorocarbon (CFC) observations are used to estimate an effective age of 20 years for North Atlantic Deep Water (NADW) at the equator. The effective age is found by subtracting a “relic” age from pCFC‐11 age. The effective age is several to 10 years younger at the equator than other methods. From the effective age equatorward “effective” spreading rates of 1–2 cm/s are found, which are similar for the NADW components. Effective spreading rates take into account recirculation gyres, along‐equator flow, and mixing, which slow spreading of climate anomalies from the North Atlantic to the Southern Hemisphere. The difference between direct velocities and tracer effective spreading rates suggests that the buffering effect due to storage, in the western North Atlantic and equatorial region, is about 20 years. Even so, warming and freshening due to greenhouse gases will be first felt in the deep waters of the western North Atlantic.

Rates of North Atlantic Deep Water formation calculated from chlorofluorocarbon inventories

Chlorofluorocarbon (CFC) inventories provide an independent method for calculating the rate of North Atlantic Deep Water (NADW) formation. From data collected between 1986 and 1992, the CFC-11 inventories for the major components of NADW are: 4.2 million moles for Upper Labrador Sea Water (ULSW), 14.7 million moles for Classical Labrador Sea Water (CLSW), 5.0 million moles for Iceland–Scotland Overflow Water (ISOW), and 5.9 million moles for Denmark Strait Overflow Water (DSOW). The inventories directly reflect the input of newly formed water into the deep Atlantic Ocean from the Greenland, Iceland and Norwegian Seas and from the surface of the subpolar North Atlantic during the time of the CFC-11 transient. Since about 90% of CFC-11 in the ocean as of 1990 entered the ocean between 1970 and 1990, the formation rates estimated by this method represent an average over this time period. Formation rates based on best estimates of source water CFC-11 saturations are: 2.2 Sv for ULSW, 7.4 Sv for CLSW, 5.2 Sv for ISOW (2.4 Sv pure ISOW, 1.8 Sv entrained CLSW, and 1.0 Sv entrained northeast Atlantic water) and 2.4 Sv for DSOW. To our knowledge, this is the first calculation for the rate of ULSW formation. The formation rate of CLSW was calculated for an assumed variable formation rate scaled to the thickness of CLSW in the central Labrador Sea with a 10 : 1 ratio of high to low rates. The best estimate of these rates are 12.5 and 1.3 Sv, which average to 7.4 Sv for the 1970–1990 time period. The average formation rate for the sum of CLSW, ISOW and DSOW is 15.0 Sv, which is similar to (within our error) previous estimates (which do not include ULSW) using other techniques. Including ULSW, the total NADW formation rate is about 17.2 Sv. Although ULSW has not been considered as part of the North Atlantic thermohaline circulation in the past, it is clearly an important component that is exported out of the North Atlantic with other NADW components.

Tracing the flow of North Atlantic Deep Water using chlorofluorocarbons

Chlorofluorocarbon (CFC) and hydrographic data collected in the North Atlantic in the late 1980s and early 1990s are used to confirm and add to earlier work on the large‐scale circulation pathways and timescales for the spreading of North Atlantic Deep Water (NADW) components and how these components relate to the hydrographic structure. Throughout the western North Atlantic, high CFC concentrations are coincident with newly formed NADW components of Upper Labrador Sea Water (ULSW), Classical Labrador Sea Water (CLSW), and Overflow Waters (OW). ULSW is marked by a CFC maximum throughout the western subtropical and tropical Atlantic, and CLSW is marked by a CFC maximum north of 38°N in data collected in 1990–1992. Iceland‐Scotland Overflow Water (ISOW) splits into two branches in the eastern basin, with one branch entering the western basin where it mixes with Denmark Strait Overflow Water (DSOW) and the densest branch flows southward along the bottom in the eastern basin. DSOW contributes the largest portion of the CFC signal in OW. It is estimated that these NADW components are at 60–75% equilibrium with the CFC concentration in the atmosphere at the time of formation. The large‐scale data set confirms that NADW spreads southward by complex pathways involving advection in the Deep Western Boundary Current (DWBC), recirculation in deep gyres, and mixing. Maps of the CFC distribution show that properties within the gyres are relatively homogeneous, particularly for OW, and there is a profound change at the gyre boundaries. The density of the core of ULSW increases in the equatorward direction because of entrainment by overlying northward flowing Upper Circumpolar Water and at the equator, ULSW has the same density as CLSW in the subtropics but is warmer and saltier. The density of OW decreases between the subpolar region and the subtropics. This is caused by the least dense part of OW exiting the subpolar region in the DWBC, while the densest component recirculates in the subpolar basins. Some variability is observed in OW density in the subtropics and tropics because of variability in mixing with Antarctic Bottom Water and changes in the subtropics that are probably related to the transport of different vintages of DSOW. Ages derived from CFC ratios show that the NADW components of northern origin spread throughout the western North Atlantic within 25–30 years. This corresponds to a spreading rate of 1–2 cm s−1 and is comparable to the time a climate anomaly introduced into the subpolar North Atlantic will take to penetrate the entire western North Atlantic Ocean.

Instabilities in the Labrador Sea water mass structure during the last climatic cycle

In the modern Labrador Sea, the North Atlantic deep water components are found below the ~2 km deep, intermediate Labrador Sea water (LSW) mass, which is renewed locally through winter convective mixing. This water mass structure remained relatively stable since ~9.5 14C ka BP, as indicated by isotopic studies of foraminifer assemblages from deep-sea cores. Almost constant differences in δ18O values are observed between major species. These average -0.5‰ between the epipelagic species Globigerina bulloides and the mesopelagic species Neogloboquadrina pachyderma, left coiled, and -1‰ between Neogloboquadrina pachyderma and the benthic species Cibicides wuellerstorfi, after correction for Cibicides wuellerstorfi specific fractionation. These isotopic compositions represent thermohaline conditions in surface waters, in the pycnocline with the LSW, and in the deep component of the North Atlantic deep water, respectively. A drastically different structure characterized the glacial Labrador Sea. Differences in δ18O values of ~ -2 to -2.5‰ are then observed between Globigerina bulloides and benthic species, indicative of a strong halocline between the corresponding water masses, thus for reduced production of intermediate waters. During the same interval, Neogloboquadrina pachyderma shows 13C and 18O fluctuations of 1 to 1.5‰ amplitude, in phase with Heinrich-Bond events and higher frequency climate oscillations. The δ18O values in Neogloboquadrina pachyderma vary between those of Globigerina bulloides and of benthic foraminifers, suggesting large amplitude bathymetric fluctuations of the halo-thermocline above and below the bathymetric range occupied by Neogloboquadrina pachyderma. Minimum δ18O values in Neogloboquadrina pachyderma match intervals of maximum ice rafting deposition, such as the late Heinrich events, thus intervals with a deeper, more dilute buoyant surface water layer.