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To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2008.05.016 Byline: Oliver Huhn, Wolfgang Roether, Reiner Steinfeldt Keywords: Transient tracers; Tracer ages; Transit time distributions; North Atlantic Deep Water; Tropical Atlantic; South Atlantic Abstract: An extensive set of transient-tracer observations has been used to determine age spectra in Upper and Lower North Atlantic Deep Water (UNADW, LNADW) along the western continental margin between [approximately equal to]10[degrees]N and 30[degrees]S. The tracers are CFC-11, CFC-12, CCl.sub.4, and tritium, and the observations cover 20 years. The procedure was to deduce from the data the free parameters of a transit time distribution (TTD; mean age or transit time I, age dispersion I, and dilution factor by tracer-free water f) of the common functional form (Inverse Gaussian). A novel northern boundary condition for the tracers was employed; it allows for enhanced halocarbon input during convective deep-water formation in winter and yields minimized biases between observed and TTD-based tracer concentrations in downstream NADW. We found that the common boundary condition of time-invariant apparent saturations gave inconsistent results. The TTD parameter fits covered the entire region and used all tracer data jointly, the parameters being prescribed to increase linearly along-stream and the Peclet number (Pe=2I.sup.2 I.sup.-2) to be constant. For the southward transit up to the equator, we obtained for UNADW I=54 years, I=33 years, and f=1.4, with uncertainties of about [+ or -]20% estimated on the basis of a thorough error analysis. A limiting factor was a significant scatter in the NADW tracer fields. The LNADW parameter values are similar but less well constrained than for UNADW. The age spectra are broad; they correspond to large-scale along-core diffusivities on the order of 10.sup.4 m.sup.2/s. The TTD-based mean NADW flow velocities are less than 1cm/s. The relative southward increases of the TTD-based ages exceed those of CFC-11 to CFC-12 ratio ages about three-fold. An additional result is an improved CCl.sub.4 decomposition rate of 0.011[+ or -]0.002year.sup.-1 at UNADW temperatures. Our TTDs can be used to estimate the NADW-related transfer of dissolved constituents. We present such estimates for.sup.3He generated by tritium decay from the pre-nuclear period onward and for anthropogenic dissolved inorganic carbon (DIC) as examples. The latter estimate yields a NADW-related transport of anthropogenic DIC southward across the equator of about 50Mt in 1994. A critical aspect of the TTD methodology is that the older part of the age spectra is essentially unconstrained by observations. The TTDs can furthermore be used as a diagnostic tool to check certain aspects of NADW transfer in numerical ocean circulation models. Author Affiliation: Institut fur Umweltphysik (IUP), Abt. Ozeanographie, Universitat Bremen, D-28359 Bremen, Germany Article History: Received 17 October 2007; Revised 14 May 2008; Accepted 27 May 2008

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2007.03.004 Byline: Reiner Steinfeldt, Monika Rhein, Maren Walter Abstract: The variability of North Atlantic Deep Water (NADW) properties in the western tropical Atlantic between 66.sup.aW/20.sup.aN and 60.sup.aW/10.sup.aN 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 50km at 16.sup.aN to 500km at 10.sup.aN, 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.sup.aN is estimated by the salinity signal to be 10-13 years, corresponding to a mean velocity of 2.0-2.5cm/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.sup.aN 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.sup.aN, 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.sup.aN and gradually declining at 10.sup.aN, leading to a broadening of the upper portion of the boundary current between the two sections. The NADW at 10.sup.aN is a composite of the water from the western boundary at 16.sup.aN and about one third admixture of water from the interior. During the spreading from 16.sup.aN to 10.sup.aN, 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.sub.I is of the order of 10.sup.-4m.sup.2/s. Author Affiliation: Institut fur Umweltphysik, Abt. Ozeanographie, Universitat Bremen, Postfach 330440, 28334 Bremen, Germany Article History: Received 21 July 2006; Revised 2 March 2007; Accepted 9 March 2007

To link to full-text access for this article, visit this link: http://dx.doi.org/10.1016/j.dsr.2005.08.003 Byline: Monika Rhein, Kerstin Kirchner, Christian Mertens, Reiner Steinfeldt, Maren Walter, Ulrich Fleischmann-Wischnath Keywords: Ocean circulation; Water mass spreading; Western tropical Atlantic; Western Atlantic; Caribbean Abstract: CTD, vessel-mounted ADCP and LADCP measurements in the Caribbean passages south of Guadeloupe (three repeats) and along 16[degrees]N (five repeats) were carried out between December 2000 and July 2004. The CTD data were used to calculate the contribution of South Atlantic water (SAW) in the upper 1200m between the isopycnals I.sub.I[cedilla]=24.5 and 27.6. Northern and southern source water masses are defined and an isopycnal mixing approach is applied. The SAW fractions are then combined with the ADCP flow field to calculate the transport of SAW into the Caribbean and across 16[degrees]N. The SAW inflow into the Caribbean through the passages south of Guadeloupe ranges from 7.6 to 11.6Sv, which is 50-75% of the total inflow. The mean (9.1[+ or -]2.2Sv) is in the range of previous estimates. Ambiguities in the northern and southern source water masses of the salinity maximum water permitted us only to calculate the contribution of SAW from the eastern source in this water mass. We estimated the additional SAW transport by the western source to be of the order of 1.9[+ or -]0.7Sv. The calculation of the SAW transport across 16[degrees]N was hampered by the presence of several anticyclonic rings from the North Brazil Current (NBC) retroflection region, some of the rings were subsurface intensified. Provided that the rings observed at 16[degrees]N are typical rings and that all rings which are annually produced in the NBC retroflection area (6.5-8.5 per year) reach 16[degrees]N, the SAW ring transport across 16[degrees]N is calculated to 5.3[+ or -]0.7Sv. From the 5 repeats at 16[degrees]N, only two showed a net northward flow, suggesting that the mean northward SAW transport is dominated by ring advection. The joint SAW transports of the Caribbean inflow (9.1Sv) and the flow across 16[degrees]N (5.3Sv) sum up to 14.4Sv. The transport increases to 16.3Sv if the additional SAW transport from the western source of SMW (1.9[+ or -]0.7Sv) is included. These transport estimates and the following implications depend strongly on the assumption that the surface water in the Caribbean inflow is of South Atlantic origin. The transport estimates are, however, in the range of the inverse model calculations for the net cross-hemispheric flow. About 30-40% of this transport is intermediate water from the South Atlantic, presumably supporting studies which found the contributions of intermediate and upper warm water to be of a comparable magnitude. For the upper warm water (I.sub.I[cedilla] Author Affiliation: Institut fur Umweltphysik, FB1, Abt. Ozeanographie, Universitat Bremen, Postfach 330 440, D-28334 Bremen, Germany Article History: Received 10 November 2004; Revised 2 May 2005; Accepted 2 August 2005

CTD, vessel-mounted ADCP and LADCP measurements in the Caribbean passages south of Guadeloupe and along 16 degrees North were carried out between December 2000 and July 2004. The joint South Atlantic Water (SAW) transport from the Caribbean inflow and the flow across 16 degrees North amounts to 14.4 Sv, which is in the range of the inverse model calculations.