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  • 1
    Publication Date: 2018-02-20
    Description: The total of 13 existing cross-equatorial shipboard current profiling sections taken during the WOCE period between 1990 and 2002 along 35°W are used to determine the mean meridional structure of the zonal top-to-bottom circulation between the Brazilian coast, near 5°S, and 5°N and to estimate mean transports of the individual identified shallow, intermediate and deep current branches. One of the results is that, on the equator, a mean westward Equatorial Intermediate Current below the Equatorial Undercurrent exists.
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  • 2
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    AGU (American Geological Union)
    In:  Geophysical Research Letters, 26 (5). pp. 587-590.
    Publication Date: 2018-02-13
    Description: During May - August, 1997, the distributions of dissolved methane and CCl3F (CFC11) were measured in the Atlantic between 50° and 60°N. In surface waters throughout the region, methane was observed to be close to equilibrium with the atmospheric mixing ratio, implying that surface ocean methane is tracking its atmospheric history in regions of North Atlantic Deep Water formation. Despite the different atmospheric history and ocean chemistry of CH4 and CFC11, their spatial distribution patterns in the water column are remarkably similar. One-dimensional distributions have been simulated with an advection-diffusion model forced by the atmospheric histories. The results suggest that the similar patterns result from the increasing input of CH4 and CFC11 to newly formed deep waters over time, combined with the effect of horizontal mixing and the oxidation of methane on a 50 year time scale.
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  • 3
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    AGU (American Geological Union)
    In:  Geophysical Research Letters, 32 (10). L10605.
    Publication Date: 2018-03-28
    Description: In 1996, about 320 kg of SF6 were introduced in the center of the Greenland Sea gyre. We use this signal together with the CFC distribution to follow the spreading of Greenland gyre water from the Denmark Strait through the Irminger Basin and the Labrador Sea to the Grand Banks. In the summer of 2003 Denmark Strait Overflow Water tagged with deliberately released SF6 could be traced throughout the Irminger Basin to the central Labrador Sea, confirming that water with potential density of 28.045 contributes to the Denmark Strait Overflow. The upper limit of the transfer time from the central Greenland Sea to the Labrador Sea was found to be 7 years. This study suggests that roughly 4 kg of excess SF6 has been transported over the Denmark Strait and confirm earlier reported transport through the Faroe Bank Channel. These results should be considered when using SF6 as a transient tracer.
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  • 4
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    AGU (American Geological Union)
    In:  Journal of Geophysical Research: Oceans, 113 . C03028.
    Publication Date: 2018-04-25
    Description: A major pathway of the Atlantic meridional overturning circulation (MOC) is the warm inflow into the Caribbean Sea. The transport and the contribution of water from the South Atlantic is calculated from observations (ADCP data and hydrography) and compared to the results of the equation image° FLAME model. The model and the observations show high consistency in the strength of the mean total inflow and its range of variability as well as in the general distribution of water from South Atlantic origin. The measurements give an annual mean South Atlantic Water (SAW) transport into the Caribbean of 9.3 Sv with high variability. This estimate has to be regarded as a lower bound since the present method (using temperature and salinity data) cannot identify the SAW included in the North Equatorial Current (NEC), which recirculated and was transformed in the interior tropical Atlantic. The model transport reproduces the observational values rather closely, with an annual mean inflow of 8.6 Sv and similar high variability. Closer inspection of the SAW pathways in the model suggest that the additional contribution by the NEC‐pathway is only about 2 Sv. The model results confirm the relative importance of the MOC pathways suggested by observations: the Caribbean inflow seems to be the main pathway (63%) for the warm and central water (σθ 〈 27.1 kg m−3), whereas for the intermediate water a larger fraction (59%) is transported northward at the eastern side of the Lesser Antilles.
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  • 5
    Publication Date: 2019-09-23
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  • 6
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    AGU (American Geological Union)
    In:  Journal of Geophysical Research: Oceans, 104 (C10). 23,495-23,508.
    Publication Date: 2018-04-17
    Description: Owing to its nearly enclosed nature, the Tyrrhenian Sea at first sight is expected to have a small impact on the distribution and characteristics of water masses in the other basins of the western Mediterranean, The first evidence that the Tyrrhenian Sea might, in fact, play an important role in the deep and intermediate water circulation of the entire western Mediterranean was put forward by Hopkins [1988]. There, an outflow of water from the Tyrrhenian Sea into the Algero Provencal Basin was postulated in the depth range 700-1000 m, to compensate for an observed inflow of deeper water into the Tyrrhenian Sea. However, this outflow, the Tyrrhenian Deep Water (TDW), was undetectable since it would have hydrographic characteristics that could also be produced within the Algero-Provencal Basin. A new data set of hydrographic, tracer, lowered Acoustic Doppler Current Profiler (LADCP), and deep float observations presented here allows us now to identify and track the TDW in the Algero-Provencal Basin and to demonstrate the presence and huge extent of this water mass throughout the western Mediterranean. It extends from 600 m to 1600-1900 m depth and thus occupies much of the deep water regime. The outflow from the Tyrrhenian is estimated to be of the order of 0.4 Sv (Sv=10(6) m(3) s(-1)), based on the tracer balances. This transport has the same order of magnitude as the deep water formation rate in the Gulf of Lions. The Tyrrhenian Sea effectively removes convectively generated deep water (Western Mediterranean Deep Water (WMDW)) from the Algero-Provencal Basin, mixes it with Levantine Intermediate water (LIW) above, and reinjects the product into the Algero-Provencal Basin at a level between the WMDW and LIW, thus smoothing the temperature and salinity gradients between these water masses. The tracer characteristics of the TDW and the lowered ADCP and deep float observations document the expected but weak cyclonic circulation and larger flows in a vigorous eddy regime in the basin interior
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  • 7
    Publication Date: 2020-02-06
    Description: The North Atlantic Current (NAC) is subject to variability on multiannual to decadal time scales, influencing the transport of volume, heat, and freshwater from the subtropical to the eastern subpolar North Atlantic (NA). Current observational time series are either too short or too episodic to study the processes involved. Here we compare the observed continuous NAC transport time series at the western flank of the Mid-Atlantic Ridge (MAR) and repeat hydrographic measurements at the OVIDE line in the eastern Atlantic with the NAC transport and circulation in the high-resolution (1/20°) ocean model configuration VIKING20 (1960–2008). The modeled baroclinic NAC transport relative to 3400 m (24.5 ± 7.1 Sv) at the MAR is only slightly lower than the observed baroclinic mean of 27.4 ± 4.7 Sv from 1993 to 2008, and extends further north by about 0.5°. In the eastern Atlantic, the western NAC (WNAC) carries the bulk of the transport in the model, while transport estimates based on hydrographic measurements from five repeated sections point to a preference for the eastern NAC (ENAC). The model is able to simulate the main features of the subpolar NA, providing confidence to use the model output to analyze the influence of the North Atlantic Oscillation (NAO). Model based velocity composites reveal an enhanced NAC transport across the MAR of up to 6.7 Sv during positive NAO phases. Most of that signal (5.4 Sv) is added to the ENAC transport, while the transport of the WNAC was independent of the NAO.
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  • 8
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    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 56 . pp. 1656-1674.
    Publication Date: 2016-11-01
    Description: The variability of two modes of Labrador Sea Water (LSW) (upper and deep Labrador Sea Water) and their respective spreading in the interior North Atlantic Ocean are investigated by means of repeated ship surveys carried out along the zonal WOCE line A2/AR19 located at 43–48°N (1993–2007) and along the GOOS line at about 48–51°N (1997–2002). Hydrographic section data are complemented by temperature, salinity, and velocity time series recorded by two moorings. They have been deployed at the western flank of the Mid-Atlantic Ridge (MAR) in the Newfoundland Basin during 1996–2004. The analysis of hydrographic anomalies at various longitudes points to a gradual eastward propagation of LSW-related signals, which happens on time scales of 3–6 years from the formation region towards the MAR. Interactions of the North Atlantic Current (NAC) with the Deep Western Boundary Current (DWBC) close to Flemish Cap point to the NAC being the main distributor of the different types of LSW into the interior of the Newfoundland Basin. Comparisons between the ship data and the mooring records revealed that the mooring sites are located in a region affected by highly variable flow. The mooring time series demonstrate an elevated level of variability with eddy activity and variability associated with the NAC considerably influencing the LSW signals in this region. Hydrographic data taken from Argo profiles from the vicinity of the mooring sites turned out to mimic quite well the temporal evolution captured by the moorings. There is some indication of occasional southward flow in the LSW layer near the MAR. If this can be considered as a hint to an interior LSW-route, it is at least of minor importance in comparison to the DWBC. It acts as an important supplier for the interior North Atlantic, distributing older and recently formed LSW modes southward along the MAR.
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  • 9
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    AGU (American Geological Union)
    In:  Journal of Geophysical Research: Oceans, 114 . C05019.
    Publication Date: 2018-04-25
    Description: The upper branch of the meridional overturning circulation in the North Atlantic is fed by cross‐equatorial transport of various water masses from the Southern Hemisphere. Here, we study the large‐scale spreading of South Atlantic Water (SAW) into the western tropical North Atlantic from the equator to 25°N. The fractions of SAW in the upper ocean water masses are quantified using a water mass analysis applied on a data set of conductivity‐temperature‐depth data from the Hydrobase project and the Argo float program. To fill gaps in the data coverage and to gain insight into the mechanisms involved, the observations are complemented with results from the high‐resolution Family of Linked Atlantic Model Experiments model (equation image°), which has been shown to realistically simulate the inflow of SAW into the Caribbean. The analysis reveals the mean SAW propagation pathways in the North Atlantic and identifies the regions of largest variability. High SAW fractions in the thermocline and central water layers are limited to the region south of 10°N, where the water body consists of 80%–90% SAW. Thus, the zonal currents in the equatorial gyre are mainly formed of SAW. The weaker currents in the intermediate layer combined with a northward excursion of the North Equatorial Current allow the SAW in this layer to intrude farther north compared to the layers above. The transition into North Atlantic Water occurs gradually from 12°N to 20°N in the intermediate layer.
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  • 10
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    Elsevier
    In:  Deep Sea Research Part I: Oceanographic Research Papers, 45 (4-5). pp. 507-527.
    Publication Date: 2016-10-20
    Description: Hydrographic and tracer [chlorofluorocarbon (CFC), component F11] data in the tropical Atlantic off Brazil taken in spring 1994 are used to describe the development of the water mass characteristics of Antarctic Bottom Water (AABW) between 10 degrees S and 11 degrees N. To compute the AABW transports, geostrophic computations and directly measured velocity fields are combined. Velocity profiles were measured with the Pegasus profiling system and an ADCP attached to the CTD. The F11 increase from 10 degrees S to 11 degrees N, mainly in the upper part of the tracer-poor AABW, reveals the mixing of AABW along its path with the overlying North Atlantic Deep Water, which carries a significant F11 signal in the equatorial Atlantic. While propagating north of 5 degrees S, the AABW shifts to higher salinities at a given temperature. About one-third of the northward flowing AABW at 10 degrees S (4.8 Sv) and at 5 degrees S (4.7 Sv) west of about 31 degrees 30'W enters the Guiana Basin, mainly through the southern half of the Equatorial Channel at 35 degrees W (1.5-1.8 Sv). The other part recirculates and some of it flows through the Romanche Fracture Zone into the eastern Atlantic. In the Guiana Basin, west of 40 degrees W, the sloping topography and the strong, eastward flowing deep western boundary current might prevent the AABW from flowing west: thus it has to turn north at the eastern slope of the Ceara Rise (2.2 Sv). At 44 degrees W, north of the Ceara Rise, AABW flows west in the interior of the basin in a main core near 7 degrees 15'N (1.9 Sv). A net return how of about 0.5 Sv was found north of 8 degrees 43'N. A large fraction of the AABW (1.1 Sv) enters the eastern Atlantic through the Vema Fracture Zone, leaving only 0.3 Sv of AABW for the western Atlantic basins
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