Roether, Wolfgang
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Roether, Wolfgang
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Roether, Wolfgang
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Item-typ:Veröffentlichung, Interpretation von FCKW-Datensätzen im Weddellmeer(2002-04-24); ; ; The aim of this thesis is to generate a circulation scheme of the deep water masses of the Weddell Sea with the primary objective to estimate production rates of different sources of these water masses using chlorofluorocarbon (CFC) data. Repeated observations of CFC for a section along the Prime Meridian from Antarctica (70°S) to approximately 50°S are presented for the period 1984 - 1998. In addition CFC data from the Weddell Sea section (Kapp Norvegia - Joinville Island) are considered. For various CFC features at this sections CFC time series have been generated, in particular for two deep cores of CFC-11 maxima adjoining the southern and northern margins of the Weddell Basin and elevated CFC concentrations over the Middle Atlantic Ridge. A new method of analyzing such CFC data has been developed. The outcome of this approach is the best fit of the age distribution, i.e. the fraction and the age of recently ventilated water. In contrast to previous methods more information about the age structure and a better error estimation could be derived. The convolution of the CFC and the velocity field results in the CFC transport through the section in this cores. The ratio of this value and the CFC surface concentration at the time of the water mass formation is an estimation of the transport of recently ventilated water within the core. Applying this to the deep cores at both sections a circulation scheme of the deep recently ventilated water could be estimated. From this scheme the production rates of the source in the western Weddell Sea and east of it (off Amery Ice Shelf) have been derived. Furthermore, the transport of the ventilated water of the mid-depth core has given an estimation of the productivity of the northern Weddell Sea. The comparison of these three sources has shown a surprising result: The production rates of all sources are in the same range (2.1±0.8 Sv western Weddell Sea; 2.4±0.8 Sv Amery Ice Shelf, about 3 Sv northern Weddell Sea).Dissertation479 134 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Wassermassentransformation im Weddell-Wirbel: Tracerstudien mit dem hydrostatischen, regionalen Ozeanmodell BRIOS(2003-09-08); ; ; The water mass transformation and the associated Deep/Bottom Water (DBW) formation are analyzed with a regional ocean circulation model of theSouthern Ocean. Simulated CFC concentrations and saturation of the mixed layer are in good agreement with observations. Sea ice,temperature and salinity and their fluxes influence the flux of tracers. High fluxes of salinity and temperature perturb the stability of the water column, initiates convection and entrain tracer poor water into the mixed layer. High Fluxes are essential for the DBW formation. DBW is primarily formed in the WeddellSea, in front of the Ross Ice Shelf (ROSS) and Amery Ice Shelf (AMERY) and spread with the boundary currents: the boundary current from ROSS develops two branches: one ventilates the Ross Gyre and the other Antarctic Coastal Current; AMERY ventilates the Antarctic Coastal Current; DBW formed in front of the Filchner Ronne Ice Shelf (FRIS) flows along the Antarctic Peninsula and partly leaves the Weddell Sea. Cross sections shows that the model doesn´t reproduce CFC cores. However, at the continental slope in the depth of the core the simulated concentrations correspond to the observed concentrations. The discrepancy can be traced back to input of simulated containing CFC water masses at ocean depths between 150-1200m instead to the abyssal ocean.:p:The DBW formation rates of the Southern Ocean were determined by tracer budget calculations. The DBW formation rates of the SouthernOcean average 5-15Sv according to water masses. However, the formation rates for a given water mass and constant overturning depend on the chosen tracer and time observation.:p:The explicitly formulated interaction between ocean and ice shelf is used to describe the input of He and Ne. The good agreement between Simulation and Observation values confirms the approach. Increase of measurement accuracy to ±0.2% would improve the detection of Ice Shelf Water from ROSS and AMERY.Dissertation467 427 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Transfer von Nordatlantischem Tiefenwasser durch den Südatlantik mit Tracer-Verteilungen und Transitzeit-Verteilungen(2005-02-11); ; ; We examine the spreading of North Atlantic Deep Water (NADW) in the tropical Atlantic and subtropical South Atlantic by means of transit time distributions (TTDs) which are fitted to observations of the transient tracers CFC-11, CFC-12, CCl4 and tritium. The TTDs are derived under the assumption of steady-state flow, and the flow being formally represented by one-dimensional advection and mixing. The latter includes all non-advective processes of the transport, such as eddies, recirculations and temporal variability of the transport field. In comparison with conventional tracer dating methods, which use the approach of a purely advective transport, this method represents the next higher approximation.With the approach chosen here, the TTDs depend on a limited number of parameters only: the mean age t, and the Péclet number Pe as the ratio of advection and mixing. Additionally an explicitly tracer-free dilution V is used to account for mixing with waters that are old enough to contain little or no tracer. It has turned out that the use of different tracers and of repeated observations usually does not provide enough independent information to determine the parameters of the TTDs consistently. Therefore we assume t and V to vary linearly with the distance and that Pe is constant. This allows us to use all relevant tracer observations at different locations and times to determine the free parameters of the TTDs. Conventionally a constant apparent tracer saturation in the source waters is used as a tracer boundary condition. To obtain a more realistic boundary condition a simple mixing model for the source water masses has been devised. This approach ensures a realistic approximation of the time evolution of the tracer concentration ratios in the source waters.Using these TTDs we examine the transport of upper and lower NADW in the South Atlantic in the deep western boundary current, along the equator, and along a zonal section at 11°S.Dissertation787 133 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Ausbreitung von Island-Schottland-Überstromwasser im östlichen Nordatlantik(2001-12-19); ; The spreading of Iceland Scotland Overflow Water (ISOW) in the eastern North Atlantic is the focus of this thesis. It is shown that the classical pathway of ISOW through the Gibbs Fracture Zone into the western North Atlantic is not the only one on which it is spreading. Anomalies of salinity, oxygen and silica are used to reveal the influence of ISOW on the northeastern North Atlantic. It can be seen from the anomalies that ISOW has an influ-ence on the water of the Iceland Basin that extends widely over the ISOW core itself. The ISOW influenced water formed by vigorous mixing in the Iceland Basin partially undergoes a anticyclonic recirculation in the northern Westeuropean Basin. Another part is exported to the south along the Midatlantic Ridge.The variability of the ISOW influence on the deep water is examined for the first time using repeat sections. The eastern boundary of the ISOW influence is subject to the biggest changes. These are strongly correlated with the NAOI (correlation coefficients: -0.93 and -0.92). This emerges from a strong coupling of the deep water spreading to the surface currents via baro-tropic velocity components in the northern part of the area of investigation.A quantitative estimate of the transport from the Iceland Basin to the Westeuropean Basin is based on a mixing and transport analysis relying on transient tracers. The transport is calcula-ted to be (1.63 ± 0.32)·10:sup:6:/sup: m³/s in the density range for which the analysis yields stable results (41,37 < :img:=sigma.gif:/img::sub:3:/sub:/(kg/m³) <41,475). The transport for the density range usually considered as deep water (:img:=sigma.gif:/img::sub::img:=Theta.gif:/img::/sub: > 27.8 kg/m³) is estimated to be 2.4 - 3.5·10:sup:6:/sup: m³/s. Considering oxygen and silica in the mixing and transport analysis confirms the results. This means that there is a transport of deep water along this route that is approximately equal to the transport through the Gibbs Fracture Zone.Dissertation317 75 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Bestimmung von Verweilzeiten und Transportraten in der Warmwassersphaere des Suedatlantiks aus Tracerdaten(1999-07-02); ; ; Tritium and CFC-11 data obtained from the programmes WOCE (1990-1995) and SAVE (1987-1989, CFC-11 data only) are used for an analysis of the upper level circulation in the South Atlantic Ocean (density range sigma-0 = 26.0-27.1 kg/m**3). A new bomb tritium input function for the South Atlantic is presented. The atmospheric deposition is based on previous work by Doney et al. [1992]. The new input function also accounts for tritium deposition due to river runoff and for tritium exchange with other ocean basins. The calculated total tritium input for 1960-1995 between 0°S-50°S and the observed tritium inventory in 1995 agree well (within 1%). This agreement supports a value of 0.70 for the liquid/vapor isotopic fractionation factor of tritium, disproving GCM model calculations by Koster et al. [1989] that yielded a fractionation factor of 0.35. Tritium surface water concentrations are inferred from a simple mixed layer model driven by atmospheric tritium fluxes and calibrated using GEOSECS and WOCE data. The surface water concentrations of the South Atlantic are lower than in the North Atlantic. The highest surface values in the 1990´s are found in the subtropics where the surface water concentrations are still slowly increasing. As boundary condition for CFC-11 it is assumed that the sea surface is in a solubility equilibrium with the atmosphere. For quasi synoptic analyses the tritium and CFC-11 data are projected to the year 1993 using a 1D advection-diffusion model. Meridional sections show tracer concentrations decreasing with depth. Maximum penetration of tritium and CFC-11 is found in the subtropical region. North of about 20°S there is a concentration decline in both tracers indicating upwelling processes. Distributions on isopycnal surfaces show high tritium values in the western South Atlantic which may be due to the formation of Subtropical and Subantarctic Mode Waters.Dissertation526 90 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Realistische Modellierung der tiefreichenden Zirkulation des Mittelmeers durch Simulation von Tracerverteilungen(2001-09-25); ; ; The thermohaline circulation of the Mediterranean is simulated by means of the OGCM (ocean general circulation model) MOMA. Special focus lies on the eastern basin, where simulated concentrations of transient tracers, especially the chlorofluorocarbon CFC-12, is compared with observations. The inner oceanic concentrations of these tracers provide a good means to identify pathways of newly ventilated water in the ocean and therefore allow to assess the model results. After 1987 the deep circulation in the eastern Mediterranean changed dramatically. The Adriatic was replaced by the Aegean as the main source of deep water. This process is simulated in the model via a decrease of sea surface temperature and an increase of sea surface salinity over the Aegean during winter. Realistic ventilation rates of the deeper parts of the Levantine and Ionian basin can be obtained by adjusting the rate of this extra buoyancy forcing. The resulting transient model circulation is compared with observational data and an explanation for the remarkable increase of salt content in the eastern basin of the Mediterranean is given.The concepts of age and age distributions are applied within the model for the old and new circulation state of the Mediterranean. Recently ventilated waters are characterized by young ages close to zero. During the spreading of the water into the ocean interior the ideal age increases, so it gives information on the circulation similar to those derived from transient tracers. The oldest water in the eastern Mediterranean is present in the Levantine basin, where upwelling of the deep water takes place, which reaches close to the surface.The ideal age is compared with concentration and ratio ages obtained from the modelled concentrations of transient tracers.The relation between ideal and tracer derived ages as well as between tracer concentrations and age distribution is shown.Dissertation553 127