Rhein, Monika
Lade...
Preferred name
Rhein, Monika
Official Name
Rhein, Monika
7 Ergebnisse
Einstellungen
Gerade angezeigt 1 - 7 von 7
- Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, North Atlantic Current in model and observations: Transport variability, flow paths and hydrography(2017-07-25); ; ; 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 comprehensively study the processes involved, therefore models are used to complement the observations. For this study, moored and ship-based observations from three sections along the NAC pathway, satellite altimetry data and 1/20 degree hindcast VIKING20 model configuration (1960-2008) output are used. They are investigated and compared to describe hydrography, flow field and transport time series. The flow into and out of the NA is monitored at 47 degree N, the flow from the western into the eastern basin of the NA is captured by a section at the western flank of the Mid-Atlantic Ridge (MAR), and the distribution in the eastern basin of the NA is observed at the OVIDE line. These three sections thus provide an extensive picture of the flow.Dissertation1440 217 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Water Mass Circulation and Variability in the Subpolar North Atlantic(2005-03-15); ; ; This study focuses on estimating the variability in the formation of Upper and classical Labrador Sea Water (ULSW and LSW). Both are formed by winterly convection and spread into the world ocean as part of the cold limb of the thermohaline circulation. Analyses are based on a large-scale hydrography/tracer data set from the years 1997, 1999, and 2001. Horizontal fields of water mass layer thickness and mean concentrations of chlorofluorcarbon (CFC) have been constructed to determine the CFC inventory of each water mass and to infer water mass formation rates. The years 1997-2001 showed a significant increase in the CFC inventory of ULSW, while the inventory of classical LSW reduced. During 1997-1999 formation of ULSW was strong (6.9-9.2 Sv). From 1999 to 2001 the ULSW formation rate reduced to 3.7-4.0 Sv. LSW formation was absent during these four years. Historical hydrographic data from the Labrador Sea have been used to compare water mass properties of ULSW and LSW on longer time scales. Time series indicate strong variability and a significant anti-correlation of ULSW and LSW formation. Coinciding with weakening convection the density surface that separates ULSW from classical LSW shifted to greater depths. Water layer lying on top of LSW revealed an increasing stratification which is presumably strengthened by warm and saline water intruding from the West Greenland Current into the interior Labrador Sea. Time series of sea surface fluxes indicated a change in the atmospheric conditions after 1995/96. The convection activity at that time was, however, sufficient to ventilate the ULSW layer. Analyses of deep and bottom water properties provided evidence for the existence of export pathways in the Newfoundland Basin that are additional to the Deep Western Boundary Current (DWBC). LSW spreading time scales point to a fast spreading in the DWBC (1-2 years from the Labrador Sea to 43°N) and a slow spreading in the interior Newfoundland Basin (3-6 years).Dissertation803 183 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Diapycnal mixing in the subpolar North Atlantic(2013-10-16); ; ; Model simulations of climate change and global overturning circulation are quite sensitive to the strength and distribution of mixing. However, the field observations are only sparsely available. The strength of diapycnal mixing was estimated from more than 700 profiles of hydrographic and velocity measurements in the subpolar North Atlantic (SPNA). These measurements were collected during hydrographic surveys from 2003 to 2011, ranging from 40 N to 62 N in latitude. Furthermore, 28 Micro-scale structure profiles were collected at 7 stations over the Mid-Atlantic Ridge and at the western boundary during the cruise in 2008, providing supplement of direct measurements and an agent to evaluate the overall estimation. Under the assumption of a steady state, spatial distribution and vertical structure of diapycnal diffusivity were mapped in this area. The inferred diffusivity is generally elevated compared to the background diffusivity in the open ocean and shows large variability in the SPNA. diffusivity of at least one magnitude larger can be seen nearly in the whole area several hundread of meters within seafloor. Strong mixing at mid-depth of one to two orders larger than the background value are found at western boundary, over Mid-Atlantic ridge and in the pathway of North Atlantic Current and deep currents. The possible connections between enhanced mixing and several environmental parameters including seafloor roughness, geostrophic currents and meso-scale eddies are analysed. Conversions between components of the North Atlantic Deep Water associated with mixing are estimated from vertical motion inferred from density field and turbulent diffusivity based on an advection-diffusion balance model. In vertical direction, averaged diffusivity is found to decrease with the height above seafloor within the deepest 1500 m and to be constant. A transformation of around 1.6 Sv from Gibbs Fracture Zone Water to overlying Labrador Sea water is derived; the transformation between lowest Denmark Strait Overflow Water to upper Gibbs Fracture Zone Water is about 3.5 Sv.Dissertation718 146 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Warming of Greenland Sea Deep Water Induced by Abyssal Mixing(2004-02-04); ; ; In the absence of deep convection, the Greenland Sea Deep Water has experienced a slow but significant warming during the 1980s and ´90s. Enhanced vertical mixing can explain the observed trends of several propertiesincluding anthropogenic tracers, but the necessary mixing ratesare at the higher end of those observed elsewhere in the ocean.In this work, the mixing is studied by means of the spatial distribution and strength of eddy diffusivities. These are calculated from energy dissipationrates, which in turn are estimated with two complementary methods.Thorpe scales provide a direct estimate of the dissipation from densityoverturns. The second method uses a spectral estimate of the finescale variances of velocity shear density strain as proxies for the energy content of the internal wave field, and the equlibrium energy dissipation rate. Both methods are applied on a data set obtained duringsummer 1998, comprising loweredADCP measurements of the velocity field in the central Greenland Sea and near the surrounding ridge systems, as well assupplementary temperature and salinity measurements.The diapycnal diffusivities observed in the Greenland Sea are highenough to account for the changes in deep water. The mean diffusivity across the 2000~m isobath is 1.2 x 10:sup:-3:/sup: m:sup:2:/sup:/s,two orders of magnitude larger than the typical deep ocean background.The highest values occur in the deep basin, with a moderate amplification in the vicinity of rough topography at mid depth.In the upper layer, the locations of strongest mixing are close tothe fronts of the boundary currents.Enhanced mixing in the deeper layers is not confined to rough topography, but occurs throughout the whole basin. The critical latitudes of most semidiurnal tides are located in the Greenland Sea, therefore thisdistribution is interpreted as a result of the resonant breakdown ofthe tidal waves.Dissertation794 205 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Long-term variability of internal waves and diapycnal mixing: The role of the North Atlantic Deep Western Boundary Current(2013-12-09); ; ; Five years of continuous mooring data combined with CTD/LADCP measurements from five cruises are used to investigate the influence of the Deep Western Boundary Current (DWBC) on the internal wave field and associated vertical mixing at the continental slope at 16°N in the western Atlantic. As the temporal variability in current strength and corresponding flow speeds within the DWBC is very high, this geographic location provides an ideal setting to analyze the direct influence of strong currents or their interaction with topography on the generation of internal waves and the magnitude of vertical mixing rates. The mooring data include two-hourly rotor current meter measurements and temperature/conductivity time series with high temporal resolution of 5 - 20 minutes. Thus, the data resolve timescales ranging from the low-frequency variability of the large scale DWBC that generates internal waves due to interactions with the topography, to high frequency vertical mixing induced by breaking internal waves. Diapycnal diffusivities obtained from a finescale parameterization show elevated mixing rates of up to 10^(-3)m^2/ s in the bottommost 1500m during times of a strong DWBC where velocities at the mooring site reach up to 50cm/s. Enhanced shear to strain ratios during these times denote an increase in low frequency waves during phases of strong flow. Variability in the high frequency range calculated from mooring data, considered as a proxy for turbulent mixing, is significantly correlated with the DWBC strength above the continental slope which also indicates a pronounced increase of vertical mixing during strong flow. During these periods spectra of horizontal velocity and internal wave available potential energy change substantially at depths below 1200m and show a strong increase of energy in internal waves particularly in the near inertial frequency band. This increase is stronger at the mooring over the continental slope than slightly more offshore over the continental rise. The generation of low frequency, near inertial waves due to the interaction of the DWBC with the slope topography to the west of the moorings where the local water depth equals the depth of the DWBC core is the mechanism proposed for the generation of the observed intensification of low frequency waves and enhanced vertical mixing rates; ray paths estimated for internal waves generated at the continental slope agree well with the observed spectral changes at different depths. Furthermore the vertical energy propagation direction estimated from rotary spectra of shipboard LADCP measurements shows a divergence at depths approximately corresponding to the depth of the DWBC core. Above the core the energy propagation is dominantly upward whereas it is downward below. This is consistent with the estimated ray path for (near inertial) internal waves generated by the interaction of the DWBC core with the slope topography to the west of the moorings.Dissertation611 215 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Observed and modeled MOC related flow into the Caribbean Sea and the North Atlantic Ocean(2007-11-19); ; ; The transport of South Atlantic Water (SAW) into the northern hemisphere is investigated in this work. This flow represents the upper branch of the Meridional Overturning Circulation (MOC) and thus has a direct influence on the global heat budget. A variety of data is used: direct hydrographic and velocity observations from ship surveys, profiles from Argo floats and data from an ocean model (FLAME). The model and the observations show high consistency in the strength of the mean total inflow into the Caribbean, its range of variability, and the distribution of water from the South Atlantic. During the ship cruises large rings were observed at 16 degrees N. The ring propagation is investigated in FLAME and the complex interaction of the rings with the Lesser Antilles discussed. The spreading of SAW into the North Atlantic is analyzed. The mean SAW transport into the Caribbean derived from observations is estimated to be 9.3 Sv. The analysis with FLAME yields a similar transport. When the rings observed at 16 degrees N are taken into account the total SAW transport is 15.3 Sv. Since the upper layer MOC transport is in the order of 16-18 Sv, the remaining SAW transport by the STCs is less than 3 Sv.Dissertation742 181 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Observed subpolar gyre transports at the Mid-Atlantic Ridge(2013-01-28); ; ; The subpolar gyre of the North Atlantic Ocean is an integral component for the climate relevant oceanic circulation. To measure the variability of the North Atlantic Current (NAC) and thus the strength of the subpolar gyre, an array of four inverted echo sounders with bottom pressure sensors (PIES) was deployed along the Mid-Atlantic Ridge between 47° and 53°N in August 2006. The locations of the individual PIES allow the separation of the main spreading paths of the NAC. The array was deployed at ground track crossing points of altimetry satellites. The PIES delivered daily data that were retrieved by acoustic telemetry each year, while the array remained at the seafloor. The first four year long time series is analysed regarding the transport variability of the NAC and the underlying water masses. The transports are calculated using the Gravest Empirical Mode technique, which has been adapted and tested. The daily uncertainties of transports across the full PIES array are 1.9 Sv. The mean transport across the full array is 29.2 Sv with a standard deviation of 8.6 Sv. Two thirds of the observed mean transport can be assigned to the NAC, while the other third is made up of Labrador Sea Water and a mixture of Denmark Strait Overflow Water and Iceland Scotland Overflow Water. The power spectral density reveals that most of the variability of the barotropic component is between 2 and 20 days. The baroclinic component has its largest energy in the period of 100 to 300 days, which seems to be generated by north-eastward carried eddies. The favoured position of the NAC within the array is corresponding to a crossing of the Mid-Atlantic Ridge at the Faraday and Maxwell Fracture Zones. The surface velocities from altimetry were analysed and used to calculate a baroclinic transport time series for the entire 19 years of satellite measurements. The analysis of the surface velocities confirms the results from the PIES time series and adds an interannual signal to the variability. The preference of the Faraday Fracture Zone as the crossing point for the NAC across the Mid-Atlantic Ridge could be further confirmed by considering the mean and standard deviation of Eddy Kinetic Energy as a direct indicator for the position of the NAC. The 19 year long transport time series of the three subsections present an opposing trend in the two northern sections. No trend for the transport across the full section can be found but only a hint to a slight southward shift of the NAC. The baroclinic mean transport across the full section is 27.6 Sv with a standard deviation of 4.6 Sv. The comparison of the 3-year low-pass filtered satellite based transport time series with the mean Eddy Kinetic Energy (EKE) across the full section and the winter North Atlantic Oscillation (NAO) index proves that the NAO leads the EKE and transports by 2 to 3 years (R = 0.5) and modulates the transport in the range of 6 to 7 Sv (peak-to-peak).Dissertation616 214