Rhein, Monika
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Rhein, Monika
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Rhein, Monika
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Rhein, M.
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Item-typ:Veröffentlichung, Observations of damping and scattering of low mode internal waves in the ocean(2020-12-17); ; ; Interne Schwerewellen treten überall im geschichteten Ozean auf und können sich Tausende von Kilometern von ihrem Entstehungsort ausbreiten, wobei sie Energie transportieren bevor sie brechen. Das Brechen interner Wellen führt zu einer diapyknischen Vermischung, die bei verschiedenen klimarelevanten Prozessen eine wichtige Rolle spielt. Ich untersuche die zeitliche Variabilität interner Wellen in der halbtägigen Frequenz M2, die durch barotrope Gezeiten erzeugt werden und in der Nähe der durch Wind generierten internen Wellen nahe der Inertialfrequenz f, in einer Region mit hoher interner Wellenenergie innerhalb eines Gezeitenstrahls südlich der Azoren. Zur Durchführung der Messungen wurde eine fast zwei jährige Verankerung eingesetzt, die die Eigenschaften der ersten beiden vertikalen Moden des internen Wellenfeldes auflösen kann. Die Ergebnisse dieser Beobachtungen wurden verwendet um Änderungen in der Amplitude, Richtung und Kohärenz des Energieflusses, der modalen Struktur des Gezeitenstrahls und die Auswirkungen mehrerer Ereignisse, die während dieser Messungen auftraten, zu analysieren. Die gesammelten Ergebnisse dieser Analysen werden mit Energieflüssen aus einem 1/10° globalem Ozeanzirkulationsmodell (STORMTIDE2) und Energieflüssen aus der Satellitenaltimetrie verglichen. Die Beobachtungen unterstützen die Hypothese, dass Wechselwirkungen mit Wirbeln den inkohärenten Teil des Energieflusses erhöhen und Energie von niedrigen Moden in höhere Moden übertragen, was zu einer erhöhten lokalen Dissipation führen kann. Daher scheint es unvermeidlich, dass zukünftige Ozeanmodelle eine energetisch konsistentere Parametrisierung der Wechselwirkungen der internen Gezeiten mit mesoskaligen Strömungen benötigen, um die zeitliche Variabilität der internen Wellen korrekt darzustellen. Diese Studie stellt einen Schritt zum Verständnis der Rolle der zeitlichen Variabilität interner Wellen zur Abschätzung der globalen Ozeanvermischung dar.Dissertation573 382 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, Pathways and variability of the circulation in the subpolar eastern North Atlantic studied with inverted echo sounders and model data(2021-07-29); ; ; The North Atlantic Current (NAC) as part of the Atlantic Meridional Overturning Circulation (AMOC) is the major pathway for warm and saline water from the subtropics into the subpolar North Atlantic. Due to buoyancy loss along its flow path and subsequent deep water formation, it connects the upper warm limb of the AMOC with the deeper cold limb. Associated volume fluxes and their variability are thus of great interest, especially in the context of climate change. The main branch of the NAC and related transports are widely studied. The NAC crosses 47°/48°N in the western North Atlantic and further north the Mid-Atlantic Ridge (MAR) before entering the eastern subpolar basin where it partly feeds the Subpolar Gyre or flows into the Nordic Seas. To quantify the meridional exchange of water between the subtropical and subpolar regime in the interior eastern North Atlantic where studies are scarce, in this work, long-term (1993 to 2017) transport time series were calculated by combining data from inverted echo sounders taken in 2016 and 2017 with satellite altimetry. The results obtained from observational data are complemented with transport time series calculated from high resolution model output of the ANHA12 configuration of the NEMO model and with the analysis of particle trajectories calculated from the Lagrangian model ARIANE. The observational data reveal an additional more direct pathway from the south across 47°/48°N into the subpolar eastern North Atlantic with a mean northward transport of +9.1 Sv ± 0.8 Sv contributing about 22% to the total inflow of +41.4 Sv into the eastern basin. The meridional transport of this pathway is significantly anticorrelated to the transport across the MAR (R = −0.7), damping the interannual variability of the total inflow into the subpolar eastern North Atlantic. Moreover, for the meridional transport in the interior eastern basin, a positive trend of +2.0 Sv ± 1.5 Sv per decade is found, partly balancing the negative decadal trend of −6.0 Sv ± 5.7 Sv observed for the interior western basin. The mean transport imbalance at the 47°/48°N transect between Newfoundland and 15°W was found to be −2.2 Sv which is likely to be compensated by the flow east of 15°W. In the model, the overall circulation pattern in the subpolar North Atlantic as well as the main regions for water mass transformation are very similar to what is found from observations. However, also substantial differences between the model and observations were found such as a surplus northward flow across 47°/48°N in the western basin, a weaker coupling between the western and eastern basin, and a smaller total inflow into the eastern subpolar North Atlantic of +24.2 Sv. Moreover, the analysis of particle trajectories reveals that about 60% of the water at 47°/48°N and the MAR originates in the subtropics and about 11% flows into the Nordic Seas.Dissertation471 305 - Some of the metrics are blocked by yourconsent settings
Item-typ:Veröffentlichung, The Atlantic Meridional Overturning Circulation in the North Atlantic, focussing on 47°N - variability, trends, and meridional connectivity(2024-03-04); ; ; The Atlantic Meridional Overturning Circulation (AMOC) plays a vital role in the climate of Europe and the North Atlantic region. Climate model studies project an AMOC decline in the 21st century. However, they disagree on the magnitude and timescales of the weakening. Thus, monitoring AMOC changes remains essential to provide benchmarks for assessing climate models and understanding the physical processes determining AMOC variability. In this thesis, basin-wide AMOC volume transports are calculated (1993-2018). Measurements from moored instruments of the NOAC array at 47°N are combined with hydrography and satellite altimetry. Variability, trends, and meridional connectivity with the RAPID array at 26°N are analyzed. The AMOC volume transport at 47°N exhibits a mean strength of 17.2 Sv and substantial variability on inter-annual and seasonal timescales but no significant long-term trend. The NOAC AMOC shows a significant correlation with the RAPID AMOC when the NOAC AMOC leads by about one year, indicating meridional connectivity. An analysis of the AMOC at the NOAC, RAPID, and OSNAP (52°N-60°N) lines in the high-resolution forced VIKING20X model simulation reveals a mean NOAC AMOC strength within the estimated error range of the NOAC observations. In disagreement with observations, the VIKING20X AMOC decreases after the mid-1990s until 2010 at all three array lines. This decrease coincides with a significant cooling and freshening in the subpolar North Atlantic. In agreement with observations, VIKING20X shows meridional connectivity between the NOAC and RAPID AMOC when the NOAC AMOC leads by about one year. This indicates a common mechanism, determining the meridional connectivity in observations and VIKING20X. An analysis of different ANHA model simulations with varying resolution underlines the importance of model resolution for accurately representing the AMOC mean strength and variability but also stresses the need for model improvements beyond resolution.Dissertation215 129