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  • 1
    Publication Date: 2016-12-05
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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  • 2
    Publication Date: 2014-10-08
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 3
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    In:  [Poster] In: EGU General Assembly 2014, 27.04.-02.05.2014, Vienna, Austria .
    Publication Date: 2015-01-15
    Description: The formation of an anticyclonic mode water eddy in Jan/Feb 2013 within the Peru-Chile Undercurrent is presented based on a multi-platform observational study. Two consecutive research cruises, a glider swarm experiment and moored measurements were conducted as part of the interdisciplinary "SFB 754 Climate-Biogeochemistry Interactions in the Tropical Ocean" project within the Peruvian upwelling regime at 12°S. The dataset allows a detailed investigation of the eddy generation process and its impacts on the near-coastal hydrography and biogeochemistry in space and time. The near-coastal horizontal circulation off Peru at 12°S changes significantly over the two months of observation. In early January, we observe a strong but clear Peru-Chile Undercurrent with maximal pole-ward velocities of ~25 cm/s in 100 - 200 m depth. A week later the vertical shear starts to increases and finally a mode water eddy forms. The eddy has a velocity maximum of ~0.3 m/s in 100 - 200 m depth and a radius of ~45 km. The eddy induced circulation strongly influences the near-coastal hydrography: Across-shore velocities result in an exchange of water masses between the shelf-break and the offshore ocean. At the eddy edge small scale salinity anomalies are found, which seem to be formed by mesoscale stirring. Energetic near-inertial oscillations are observed in the deeper water column during eddy generation that appear to be associated with this feature. After its generation close to the shelf break the eddy propagates westwards.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 4
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    In:  (Doctoral thesis/PhD), Christian-Albrechts-Universität Kiel, Kiel, Germany, 110 pp
    Publication Date: 2019-02-01
    Description: The role of meso- and submesoscale processes for the near-coastal circulation, physical and biogeochemical tracer distributions and oxygen minimum zone ventilation in the Peruvian upwelling regime is investigated in this thesis. A multi-platform four-dimensional observational experiment was carried out off Peru in early 2013 and is the basis for this thesis. Furthermore a high-resolution submesoscale permitting physical circulation model is used to study submesoscale frontal dynamics in more detail. The formation of a subsurface anticyclonic eddy and its impact on the near-coastal salinity, oxygen and nutrient distributions was captured by the observations. The eddy developed in the Peru-Chile Undercurrent downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy resulted in enhanced cross-shore exchange of physical and biogeochemical tracers due to along-isopycnal stirring and offshore transport of core waters. The core waters originated from the bottom boundary layer and were characterized by low potential vorticity and an enhanced nitrogen-deficit. The subduction of highly oxygenated surface water in a submesoscale cold filament is observed by glider-based measurements. The subduction ventilates the upper oxycline but does not reach into oxygen minimum zone core waters during the summer observations. Lagrangian floats are used to study the pathways of newly upwelled water in a regional submesoscale permitting model. The model analysis suggests a gradual warming of the newly upwelled waters due to surface heat fluxes. The associated density decrease prevents the floats to enter the density range of the oxygen minimum zone in summer. However, in winter a density increase is found due to surface cooling and thus it might be possible that submesoscale processes ventilate the oxygen minimum zone. In the model about 50 % of the newly upwelled floats leave the mixed layer within 5 days both in summer and winter emphazising a hitherto unrecognized importance of subduction for the ventilation of the Peruvian oxycline
    Type: Thesis , NonPeerReviewed
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  • 5
    Publication Date: 2012-10-09
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 6
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    In:  (Bachelor thesis), Christian-Albrechts-Universität, Kiel, Germany, 37 pp
    Publication Date: 2012-07-06
    Description: Mixing processes in the upper equatorial Atlantic at 23°W on the basis of direct microstructure measurements from June 2006, March 2008, and November 2009 have been studied. These measurements indicate large differences in mixing intensities below the mixed layer. In March 2008 vertical shear and mixing levels were comparatively low. At similar stratification in June 2006, enhanced mixing well below the mixed layer was observed. During this time a tropical instability wave passed by which was connected with strong vertical shear in upper 60 m of the water column. Throughout these two cruises, nocturnal mixing rates exceeded daily values, associated with the known deep diurnal cycle. The averaged diapycnal heat flux between 2°S and 2°N across the base of the mixed layer was 55 W/m² in June 2006. The Equatorial Undercurrent changed its position significantly over one week in November 2009. During the southward cruise, weak nocturnal mixing rates below the mixed layer were found. The deep position of the undercurrent core with its high shear zone above may explain this observation. Nocturnal measurements during the northward cruise at the equator reveal much higher vertical shear directly under the base of the mixed layer in connection with significantly increased turbulence. These results suggest that the variability of mixing processes in the investigated area might be determined to a large portion by the strength and position of the vertical shear above the undercurrent core.
    Keywords: Course of study: BSc Physics of the Earth System
    Type: Thesis , NonPeerReviewed
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  • 7
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    In:  [Poster] In: AGU Fall Meeting 2012, 03.-07.12.2012, San Francisco, USA .
    Publication Date: 2013-10-18
    Description: The water mass transformation process in the Labrador Sea during winter plays an important role for the Atlantic meridional overturning circulation and the global climate system. The Labrador Sea Water (LSW) is exported within the deep Labrador Current (LC) after the convection process. LSW takes up large amounts of atmospheric tracer gases as CO2 and oxygen, and is thus one of the major agent for ventilation of the abyssal ocean. It is shown that enhanced eddy kinetic energy (EKE) along the LC shows up in a 1/12 ° ocean model simulation during the transformation process. Moored in-situ measurements within the LC also show enhanced EKE levels during winter. This instability processes within the LC is important as it might alter the water mass properties of the (LSW) by frontal mixing processes during the water mass transformation and export within the LC. The frontal instability process, which lead to enhanced EKE along the LC during winter is investigated using ageostrophic linear stability analysis. Dense and weakly stratified water masses produced during the wintertime transformation process lead to weaker stratification and a strengthening of the lateral density gradients within the LC. Weak stratification and enhanced vertical shear result in low Richardson numbers and the growth rate of baroclinic waves increases significantly within the shelf break LC during winter. Rapid frontogenesis along the whole LC sets in resulting in enhance EKE. During the rest of the year strong stratification and weak vertical shear leads to larger Richardson numbers and smaller growth rates. Ageostrophic linear stability analysis shows that a geostrophic interior mode has similar wavelengths as the first wavelike disturbances in the model simulations. A shallow mode with lateral scales O (1 km) is also predicted, which can be associated with mixed layer instabilities and submesoscale variability but remains unresolved by the model simulation.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 8
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    AGU
    In:  Journal of Geophysical Research: Oceans, 121 (11). pp. 7973-7990.
    Publication Date: 2019-02-01
    Description: Key Points: - Two amino acid-like and three humic-like FDOM components were found in and above the oxygen minimum zone off the coast of Peru - The distribution of CDOM and amino acid-like FDOM covaried with chl a, suggesting phytoplankton as their major source - Presence of DOM microbial reworking and DOM release by anoxic sediment was illustrated by the distribution of humic-like FDOM As a result of nutrient upwelling, the Peruvian coastal system is one of the most productive regions in the ocean. Sluggish ventilation of intermediate waters, characteristic for the Eastern Tropical South Pacific (ETSP) and microbial degradation of a high organic matter load promotes deoxygenation at depth. Dissolved organic matter (DOM) plays a key role in microbial respiration and carbon cycling, but little is known on DOM distribution and cycling in the ETSP. DOM optical properties give important insights on DOM sources, structure and biogeochemical reactivity. Here, we present data and a conceptual view on distribution and cycling of chromophoric (CDOM) and fluorescent (FDOM) DOM in and above the oxygen minimum zone (OMZ) off Peru. Five fluorescent components were identified during PARAFAC analysis. Highest intensities of CDOM and of the amino acid-like fluorescent component (C3) occurred above the OMZ and coincided with maximum chl a concentrations, suggesting phytoplankton productivity as major source. High intensities of a marine humic-like fluorescent component (C1), observed in subsurface waters, indicated in situ microbial reworking of DOM. FDOM release from inner shelf sediment was determined by seawater analysis and continuous glider sensor measurement and included a humic-like component (C2) with a signature typical for terrestrially derived humic acids. Upwelling supplied humic-like substances to the euphotic zone. Photo-reactions were likely involved in the production of a humic-like fluorescent component (C5). Our data show that variable biological and physical processes need to be considered for understanding DOM cycling in a highly dynamic coastal upwelling system like the ETSP off Peru.
    Type: Article , PeerReviewed
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  • 9
    Publication Date: 2015-01-15
    Description: A swarm experiment with seven gliders equipped with sensors measuring pressure, temperature, salinity, oxygen and chlorophyll fluorescence was conducted in early 2013 within the upwelling region off Peru. The goal was to study the role of meso- and submesoscale proccesses for the near-coastal oxygen ventilation of the Peruvian oxygen minimum zone. Each glider carried out about one dive per hour measuring two multi-parameter profiles with a lateral resolution less than 300 m. About 15.000 profiles were recorded during the two-months deployment within a small spatial area to capture both the temporal and spatial variability of the physical and biochemical parameters. Two main results are presented in the talk: 1) The formation of a low oxygen mode water eddy within the Peru Chile Undercurrent is observed. The near-coastal horizontal circulation off Peru at 12°S changes significantly over the two months of observation. In early January, we observe a pronounced Peru-Chile Undercurrent with maximal poleward velocities of 25 cm/s in 100 - 200 m depth. A week later the circulation start to change and a mode water eddy forms within the glider field. The physical and biogeochemical eddy properties and impacts on the near-coastal salinity and oxygen distribution are described in detail. 2) During an upwelling event the formation and decay of a submesoscale cold water filament is present in the sea surface temperature and glider data. Near the surface, non-density compensated salinity and oxygen intrusions are observed which seem to be associated with this feature. These anomalies reach well below the mixed layer into the thermocline and frontal subduction is suggested as their formation mechanism.
    Type: Conference or Workshop Item , NonPeerReviewed
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  • 10
    Publication Date: 2017-06-15
    Description: Recent modeling results suggest that oceanic oxygen levels will decrease significantly over the next decades to centuries in response to climate change and altered ocean circulation. Hence, the future ocean may experience major shifts in nutrient cycling triggered by the expansion and intensification of tropical oxygen minimum zones (OMZs), which are connected to the most productive upwelling systems in the ocean. There are numerous feedbacks among oxygen concentrations, nutrient cycling and biological productivity; however, existing knowledge is insufficient to understand physical, chemical and biological interactions in order to adequately assess past and potential future changes. In the following, we summarize one decade of research performed in the framework of the Collaborative Research Center 754 (SFB754) focusing on climate–biogeochemistry interactions in tropical OMZs. We investigated the influence of low environmental oxygen conditions on biogeochemical cycles, organic matter formation and remineralization, greenhouse gas production and the ecology in OMZ regions of the eastern tropical South Pacific compared to the weaker OMZ of the eastern tropical North Atlantic. Based on our findings, a coupling of primary production and organic matter export via the nitrogen cycle is proposed, which may, however, be impacted by several additional factors, e.g., micronutrients, particles acting as microniches, vertical and horizontal transport of organic material and the role of zooplankton and viruses therein.
    Repository Name: EPIC Alfred Wegener Institut
    Type: Article , NonPeerReviewed
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