Variabilität von Vermischungsprozessen und diapyknischen Wärmeflüssen im äquatorialen Atlantik bei 23° West

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

Meso- and submesoscale variability within the Peruvian upwelling regime : mechanisms of oxygen supply to the subsurface ocean

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 oxyclin

Data-driven rules for multidimensional reflection problems

Over the recent past data-driven algorithms for solving stochastic optimal control problems in face of model uncertainty have become an increasingly active area of research. However, for singular controls and underlying diffusion dynamics the analysis has so far been restricted to the scalar case. In this paper we fill this gap by studying a multivariate singular control problem for reversible diffusions with controls of reflection type. Our contributions are threefold. We first explicitly determine the long-run average costs as a domain-dependent functional, showing that the control problem can be equivalently characterized as a shape optimization problem. For given diffusion dynamics, assuming the optimal domain to be strongly star-shaped, we then propose a gradient descent algorithm based on polytope approximations to numerically determine a cost-minimizing domain. Finally, we investigate data-driven solutions when the diffusion dynamics are unknown to the controller. Using techniques from nonparametric statistics for stochastic processes, we construct an optimal domain estimator, whose static regret is bounded by the minimax optimal estimation rate of the unreflected process' invariant density. In the most challenging situation, when the dynamics must be learned simultaneously to controlling the process, we develop an episodic learning algorithm to overcome the emerging exploration-exploitation dilemma and show that given the static regret as a baseline, the loss in its sublinear regret per time unit is of natural order compared to the one-dimensional case.Comment: 29 pages, 3 figure

Distribution of transparent exopolymer particles in the easter tropical south Pacific

Important for the carbon export to the Deep Sea, Transparent Exopolymer Particles (TEP) also serve as nutritional substrate and attachment surfaces for bacteria, supporting biological oxygen consumption. Spatial distribution of TEP in the Eastern South Pacific Ocean (ESP), an area influenced by a highly productive upwelling system and underlying extensive Oxygen Minimum Zone (OMZ), is largely unknown. In attempt to recognize how TEP stocks are affected by complex biogeochemistry of ESP and vice versa, we determined TEP distribution in the ESP (12°-14°S, 76°-79°W) during the Meteor 93 cruise (February - March 2013). Highest TEP concentrations (>1900µgXeq/l) were observed close to the coast, coinciding with upwelling of nutrient-rich waters. Generally TEP accumulated in the oxygenated photic layer and correlated significantly with chlorophyll-a-fluorescence (r=0.7, n=323, P<0.05). At the upper boundary the OMZ (70 m), TEP concentrations were moderate (<80µgXeq/l), slightly attenuating toward its lower border (<60µgXeq/l; 500m depth). The role of TEP for organic matter cycling in the ESP and their potential influence on oxygen and carbon fluxes in the OMZ are discussed

The formation of a subsurface anticyclonic eddy in the Peru-Chile Undercurrent and its impact on the near-coastal salinity, oxygen and nutrient distributions

The formation of a subsurface anticyclonic eddy in the Peru-Chile Undercurrent (PCUC) in January and February 2013 is investigated using a multi-platform four-dimensional observational approach. Research vessel, multiple glider and mooring-based measurements were conducted in the Peruvian upwelling regime near 12°30'S. The dataset consists of > 10000 glider profiles and repeated vessel-based hydrography and velocity transects. It allows a detailed description of the eddy formation and its impact on the near-coastal salinity, oxygen and nutrient distributions. In early January, a strong PCUC with maximum poleward velocities of ∼ 0.25 m/s at 100 to 200 m depth was observed. Starting on January 20 a subsurface anticyclonic eddy developed in the PCUC downstream of a topographic bend, suggesting flow separation as the eddy formation mechanism. The eddy core waters exhibited oxygen concentrations < 1μmol/kg, an elevated nitrogen-deficit of ∼ 17μmol/l and potential vorticity close to zero, which seemed to originate from the bottom boundary layer of the continental slope. The eddy-induced across-shelf velocities resulted in an elevated exchange of water masses between the upper continental slope and the open ocean. Small scale salinity and oxygen structures were formed by along-isopycnal stirring and indications of eddy-driven oxygen ventilation of the upper oxygen minimum zone were observed. It is concluded that mesoscale stirring of solutes and the offshore transport of eddy core properties could provide an important coastal open-ocean exchange mechanism with potentially large implications for nutrient budgets and biogeochemical cycling in the oxygen minimum zone off Peru

Chromophoric and fluorescent dissolved organic matter in and above the oxygen minimum zone off Peru

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

Chromophoric and fluorescent dissolved organic matter in and above the oxygen minimum zone off Peru

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

The formation of an anticyclonic mode water eddy within the Peru-Chile Undercurrent

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