Walter, Maren

Preferred name

Walter, Maren

Official Name

Walter, Maren

ORCID

0000-0001-5108-2278

GND

1020512083

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Plume dispersal in the Arctic Ocean - the Aurora Site at Gakkel Ridge
(2025-05-16)
Mette, Jonathan
;
Walter, Maren
This thesis assesses the dispersal of the hydrothermal plume at the Aurora Vent Site in the Arctic Ocean, based on observational data and complemented by a numerical ocean model. The theoretical background of hydrothermal plumes in the Arctic is presented and the methods used are explained. This includes the processing of data acquired from a CTD probe, water samples and an oceanographic mooring with respect to the hydrographic setting and to the tracers for identification of the hydrothermal fluid. In addition a setup for simulating the plume dispersal with the Regional Ocean Modelling System is described. The observational results reveal a plume that ascends up to a height of 1200 m and spreads laterally to at least a distance of 2500 m, although a strong core is confined to a much smaller area. The plume dispersal is highly inhomogeneous for the different investigated tracers. This, as well as the small horizontal extent is explained by the presence of slow currents that are altered by tidal or inertial oscillations. A similar vertical extent can be obtained from the model simulation. However in regards to the currents and horizontal extent, the simulation shows large discrepancies compared to the observations, therefore suggestions for improving the model setup are presented.
text::thesis::master thesis
137 72
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Process Studies on the Generation of Near-Inertial Internal Gravity Waves by Wind
(2017-03-28)
Voelker, Georg Sebastian
;
Sutherland, Bruce R
;
Walter, Maren
;
Walter, Maren
The generation of internal gravity waves in the ocean associated to wind stress forcing is studied with the aid of two numerical models. In a non-linear, axisymmetric and Boussinesq model the response to an idealized stationary wind stress pulse is investigated. The major generation mechanisms for internal gravity waves, inertial pumping, excites radially outward propagating low mode wave packets. The energy radiated relative to the wind work is found to be only about 0.02%, which is 2-3 orders of magnitude lower than generally observed. In addition to the detailed axisymmetric simulation, a simplified hybrid slab model based on the classical slab model of (Pollard and Millard, 1970) is considered. In particular, the inertial pumping resulting from the divergent, horizontal, near-inertial response of the surface slab is regarded the boundary condition to the internal gravity wave field below the mixed layer. In a set of idealized experiments, laterally moving wind stress fronts are the most efficient driver for vertical motion depending on their translation speed. The application of the hybrid slab model to the North Atlantic for the years 1989 and 1996 as examples of opposite phases of the North Atlantic Oscillation shows latitude bands with characteristic ratio of energy radiated as internal gravity waves to the wind work. These are partly associated to the wind field structure in these meridional regimes. The ratio of total energies transferred equals to 9% for both years. In an application of the hybrid slab model to the axisymmetric setup it is found that the structure of the inertial oscillations of the axisymmetric simulation are well reproduced. However, the near-inertial response of the hybrid slab model has an 8 times larger amplitude and does not include non-linear effects. The ratio of radiated energy to the wind work for the stationary storm estimated with the hybrid slab model is about 1%. This ratio is increased by a factor 30 when the same wind stress structure is moving across the domain with 21 km/h.
Dissertation
1408 260
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Mixing Induced Vertical Heat and Freshwater Fluxes in the Upper Ocean of the Subpolar North Atlantic
(2020-11-20)
Sukhikh, Natalia
;
Myers, Paul G.
;
Walter, Maren
;
Walter, Maren
This study is focused on an investigation of diapycnal diffusivity induced by internal waves breaking in the upper ocean of the subpolar North Atlantic. Turbulent diffusion plays a significant role in transferring heat and freshwater between the oceanic surface and deep ocean. The upper subpolar North Atlantic is defined as a region between 40-70N and 0-65W below the seasonal thermocline. It is the complex region where mixing is induced by many processes such as double diffusion, turbulence, or isopycnal mixing. This study is an attempt to define the contribution of turbulent diapycnal mixing to the vertical transfers of heat and freshwater. To date, measurements of dissipation rate and diapycnal diffusivity are limited in the upper subpolar North Atlantic. Often, these parameters are parametrized with CTD (Conductivity-Temperature-Depth) and Lowered ADCP (Acoustic Doppler Current Profiler) data on CTD stations. Measurements of the microscale shear and temperature gradient are relatively rare in this region. Here, I added the vertical shear from the shipboard ADCP velocity data to the analysis of parametrized turbulent mixing. The main advantage of this kind of the data is that the dataset is continuous in time and space along the ship track. It helps to increase the number of estimations of diapycnal mixing in the region. I started from the description of a method for post-processing of the collected velocity datasets. After, I describe the variability of diapycnal diffusivity in the subpolar North Atlantic. The next chapter is dedicated to understanding the role of wind forcing in energy transfer to the internal waves field and turbulent mixing. The last two chapters contain results from a numerical model and investigate the role of turbulent diffusion in the vertical transfers of heat and freshwater in comparison with vertical advection, and describe the sensitivity of background stratification to changes in external forcings. Diapycnal diffusivity is used as a measure of turbulent diffusion induced by internal waves’ breaking and estimated from the long-term shipboard observations. Vertical shear is calculated from the shipboard ADCP data, and buoyancy is described with the CTD data collected during 13 research cruises in the subpolar North Atlantic in 2003-2018. Dissipation rate is estimated from a finescale shear-based parameterization based on the properties of the internal waves’ field and the assumption of proportionality of dissipation to internal waves’ energy and buoyancy. Diffusive heat and freshwater fluxes are calculated for the CTD stations. Advective fluxes are computed with a high-resolution NEMO (Nucleus for European Modelling of the Ocean) model based on the configuration ANHA12 (Arctic and Northern Hemispheric Atlantic with 1/12 degree). Additionally, a Hybrid Slab Model is used to estimate the energy flux from wind to the internal waves’ field. Diapycnal diffusivity has no significant temporal (seasonal or interannual) variability in the subpolar North Atlantic. Two main regions with different mixing regimes are described. The Labrador Sea and the region south of Greenland are characterized by low turbulent diffusion rates. Enhanced diffusivities are detected in the central parts of the subpolar North Atlantic (the Western North Atlantic along the 47N and the PIES line along the Mid-Atlantic Ridge). The energy flux coming from wind to the internal wave energy shows strong seasonality. But a direct response of internal waves’ energy or dissipation and diffusivity to wind input energy flux is not found. Vertical advection is driven by the vertical velocity field and also contributes to vertical heat and freshwater transfers. It varies in the upper subpolar North Atlantic and follows the divergence and convergence zones of the general circulation. At the same time, the turbulent heat flux has a downward direction, while the turbulent freshwater flux is upward. Dissipation rate is estimated from a finescale parameterization that depends on two parameters, internal wave energy and background stratification. The high-resolution NEMO model output allows us to describe potential changes of the upper ocean buoyancy depending on changes in different atmospheric and hydrological forcings. In a short-term perspective, buoyancy is the most sensitive to wind and air temperature. Wind has a double contribution as it influences the mixed layer depth and contributes to the internal waves’ energy. Wind also modifies sensible and latent heat fluxes and influences the ocean-atmosphere interactions and oceanic heat content. In a long-term perspective, changes in precipitation and runoff forcings will also have a significant contribution. The signal of internal waves in the upper subpolar North Atlantic is found from the shipboard data, but double diffusion, enhanced mesoscale eddy activity and non-linear interactions of internal waves with topography, general currents or eddies should be taken into account for a complete analysis of vertical mixing in further investigations.
Dissertation
838 315
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Internal wave propagation in the Arctic Ocean
(2024-06-10)
Bracamontes Ramírez, Joel
;
Olbers, Dirk
;
Walter, Maren
;
Walter, Maren
The propagation of internal gravity waves in the Arctic Ocean is studied using direct and indirect observations, reanalysis data and two numerical models. I focus on how stratification modifies wave propagation and its connection to the pathways of internal wave energy from the surface to the deep ocean. Understanding internal wave propagation in the Arctic Ocean is crucial for better understanding wave-driven mixing and its implications for climate projections. Therefore, three specific aspects of internal wave propagation have been addressed in this thesis. First, the transient transmission of a wave packet across a density staircase is studied using a 2D Boussinesq model. A series of simulations with fixed wave frequency and varying horizontal wave number are carried out. The results show that the incident wave excites trapped modes by a near resonance mechanism, which slowly transfer energy above and below the staircase. A theoretical prediction was made using typical values for thermohaline staircases and internal waves in the Arctic Ocean. Surface-generated near-inertial internal waves that excite trapped modes should have a critical horizontal wavelength of ∼ 400 m. For higher-frequency non-hydrostatic waves, this critical horizontal wavelength decreases to ∼ 80 m. Such waves are likely to be generated by wind-driven ice floes. Secondly, the existence of turning depths for near-inertial internal waves and their effect on wind-driven deep mixing is assessed using 10 years of temperature and salinity profiles in the Canadian Basin. It is found that turning depths exist in the deep Canadian Basin at ∼ 2750 m, but with decreasing distance from the bottom towards the slope. A subsequent discussion focuses on the possible topographic interaction of internal wave reflection and dissipation, especially where turning depths are shallow and above the slope, where the evanescent perturbation of the internal waves can still interact with the topography. Finally, direct observations of near-inertial internal waves are investigated using current observations from a mooring on the Gakkel Ridge, and their surface generation is addressed using a wind and ice drift speed and ice concentration dataset. Cross correlation analysis shows that there is a correlation between ice drift speed and near inertial wave energy with a lag of < 26 days. In addition, a correlation of ∼ 15 days is observed between the wind factor and the near-inertial wave energy. This result suggests that near-inertial internal waves may be generated at the surface by an interplay of wind and ice properties and propagate from the surface to the seafloor. Evidence for wave reflection is also found, and 2D numerical simulations of waves reflected at a turning depth are performed and compared with observations, showing qualitative agreement.
Dissertation
236 163

Walter, Maren

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