Observing tidal effects on the dynamics of the Ekström Ice Shelf with focus on quarterdiurnal and terdiurnal periods

Antarctica's ice shelves stabilize the ice sheet and, therefore, understanding processes affecting the mass budgets of ice shelves is important for estimating grounded ice loss. To study the ice shelf dynamics, we analyzed seismological and GNSS data from the Ekström Ice Shelf in Dronning Maud Land. We extracted probabilistic power spectral densities (PPSD) in the frequency band 3.4-6.8 Hz, typical of icequakes, from seismological data and observed pronounced signals in the PPSD with near 3 and 4 cycles per day (cpd) corresponding to tidal overharmonics, in addition to the main tidal constituents near 1 and 2 cpd. GNSS data reveal the same components in ice flow speed but not in vertical displacements. Generally, tide-induced grounding line migration modulates the flow velocity of an entire ice shelf. We find that this velocity modulation causes the increased icequake activity in the tidal overharmonics with 3 and 4 cpd in an ice shear zone where the flow velocity drops to nearly zero

Simulation of the Laptev Sea shelf dynamics with focus on the Lena Delta region

The polar shelf zones are highly dynamic and diverse systems. They form a border between warm and fresh water of continental drain and the cold currents of the northern seas. The Lena River is one of the largest rivers in the Arctic, with the largest delta. The south-eastern part of the Laptev Sea, which includes the Lena Delta region, is the place where substantial changes in ocean circulation and ecosystem may happen in changing climate. Exploring processes there, which may serve as an indicator of climate change, acquire a special importance. The Lena freshwater plume propagation dominates many aspects of dynamics in the Laptev Sea shelf. However, the direct measurements are by far insufficient, calling for a modeling approach which would enable one to estimate the impact of different factors on the circulation dynamics and would lay the foundation for further ecosystem modeling. The complexity of the region’s geometry and insufficient data make modeling of ocean circulation in the Lena Delta vicinity a challenging technical task not solved in the necessary detail previously. The quantitative effect of various factors (tides, winds, hear exchange with the atmosphere) on the freshwater plume propagation also has not been fully explored. The main goal of this thesis is the analysis of the Lena River freshwater plume dynamics in the summer season on the basis of a full baroclinic numerical model of the Laptev Sea shelf with focus on the Lena Delta region. The setup is based on FVCOM (The Unstructured Grid Finite Volume Coastal/Community Ocean Model; Chen et al. 2006). The thesis contains a detailed description of the model setup, including the generation of an unstructured mesh, analysis of barotropic and baroclinic dynamics in the region of interest, the description of new approaches for the model elaboration and visualization of simulation results and a comparison of the impact of different atmospheric forcing products on the simulated dynamics. Special attention is paid to the Lena River hydrology regime in the basin outlet, which is taken into account in simulations. Since tides are responsible for a considerable fraction of mixing over the shallow shelf of the Laptev Sea, the first step consisted in accurate modeling of barotropic tides in the Lena Delta region of the Laptev Sea. This demanded using accurate topography data and the design of optimized open boundary conditions that would provide the best agreement with observations. The simulated tidal maps for principal semidiurnal constituents, which are the most important in the considered area, showed an improved agreement with observations as compared to other modeling efforts. Important information about barotropic currents, evolution of energy fluxes in the region and residual circulation, which affects sediment and nutrients transport, was obtained in this work. The next important step toward more realistic simulations was taking into account the Lena River hydrology. This step required substantial preliminary work on compiling and analyzing respective Lena River characteristics in the basin outlet area. The anomaly in surface water temperature was found to exist at the most downstream location in the summer season. Its description and basic analysis is presented. To sort the problem of anomaly out, the observational data in the scope of hydrology and morphology for the Lena River delta and main channel area, including data on permafrost conditions under the river channel, were considered. The third step was full baroclinic simulations with focus on the Lena River freshwater plume dynamics in the summer season. The role of tides, winds and thermohaline forcing in shaping the plume dynamics was explored by applying different sources of atmospheric forcing and switching on/off tidal dynamics. In addition, the roles of local bathymetry and techniques of freshwater distribution were assessed. A detailed comparison with the available observational data was also performed showing a good agreement. It was found that the surface salinity distribution is most sensitive to winds, with the implication that the ability of model to predict it relies on the availability of high-quality wind forcing data. Tidal mixing and residual transport are important, but only locally, whereas heat exchange with the atmosphere influences the water mass properties, but has only a weak impact on dynamics. This understanding together with the proof that the model simulations agree well with the observational data are the main results of this thesis. They demonstrate that the model can serve as a platform for future ecosystem modeling in the Lena Delta region

Modeling of Nonhydrostatic Dynamics and Hydrology of the Lombok Strait

The long-wave dynamics of the Lombok Strait, which is the most important link of the West Indonesian throughflow connecting the Pacific and Indian Ocean waters, was simulated and analyzed. A feature of the strait is its extremely complex relief, on which water transport creates a field of pronounced vertical velocities, which requires consideration of the nonhydrostatic component of pressure. The work presents a 3-D nonhydrostatic model in curvilinear coordinates, which is verified on a test problem. Particular attention is paid to the method of solving the 3-D elliptical solver for a nonhydrostatic problem in boundary-matched coordinates and a vertical σ level. The difference in transport through the Lombok Strait is determined by the difference in atmospheric pressure over the Pacific and Indian Oceans. Based on the results of the global simulation, the role of these factors in terms of their variability is analyzed, and the value of nonhydrostatic pressure in the dynamics of the Lombok Strait is revealed and evaluated. The vertical dynamics of the Lombok Strait are considered in detail based on hydrostatic and nonhydrostatic approaches

A comparison of vertical mixing parametrizations in the simulation of the ice and upper ocean state based on the Arctic Ocean model

The vertical mixing in the ocean plays an important role in regulating sea surface temperature, which is a critical oceanic parameter, controlling the atmosphere-ocean heat ,energy and momentum exchanges. Because of the small-scale turbulent processes involved, the vertical mixing usually cannot be explicitly resolved in ocean general circulation models and has to be parametrized. The three-dimensional coupled ice-ocean numerical model used in this study, is based upon the ocean model, developed in the Institute of Computational Mathematics and Mathematical Geophysics, SB RAS, and Sea ice-model- (CICE 3.14- (http://oceans11.lanl.gov/drupal/CICE) , adapted to the region of the North Atlantic (1x1 degree) and the Arctic Ocean(35 -50km). Several one-dimensional vertical mixing parametrizations were implemented from GOTM package (General Ocean Turbulence Model, http://www.gotm.net/). Among them: nonlocal K-profile parameterization (KPP, [1]), Total Kinetic Energy (TKE) with first order [2] and second order [3] coefficients . These vertical parameterizations were compared with more simple adjustment procedure based on the Richardson number, previously used in the ICMMG ocean model. The parametrization were tested in numerical experiments which were aimed to simulate the variability of the Arctic Ocean state under atmospheric forcing (NCEP/NCAR,1948-1912)

The water temperature characteristics of the Lena River at basin outlet in the summer period

The water temperature characteristics of the Lena River at basin outlet during the summer season (June–September) are considered. The analysis is based on long-term data series covering the period from the beginning of observation (1936) to the present time at Kusur (Kyusyur) station and complementary data at several stations downstream and one station upstream. These additional data are rarely used, but their analysis is important for understanding processes in the basin outlet area. The differences between the stream surface temperatures at Kusur station and 200 km downstream to the north at Habarova (Khabarova) station have almost always been an anomalously large and negative for the considered period since the beginning of observation during open water season from July to September. The description of this difference and its analysis are presented. To sort the problem out, we consider the observational data in terms of the hydrology and morphology of the Lena River delta and main channel area, apply statistical and deterministic modelling approaches

Non-linear aspects of the tidal dynamics in the Sylt-Rømø Bight, south-eastern North Sea

This study is dedicated to the tidal dynamics in the Sylt-Rømø Bight with a focus on the non-linear processes. The FESOM-C model was used as the numerical tool, which works with triangular, rectangular or mixed grids and is equipped with a wetting/drying option. As the model's success at resolving currents largely depends on the quality of the bathymetric data, we have created a new bathymetric map for an area based on recent studies of Lister Deep, Lister Ley, Højer Deep and Rømø Deep. This new bathymetric product made it feasible to work with high-resolution grids (up to 2 m in the wetting/drying zone). As a result, we were able to study the tidal energy transformation and the role of higher harmonics in the domain in detail. For the first time, the tidal ellipses, maximum tidally induced velocities, energy fluxes and residual circulation maps were constructed and analysed for the entire bight. Additionally, tidal asymmetry maps were introduced and constructed. The full analysis was performed on two grids with different structures and showed a convergence of the results as well as fulfilment of the energy balance. A great deal of attention has been paid to the selection of open-boundary conditions, model validation against tide gauges and recent in situ current data. The tidal residual circulation and asymmetric tidal cycles largely define the circulation pattern, transport and accumulation of sediment, and the distribution of bedforms in the bight; therefore, the results presented in the article are necessary and useful benchmarks for further studies in the area, including baroclinic and sediment dynamics investigations