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Simulation of the Laptev Sea Shelf Dynamics with Focus on the Lena Delta Region by Vera Fofonova

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Simulation of the Laptev Sea Shelf Dynamics with Focus on the Lena Delta Region by Vera Fofonova Simulation of the Laptev Sea shelf dynamics with focus on the Lena Delta region by Vera Fofonova A Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy in Geosciences Approved Dissertation Committee Prof. Dr. Karen Helen Wiltshire Jacobs University Bremen; Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research Dr. Sergey Danilov Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research Prof. Dr. Vikram Unnithan Jacobs University Bremen Dr. Jens Schröter Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research Date of Defense: 26th of June, 2014 School of Engineering and Science Abstract 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. List of acronyms AARI – Arctic and Antarctic Research Institute AMSR-E – Advanced Microwave Scanning Radiometer - Earth Observing System Sensor on the NASA Aqua Satellite AO – Arctic Ocean AWI – Alfred Wegener Institute, Helmholtz Centre for Polar and Marine Research AOTIM5 – Arctic Ocean Tidal Inverse Model COSMO – Consortium for Small-Scale Modeling CTD – Conductivity, Temperature, and Depth DOE – Department of Energy ECMWF – European Centre for Medium-Range Weather Forecasts EP – Erofeeva and Padman (Padman and Erofeeva, 2004) FVCOM – Unstructured Grid Finite Volume Coastal/Community Ocean Model GEBCO – General Bathymetric Chart of the Oceans GS – Gauging Station IBCAO – International Bathymetric Chart of the Arctic Ocean KP – Kowalik and Proshutinsky (Kowalik and Proshutinsky, 1993, 1994) MDS – Multidimensional Scaling Projection NAOSIM – North Atlantic/Arctic Sea Ice - Ocean Model NOAA – National Oceanic and Atmospheric Administration NCEP – National Centers for Environmental Prediction NCAR – National Center for Atmospheric Research OOBC – Optimized Open Boundary Conditions PSMSL – Permanent Service for Mean Sea Level SIC – Sea Ice Concentration TPXO6.2 (7.1) – Global Inverse Tide Model UMASSD-WHOI – University of Massachusetts - Woods Hole Oceanographic Institution i Table of contents Introduction ........................................................................................................................................... 5 1. General description of the Laptev Sea region .................................................................................. 8 2. Lena River hydrology regime ........................................................................................................ 11 3. Short model description ................................................................................................................. 14 4. Overview of the manuscripts and main steps................................................................................. 17 4.1. List of manuscripts .................................................................................................................. 17 4.2. Workflow ................................................................................................................................ 18 5. Semidiurnal tides in the Laptev Sea Shelf zone in the summer season (Manuscript 1) ................. 20 5.1. Introduction ............................................................................................................................. 21 5.2. Available solutions and data ................................................................................................... 23 5.2.1. Tidal solutions .................................................................................................................. 23 5.2.2. Observations .................................................................................................................... 24 5.3. Model, input data and experiment descriptions ...................................................................... 26 5.3.1. Model description ............................................................................................................ 26 5.3.2. Input data ......................................................................................................................... 27 5.3.3. Open boundary conditions derivation and experiment description ................................. 28 5.4. Results and Discussion ........................................................................................................... 29 5.4.1. Tidal maps and parameters of barotropic ellipses............................................................ 29 5.4.2. Sensitivity to bathymetry ................................................................................................. 38 5.4.3. Energy balance ................................................................................................................. 40 5.5. Conclusions ............................................................................................................................. 46 6. Impact of wind and tides on the Lena River freshwater plume dynamics
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