ALBERT

Description: The polar shelf zones are highly dynamic and diverse systems. In the Arctic shelf region multiple river deltas accumulate organic carbon. They host a unique and very diverse northern fauna and flora. One of the final goals of my work is following ecosystem dynamic of Lena Delta region, Laptev Sea. In framework of this purpose the main task is to tune a basic numerical model which is able to reproduce the climatic changes in the region and to estimate the variability of the climate system. To design appropriate model we are using Finite Volume Coastal Ocean Model. FVCOM is a prognostic, unstructured-grid, finitevolume, free-surface, 3-D primitive equation coastal ocean circulation model developed by UMASSD-WHOI joint efforts. The main objective of current research is to verify the model and analyze temperature and salinity local pattern of mixed layer (up to 50 meters) in the Lena Delta Region of Laptev Sea in a short period of time (less than half a year). Also assess the degree of influence the Lena River runoff temperature, local wind pattern and tides dynamics to temperature and salinity variability in the considered area. The particular attention is paid to assessment of impact local bathymetry data to temperature and salinity local pattern. The calculation processes were carried out on the basis of FVCOM. We used high quality unstructured grid, which allows to take into account complexity of coastline, characteristics of the bathymetry data and features of the problem (the necessity to reduce the size of elements on a main ways of Lena fresh water plumb). Elements sizes are vary from 400m to 3 km. The model contains 6 vertical sigma-layers with 250000 nodes on each of them. The input data (bathymetry and coastline data, atmospheric forcing, temperature and salinity fields for first time step for every depth layer, temperature and salinity on the open boundary nodes for nudging, runoff date) are taken from NOAA, GEBCO, from German and Russian Institutes.

Description: The Lena delta region of Laptev Sea can serve as an indicator of climate changing. A large number of observed data in this region suggests a strong climate change and biological data for the last thirty years. Unfortunately we still have not fully answer about extent of the changes and their causes in this region. One of the goals of my modeling is simulation of ecosystem dynamics in the Lena Delta region of the Laptev Sea. The dynamics are sensitive to the temperature and salinity variability and thus are connected to the ocean circulation dynamics. We explore them with the help of the Finite Volume Coastal Ocean Model. Local patterns of temperature and salinity in the mixed layer (up to 50 meters) in the Lena Delta Region of the Laptev Sea are analyzed for different regimes of atmospheric circulation (anticyclonic and cyclonic) with time scales of 5 – 30 days. We also assess the influence of Lena runoff temperature, local wind pattern and tidal dynamics on the temperature and salinity variability in the area. A particular attention is paid to the impact of local bathymetry data to temperature and salinity local pattern. May of 2008 and 2009 were chosen as test periods. Simulations were carried out with the Finite Volume Coastal Ocean Model (FVCOM). FVCOM is a prognostic, unstructured-grid, finite-volume, free-surface, 3-D primitive equation coastal ocean circulation model. It was used by us in a variant including wetting and drying, GOTM (turbulence model) and nudging of temperature and salinity at open boundaries. We used a high quality unstructured mesh, which allowed us to take into account the complex structure of coastline and bathymetry and other aspects of the problem (the necessity to refine mesh elements along main ways of Lena fresh water plumb). Element sizes vary from 400m to 3 km. The mesh has 6 vertical sigma layers with 250000 nodes on each of them. The input data (bathymetry, coastline, atmospheric forcing, initial temperature and salinity, climatological values for nudging at the open boundary, and runoff) are taken from NOAA, GEBKO and several other institutions.

Description: The Laptev Sea and the East Siberian Sea have the most severe climate and the lowest salinity of all the seas of the northern coast of Asia. Among the seas of the Arctic Ocean the Laptev Sea receives the input from the largest number of rivers such as Lena (provides approximately 70% of total runoff to the Laptev Sea), Olenyok, Khatanga, Anabar, Yana, Omoloy, Gusiha and other small rivers. The Lena delta region of Laptev Sea can serve as an indicator of climate changing. A large number of observed data in this region suggests a strong climate change and biological data for the last thirty years. Thus it is timely to estimate the role of different factors to dynamics in the region. One of the goals of my PhD is to evaluate dynamics of the hydrography-carbon transport in Lena Delta region of the Laptev Sea, in recent decades. In framework of this purpose the first task is to tune a basic numerical model which is able to reproduce the climatic changes in the region; the second task is to estimate the variability of the climate system; the third task is to implement a biological module in to the Laptev Sea shelf 3D model. In framework of the first and second tasks in this paper were examined the role of local wind pattern, tidal dynamics, structure and temperature of freshwater runoff, characteristics of heat fluxes to explain the features of the temperature and salinity distributions in the region. In addition was estimated the impact bathymetry data of the Lena Delta vicinity on temperature and salinity local patterns. The numerical simulations were based on Finite Volume Coastal Ocean Model (FVCOM).

Description: The Lena delta region of Laptev Sea serves as an indicator of climate changing. A large number of observed data in this region suggests a strong climate change and biological data for the last fifty years. Unfortunately we still have not fully answer about extent of the changes and their causes. One of the main task is lead out model, which will provide information about salinity, temperature, water elevation, sea ice fraction and ice flow, dynamic of fresh water distribution (Lena river) in a Laptev Sea. Based on this information we will try to estimate observation data and explain the changes in this region.

Description: Tidal processes play an important role in the dynamics of shelf circulation in the Laptev Sea. The Unstructured Grid Finite Volume Coastal Ocean Model (FVCOM) is used to simulate the tidal dynamics in the Lena Delta region of the Laptev Sea in ice-free barotropic case. The grid size is ranging from 400 m to 5 km. The major semidiurnal tidal waves M2 and S2 are investigated with the M2 being the most important in generating large sea level amplitudes and currents over the shallow areas. A correction to the tidal elevation at the open boundary is proposed which minimizes the discrepancy between the model prediction and observations. They include both recent mooring data and the standard set of tide gauge measurements used in previous studies. The comparison of results to known tidal solutions is carried out. The paper also discusses the residual circulation and energy fluxes and assesses the impact of additional bathymetric information.

Description: The article describes the modeling processes of the shelf circulation dynamics in the Laptev Sea with focus on the Lena Delta region. We try to estimate the role of different factors such as heat exchange with atmosphere, Lena runoff and tidal forcing on the dynamics in the region. An unstructured-grid Finite Volume Coastal Ocean Model (FVCOM) is used as a modeling tool.

Description: 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.