Description: In austral winter, biological productivity at the Angolan shelf reaches its maximum. The alongshore winds, however, reach their seasonal minimum suggesting that processes other than local wind‐driven upwelling contribute to near‐coastal cooling and upward nutrient supply, one possibility being mixing induced by internal tides (ITs). Here, we apply a three‐dimensional ocean model to simulate the generation, propagation, and dissipation of ITs at the Angolan continental slope and shelf. Model results are validated against moored acoustic Doppler current profiler and other observations. Simulated ITs are mainly generated in regions with a critical/supercritical slope typically between the 200‐ and 500‐m isobaths. Mixing induced by ITs is found to be strongest close to the coast and gradually decreases offshore thereby contributing to the establishment of cross‐shore temperature gradients. The available seasonal coverage of hydrographic data is used to design simulations to investigate the influence of seasonally varying stratification characterized by low stratification in austral winter and high stratification in austral summer. The results show that IT characteristics, such as their wavelengths, sea surface convergence patterns, and baroclinic structure, have substantial seasonal variations and additionally strong spatial inhomogeneities. However, seasonal variations in the spatially averaged generation, onshore flux, and dissipation of IT energy are weak. By evaluating the change of potential energy, it is shown, nevertheless, that mixing due to ITs is more effective during austral winter. We argue that this is because the weaker background stratification in austral winter than in austral summer acts as a preconditioning for IT mixing.

Description: The Yedoma region is unique in the permafrost region of the Northern Hemisphere and is characterized by a particularly high ground ice content in the sediment. These frozen deposits store a large amount of carbon and thus have the potential to influence the global climate. Especially the upper layers are susceptible to thaw processes, as they are exposed to increasingly rising mean annual air temperatures. The Northeastern Siberian Yedoma domain is of particular interest in this study. The morphology of ground ice is highly variable and the exact abundance and distribution is still unknown in large parts of Siberia. For an accurate overview of the distribution of intrasedimentary ground ice content, data from 26 sites in Northeastern Siberia were examined. The data were taken from data repositories (e.g., PANGAEA), expedition reports, scientific papers etc. and has been synthesized in a template in Excel. Of relevance was the absolute ice content (wt%) at different depths. Five depth classes were investigated: depth class 1: 0-0.99m; depth class 2: 1-1.99m; depth class 3: 2-2.99m; depth class 4: 3-24.99m; depth class 5: 25-65m. Using the mean absolute ice content for each depth class, ArcGIS was used to create a map for the distribution of ice content. R was applied to represent the ground ice content distribution at the different depths. Furthermore, the focus was on other parameters such as stratigraphy, total organic carbon content and landscape types, which were also examined with respect to the absolute ice content. The ice content is distributed very heterogeneously in Northeastern Siberia, averaging between 30 and 60 wt% over all depths. In large parts of the study area, the ice content in the upper three meters is with 40 to 65 wt% much higher than in the deeper sediment layers. In the depths of 3-65m, the ice content ranges from 20 to 50 wt%. Investigations of the age classes showed that the mean absolute ice content in thermokarst deposits (MIS 1) is with 48.60 wt% higher than in older sedimentary units. The TOC content also decreases significantly with depth. The Yedoma sediment composition and depositional regimes are highly variable. Even on a small scale, large differences in ice content could be observed. With the given data basis, no concrete statements about the vertical and horizontal ice content could be made for the whole study area. The model created in this study can be applied to model the absolute ground ice content based on the TOC content. Assessing the nature and content of ground ice in the upper layers in Northeastern Siberia is fundamental to environmental assessment and important for quantifying carbon fluxes and understanding permafrost response to climate change.