| Summary: | As the Arctic Circle opens up due to the effects of global warming, the surrounding regions, and in particular the subarctic zone, experience both climatic and societal changes, which makes it imperative to understand the existing ecosystems. Lake Lagarfljót, an Icelandic fjord lake with glacial inflow, is used to evaluate processes occurring in weakly stratified water bodies in the sub-Arctic. Field observations during the summers of 2010 and 2011 show that the lake experiences a short period of stratification, with overturning occurring when the heat fluxes become negative. The internal-wave regime evolves rapidly as the stratification strengthens, higher vertical modes and Kelvin waves are identified. Glacial inflow to Lake Lagarfljót delivers large volumes of fine sediment. It is expected that most of the sediments remain in suspension and this can alter the density of the water-column, resulting in inverse thermal gradients. Additionally, the glacial river is deflected to the eastern edge of the lake by Coriolis forces, causing the inflow to remain in shallow water before entering the center of the lake. Results from the Münnich model demonstrate that the natural oscillation of the lake basin is close to the oscillation period of the wind. The 3D hydrodynamic model Si3D was adapted to account for suspended sediment, and it successfully reproduced the motions observed in the lake, showing its utility for the study of weakly stratified lakes. Results suggest that wind is the main driver of internal waves in Lake Lagarfljót, but that the river influences stratification, meaning that it cannot be ignored when considering lake circulation. The model confirms the presence of Kelvin waves in the basin. Tracking the pathways of the inflowing glacial water, it appears that the initial intrusion depth is dependent on internal waves. During summertime stratification, the river penetrates the lake as interflows or underflows. Underflows can become trapped, causing them to oscillate and becoming flushed episodically ...
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