Seasonal thermal regime and climatic trends in lakes of Tibetan Highlands
Other literature type
The hydrology of the lake-rich Tibetan Plateau is important for the global climate yet little is known about the thermal regime of Tibetan lakes due to scant data. We (i) investigated the characteristic seasonal temperature patterns and recent trends in thermal and stratification regime of lakes on the Tibetan Plateau and (ii) tested theperformance of the one-dimensional lake parameterization scheme FLake for the Tibetan lake system. For this purpose we combined three years of in situ lake temperature measurements, several decadesof satellite observations and the global reanalysis data. We chose the two largest freshwater Tibetan lakes – Ngoring and Gyaring – as study sites. The lake model FLake faithfully reproduced the specific features of the high-altitude lakes and was subsequently applied to reconstruct the vertically resolved heat transport in both lakes during the last four decades. The modelsuggested Ngoring and Gyaring were ice-covered forabout 6 months and stratified in summer for about 4 months per year with a short spring overturn and longer autumn overturn. In summer the surface mixed boundary layer extended to 6–8 m depth and was about 20 % shallower in the more turbid Gyaring. The thermal regime of transparent Ngoring responded more strongly to atmospheric forcing than Gyaring, where the higher turbidity dampened the response. According to reanalysis data, air temperatures and humidity increased, whereas solar radiation decreased since the 1970s. Surprisingly, mean lake temperatures did not change, nor did the phenology of ice cover or stratification. Lake surface temperatures in summerincreased only marginally. The reason is that the increase in air temperature was offset by the decrease in radiation, probably due to increasing humidity. This study demonstrates that air temperature trends are not directly coupled to lake temperatures and underscores the importance of short-wave radiation for the thermal regime of high-altitude lakes.