Showing papers by "Andreas Lorke published in 2010"

Internal waves and the generation of turbulence in the thermocline of a large lake

Abstract: High-resolution thermistor chain data collected between 5 and 20 m in a large stratified freshwater lake (Lake Constance) at a water depth of 60 m reveal the frequent occurrence of large-amplitude (2: I-m) vertical density inversions that indicate overturns in the pelagic thermoc1ine. Velocity data collected simultaneously with the temperature measurements indicate that the density inversions are mainly produced by shear instabilities. A comparison between the timing of the passage of the basin-scale internal Kelvin wave and the density inversions demonstrates a pronounced phase relationship, implying that the processes leading to the occurrence of turbulence and mixing are connected to the passage of the Kelvin wave. Two different processes are identified during which density inversions were particularly common. An increased number of density inversions and especially high dissipation rates of turbulent kinetic energy were observed when Kelvin wave-induced critical shear supported the generation of large Kelvin-Helmholtz billows. A particularly large number of density inversions was also associated with the passage of nonlinear high­ frequency waves of large amplitudes. The density inversions typically occurred at the wave troughs, which indicates breaking of these waves. These observations indicate that self-induced shear generated by the basin­ scale seiche and by high-frequency internal waves leads to a significant amount of turbulence and mixing in the pelagic thermoc1ine.

Impact of lake level change on deep-water renewal and oxic conditions in deep saline Lake Van, Turkey

Abstract: [1] Changes in the hydrological regime of the saline closed basin Lake Van, a large, deep lake in eastern Turkey, resulted in a lake level increase by about 2 m between 1988 and 1995, followed by a 1.5 m decrease until 2003 and a relatively constant lake level thereafter. Based on measurements of transient tracers (sulfur hexafluoride, CFC-12, 3H, 3He, 4He, Ne), dissolved oxygen, light transmission, conductivity-temperature-depth profiles, and thermistor data, we investigate the implications associated with lake level fluctuations for deep-water renewal and oxygenation. Our data suggest that deep-water renewal was significantly reduced in Lake Van between 1990 and 2005. This change in mixing conditions resulted in the formation of a more than 100 m thick anoxic deep-water body below 325 m depth. Apparently, the freshwater inflows responsible for the lake level rise between 1988 and 1995 decreased the salinity of the surface water sufficiently that the generation of density plumes during winter cooling was substantially reduced compared to that in the years before the lake level rise. Significant renewal and oxygenation of the deep water did not occur until at least 2005, although by 2003 the lake level was back to almost the same level as in 1988. This study suggests that short-term changes in the hydrological regime, resulting in lake level changes of a couple of meters, can lead to significant and long-lasting changes in deep-water renewal and oxic conditions in deep saline lakes.