Showing papers by "Alfred Wüest published in 2015"

Characterisation of the subaquatic groundwater discharge that maintains the permanent stratification within Lake Kivu; East Africa.

Abstract: Warm and cold subaquatic groundwater discharge into Lake Kivu forms the large-scale density gradients presently observed in the lake. This structure is pertinent to maintaining the stratification that locks the high volume of gases in the deepwater. Our research presents the first characterisation of these inflows. Temperature and conductivity profiling was conducted from January 2010 to March 2013 to map the locations of groundwater discharge. Water samples were obtained within the lake at the locations of the greatest temperature anomalies observed from the background lake-profile. The isotopic and chemical signatures of the groundwater were applied to assess how these inflows contribute to the overall stratification. It is inferred that Lake Kivu’s deepwater has not been completely recharged by the groundwater inflows since its turnover that is speculated to have occurred within the last ~1000 yrs. Given a recent salinity increase in the lake constrained to within months of seismic activity measured beneath the basin, it is plausible that increased hydrothermal-groundwater inflows into the deep basin are correlated with episodic geologic events. These results invalidate the simple two-component end-member mixing regime that has been postulated up to now, and indicate the importance of monitoring this potentially explosive lake.

Drivers of deep-water renewal events observed over 13 years in the South Basin of Lake Baikal

Abstract: Lake Baikal, with a depth of 1637 m, is characterized by deep-water intrusions that bridge the near-surface layer to the hypolimnion. These episodic events transfer heat and oxygen over large vertical scales and maintain the permanent temperature stratified deep-water status of the lake. Here we evaluate a series of intrusion events that reached the bottom of the lake in terms of the stratification and the wind conditions under which they occurred and provide a new insight into the triggering mechanisms. We make use of long-term temperature and current meter data (2000-2013) recorded in the South Basin of the lake combined with wind data produced with a regional downscaling of the global NCEP-RA1 reanalysis product. A total of 13 events were observed during which near-surface cold water reached the bottom of the South Basin at 1350 m depth. We found that the triggering mechanism of the events is related to the time of the year that they take place. We categorized the events in three groups: (1) winter events, observed shortly before the complete ice cover of the lake that are triggered by Ekman coastal downwelling, (2) under-ice events, and (3) spring events, that show no correlation to the wind conditions and are possibly connected to the increased spring outflow of the Selenga River.

Minimal model for double diffusion and its application to Kivu, Nyos, and Powell Lake

Abstract: Double diffusion originates from the markedly different molecular diffusion rates of heat and salt in water, producing staircase structures under favorable conditions. The phenomenon essentially consists of two processes: molecular diffusion across sharp interfaces and convective transport in the gravitationally unstable layers. In this paper, we propose a model that is based on the one-dimensional description of these two processes only, and—by self-organization—is able to reproduce both the large-scale dynamics and the structure of individual layers, while accounting for different boundary conditions. Two parameters characterize the model, describing the time scale for the formation of unstable water parcels and the optimal spatial resolution. Theoretical relationships allow for the identification of the influence of these parameters on the layer structure and on the mass and heat fluxes. The performances of the model are tested for three different lakes (Powell, Kivu, and Nyos), showing a remarkable agreement with actual microstructure measurements.

Impact of deepwater mixing on hypoxia in Lake Geneva

Abstract: Lake Geneva is recovering from its eutrophic past but no significant trend in the areal hypolimnetic oxygen depletion rate has been observed over the last 30 years. Due to the large depth of 309 m, the lake is not mixed completely every winter which can lead to severe hypoxic condition in the deeper layer. An analysis of field data measured between 1970-2012 shows, that the severity of hypoxia is strongly related to mixing depth and in turn on the mean Schmidt-Stability during winter. We used the one dimensional k-eps model Simstrat (Goudsmith, 2002) to predict the change in deep-water mixing with increasing temperature. To improve the simulation of deep-water mixing, a new method of calculating the amount of wind energy transferred into internal waves is presented. The model was validated with the period 1981-2013 and different scenarios from the predicted temperature change from the Swiss Climate Change Scenario CH2011 (2011) were used to simulate the period 2040-2085. The results show a significant decrease in deep-water mixing with increasing temperature and also a strong impact on the lake’s oxygen budget. References CH2011 (2011), Swiss Climate Change Scenarios CH2011, 88 pp., C2SM, Zurich, Switzerland, ISBN: 978-3-033–03065-7. Goudsmith (2002) et al, Application of k-ϵ turbulence models to enclosed basins: The role of internal seiches, Journal of Geophysical Research 107(C12) 3230.