Deep-water coastal upwelling events during wintertime in a large lake (Lake Geneva)

Convective winter cooling is an important process in the annual cycle of a lake ecosystem, because it brings oxygen to the deeper layers. However, in deep lakes, convective processes are often not strong enough to extend the mixing down to the deepest layers. This may become even more problematic in the future considering climate change and potentially milder winters. In large deep lakes, two other processes originating in the coastal zone can significantly contribute to winter deep water renewal: cold water density currents on the lateral slopes caused by differential cooling between the shallow literal zone and the deep off-shore region, and upwelling of deep water masses over the lateral slopes which results from wind forcing and may be influenced by the Coriolis force. Previous field experiments in Lake Geneva, the largest freshwater lake in Western Europe (max. depth 310 m), have shown that cold-water density currents are a common phenomenon during wintertime, frequently reaching the thermocline at O(100m) depth. As part of a project investigating winter mixing dynamics in Lake Geneva, we examine wind-driven, Coriolis-influenced upwelling in order to better understand its role in the deep-water exchange during the weakly stratified period (December-March). The study is based on field observations taken on the northern shore of the lake, 20 km west of Lausanne in the winters of 2017-19. Near-bottom water temperatures are inferred from a fiber-optic based Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1m) and O(0.1°C), respectively. Temperature and current velocities in the water column are recorded by several ADCP and thermistor chain moorings. The data set is completed by regular CTD campaigns following wind events favoring upwelling. These field measurements were combined with a validated 3D hydrodynamic model (MITgcm) in order to analyze and quantify the exchange between the littoral zones and deeper parts of the lake. It was observed that following medium to strong northeast-bound wind events (i.e. winds parallel to the main axis of the central lake) lasting for at least one day, near-shore temperatures at all instruments suddenly dropped to the same temperatures as those in the deep open waters and remained low for several days. This indicates that, triggered by persistent wind-stress and under the influence of the Coriolis force, hypolimnetic water reached the surface in the near shore zone, where it is subject to heat and gas exchange with the atmosphere, before returning to the deep layers. Numerical modeling of these events showed comparable results and allowed to trace the origin of this cold water to the deep layers. Our field observations and the numerical results suggest that in addition to the previously observed cold-water density currents, wind-driven upwelling is a common phenomenon in Lake Geneva during the weakly stratified winter period and potentially a significant mechanism for deep-water renewal.

Wind-driven interbasin exchange and hypolimnetic upwelling during wintertime in a large, deep lake (Lake Geneva)

David Andrew Barry, Ulrich Lemmin, Rafael Sebastian Reiss

Distinct sub-basins and large embayments are a ubiquitous feature of many lakes. Horizontal gradients in water quality between basins can result from a number of processes. For example, different seasonal mixing regimes between basins with different maximum depths can produce biochemical gradients between their hypolimnia. Consequently, interbasin exchange can be an important process with significant ecological consequences. Combining field observations, 3D hydrodynamic modeling, and model-based Lagrangian particle tracking, we investigated wind-driven interbasin exchange between the shallow Petit Lac (max. depth 75 m) and deep Grand Lac (max. depth 309 m) basins of Lake Geneva, Western Europe’s largest lake, during early winter. In addition to CTD casts conducted in the Petit Lac, several ADCP and thermistor chain moorings were deployed at the confluence between the two basins during the winter 2018/2019. Following a strong northeast-bound, along-axis wind event lasting from 7 to 10 December 2018, a two-layer flow pattern established at the confluence: epilimnetic water from the Petit Lac was pushed by the wind into the Grand Lac and was compensated for by a bottom inflow of deep hypolimnetic waters from the Grand Lac into the Petit Lac. Consequently, temperatures in the lower part of the water column gradually decreased at all moorings, with the lowest temperatures corresponding to values found at 180 m depth, as indicated by full-depth temperature profiles taken in November and December 2018. For approximately 3.5 days, deep Grand Lac water was continuously transported into the Petit Lac, with observed inflowing current velocities near the bottom exceeding 27 cm/s. Approximately 1.5 d after the wind subsided, the current patterns at the confluence reversed and the previously upwelled Grand Lac water was drained again from the Petit Lac in a bottom-hugging current with measured velocities reaching 19 cm/s. The current and temperature patterns at the confluence were well represented by a 3D hydrodynamic model (MITgcm). Model-based particle tracking confirmed the deep origin of the upwelled Grand Lac waters. Furthermore, it revealed that the interbasin upwelling event effectively formed a current loop, during which, over the course of more than one week, hypolimnetic water from below 150 m depth first upwelled into the Petit Lac, intruding approximately 10 km into the shallow basin, and subsequently descended back into the Grand Lac hypolimnion. Moreover, low model-based gradient Richardson numbers and temperature inversions observed in the CTD profiles indicate turbulent mixing between the deep, upwelled Grand Lac waters and the “fresher,” i.e., better quality Petit Lac waters. Our field observations and modeling results show that enhanced wind-driven interbasin exchange and deep hypolimnetic upwelling between the shallow Petit Lac and deep Grand Lac basins of Lake Geneva frequently occur during early winter. Furthermore, our results suggest that these hypolimnetic interbasin upwelling events may present a potentially important mechanism for hypolimnetic-epilimnetic exchange and deep-water renewal in Lake Geneva and possibly in other deep multi-basin lakes under similar wind conditions; especially, when considering the expected weakening of the classical deep convective cooling during wintertime due to climate change effects.

2021

Detailed hydrodynamics of cold density currents generated by differential cooling along the sloping bed of Lake Geneva

David Andrew Barry, Ulrich Lemmin, François Mettra, Rafael Sebastian Reiss

Deep water ventilation and mixing are of great importance for large lake ecosystems. Relevant processes include cold density currents due to differential cooling, inter-basin exchange and upwelling. At present, a good understanding of these processes in large lakes under realistic external forcing is still missing, which prevents, for example, to assess their relative contribution to and their evolution with climate change. Cold density currents bring cold surface water with oxygen and potential pollutants from coastal regions to the deep layers of the lake along the sloping boundaries. Earlier hydrodynamic studies allowed determination of the general behavior of those currents in Lake Geneva (Fer et al., 2001, 2002). With the recent advances in instrument capabilities, we were able to collect more detailed field data in the bottom boundary layer. In this study, we present preliminary results on the hydrodynamics of cold density currents generated by differential cooling using a unique high spatial and temporal resolution dataset. Acoustic Doppler Current Profilers (ADCPs) and vertical thermistor lines were deployed during the winter season on the northern shore of Lake Geneva on the shallow shelf and along the sloping lakebed. In addition, CTD (Conductivity- Temperature-Depth) profiles were taken during periods of strong cooling in order to obtain a broader view of the temperature field along a cross-shore transect. After a cold, calm night, strong differential cooling forms between the shallow shelf and the open lake, initiating cold dense water from the shelf that flows as pulses along the sloping lakebed. Analysis of the velocity profile shows that the maximum velocity is relatively close to the bed, as would be expected for a density current. However, the height above the lakebed of this maximum velocity fluctuates depending on the stage of the flow (accelerating or decelerating). During the accelerating stage, i.e., at the beginning of a new pulse, the flow is thick enough to elucidate details of the velocity profiles close to the boundary – the measured profiles are logarithmic, as expected for a boundary flow (e.g., Kneller et al., 1999). Moreover, this new dataset enables us to determine the stability of the flow and the entrainment of the ambient water into the density current.

2021

Evolution of stream and lake water temperature under climate change

Damien Bouffard, Wolf Hendrik Huwald, Adrien Michel, Carl Love Mikael Råman Vinnå, Bettina Schaefli, Martin Schmid, Alfred Johny Wüest

This report presents past observations and projects the future development of water temperature in Swiss lakes and rivers. Projections are made until the end of the 21st century using the CH2018 climate scenarios. Besides climate change effects on temperature, we also discuss effects on discharge for rivers, and effects on the thermal structure, and specifically the seasonal mixing regime and ice cover of lakes. Observations over the past 40 years show a clear increase in river temperatures, with a mean trend of 0.33 ± 0.03 °C per decade, corresponding to ~80% of the observed air temperature trend. This warming has been continuous over the last four decades and impacts the health of stream ecosystems (e.g. by favouring the spread of fish diseases) and their services (e.g., the water usage for industrial cooling). The temperature rise is more pronounced in the Swiss Plateau than in the Alps, where snow and glacier melt partially mitigates (for now) the effects of increasing air temperature. Conversely, annual average discharge shows no significant trend. Similar trends have also been reported for Swiss lakes with mean summer lake surface temperature increasing by 0.40 ± 0.08 °C per decade since the 1950s. This warming trend affects lake stratification. Warm periods may for instance increase the occurrence of deep-water anoxic conditions, as observed during the 2003 heat wave in Lake Zurich. In mild winters, ice cover duration is reduced in alpine lakes, and winter deep mixing is less intense in large-peri-alpine lakes. The mild winter 2006/7 limited, for instance, the seasonal mixing of Lake Constance to about 60 m depths. Effects of warming on lake thermal structure vary within and between regions, due to both lake and watershed characteristics as well as regional climate change patterns. We simulated the future evolution of stream temperature for 10 catchments in Switzerland for a historical reference period (1990–2000) and two future periods: 2055–2065 (mid-century) and 2080–2090 (end of the century). Results show that the temperature will stabilize by the end of the century for the RCP2.6 scenario (strong CO2 emission reduction), whereas the warming will accelerate with time for the RCP8.5 scenario (business as usual scenario). This expected warming will have significant impacts on the stream ecosystems. Alpine and lowland catchments will experience a similar annual mean temperature increase but display different seasonal effects. While Swiss Plateau rivers will become warmer both in winter and summer (but more in summer), alpine rivers will experience almost no warming in winter but a strong warming exceeding that of air temperature in summer. This is explained by an abrupt decrease in discharge, and by the soil warming resulting from the absence of snow and thus a lower albedo. Lake temperature projections are based on one-dimensional, vertically resolved, hydrodynamic simulations for 29 lakes. The simulated lakes cover a wide range of sizes, depths and water quality, and an altitude range from 200 to 1800 m a.s.l. Simulations indicate substantial changes in lake thermal structure for RCP8.5 with surface temperatures increasing on average by 3.3 °C at the end of the 21st century. This increase is limited to 0.9 °C in the mitigation scenario RCP2.6. We identified an altitude-dependent evolution of the durations of summer and winter stratification as well the ice-covered period. Larger changes in stratification duration are expected to occur at higher altitude lakes. Yet, these lakes will still maintain winter stratification and a shortened icecovered period while lower altitude lakes (below ~1500 m a.s.l.) risk drastic changes in the mixing regime. e.g., a complete loss of the ice cover and winter stratification under the RCP8.5 scenarios. Such changes in the mixing regime may strongly impact lake ecosystems. These low to mid altitude lakes may therefore be considered as the most vulnerable to climate change.

2021

Wind-driven interbasin exchange and hypolimnetic upwelling during wintertime in a large, deep lake (Lake Geneva)

David Andrew Barry, Ulrich Lemmin, Rafael Sebastian Reiss

2021

Evolution of stream and lake water temperature under climate change

Damien Bouffard, Wolf Hendrik Huwald, Adrien Michel, Carl Love Mikael Råman Vinnå, Bettina Schaefli, Martin Schmid, Alfred Johny Wüest

2021

Detailed hydrodynamics of cold density currents generated by differential cooling along the sloping bed of Lake Geneva

David Andrew Barry, Ulrich Lemmin, François Mettra, Rafael Sebastian Reiss

2021