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

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.

Dynamics of wind-induced coastal upwelling and interbasin exchange in Lake Geneva during winter: Implications for deepwater renewal

Rafael Sebastian Reiss

Water quality in lakes is closely linked to hydrodynamics and often dominated by stratification, limiting the exchange between the epi- and hypolimnion. Thus, the vertical redistribution of biogeochemical tracers such as dissolved oxygen and nutrients by convective overturning during winter is an important process in annual lake cycles. In deep, monomictic lakes convective cooling often does not reach the deepest layers. Studies have shown that climate change will further reduce the efficiency of this process, motivating a good understanding of alternative deepwater renewal mechanisms.Combining field observations, three-dimensional (3D) hydrodynamic modeling, and particle tracking, this thesis explores wind-induced transport processes contributing to vertical exchange in Lake Geneva during winter. A first focus is on Ekman-type coastal upwelling at the northern shore. Then, wind-induced interbasin exchange and hypolimnetic upwelling between the Petit Lac (depth 75 m) and Grand Lac (depth 309 m) basins of Lake Geneva are investigated. Finally, the effect of stratification in the Petit Lac on the latter phenomenon is revealed by model-based momentum budget analysis.Coastal upwelling at the northern shore of Lake Geneva was investigated in the winter 2017/2018. Following a strong alongshore wind, nearshore temperatures decreased and remained low for 10 days, with the lowest temperatures corresponding to values found at 200-m depth. Current measurements at 30-m depth showed dominant alongshore currents with episodic upslope transport of cold hypolimnetic water in the lowest 10 m. Model results suggest that upwelled waters spread over ~10% of the Grand Lac's surface area. Furthermore, particle tracking confirmed that upwelled waters originated from far below the thermocline and descended back to these layers after the wind ceased.Exchange and upwelling between the lake's two basins was investigated in the winter 2018/2019. Following a strong along-axis wind, a two-layer flow established, with the downwind surface drift into the Grand Lac being balanced by opposing bottom currents into the Petit Lac and the current reversal occurring around the thermocline depth. Bottom temperatures at the confluence gradually decreased to values found in the deep Grand Lac hypolimnion. Furthermore, particle tracking revealed a current loop, whereby hypolimnetic water from below 150-m depth first upwelled into the Petit Lac, intruding ~10 km, i.e., half its length, and then descended back into the Grand Lac hypolimnion. Model results indicated a temporary doubling of the Petit Lac hypolimnetic volume during the upwelling.The observed interbasin upwelling occurred only during early winter, with the bottom inflow of deep hypolimnetic water driven by baroclinic pressure gradients at the confluence, produced by upwind upwelling at the western end of the Grand Lac. Once the Petit Lac was fully mixed, the wind-induced net volume exchange between the basins was reduced by > 50% and deep hypolimnetic interbasin upwelling was suppressed altogether.This thesis shows that wind-induced coastal upwelling and hypolimnetic interbasin exchange are complex 3D processes, regularly occurring during winter and providing important mechanisms for deepwater renewal in Lake Geneva. They cannot be represented by one-dimensional models frequently employed to predict climate change induced shifts and merit further attention when assessing deepwater renewal in large, deep lakes.

EPFL2021

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

David Andrew Barry, Andrea Cimatoribus, Ulrich Lemmin, Rafael Sebastian Reiss

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.

2019

Wind‐Induced Hypolimnetic Upwelling Between the Multi‐Depth Basins of Lake Geneva During Winter: An Overlooked Deepwater Renewal Mechanism?

David Andrew Barry, Ulrich Lemmin, Rafael Sebastian Reiss

Combining field observations, 3D hydrodynamic modeling, and particle tracking, we investigated wind-induced interbasin exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in early winter. Following a strong 2.5-day wind event, a two-layer flow field developed, where the downwind surface drift into the GL was balanced by counterflowing hypolimnetic currents into the PL. Velocities in both layers exceeded 20 cm/s, with the highest values (27 cm/s) found near the bottom. For 3.5 days, hypolimnetic temperatures at the confluence decreased to values found in the deep GL hypolimnion at 180-m depth. Approximately 1.5 days after the wind event ceased, currents reversed and upwelled waters returned into the deep GL hypolimnion. The Coriolis force strongly modified the interbasin exchange dynamics, which were well represented by the model. Particle tracking revealed a “current loop,” that is, water from below 150-m depth first upwelled into the PL, intruded approximately 10 km (half its length), and then descended back into the GL hypolimnion. Model results showed that the PL hypolimnetic volume doubled during the upwelling. Low model-based gradient Richardson numbers and temperature inversions in CTD profiles indicated turbulent mixing between the upwelled GL and surrounding PL waters. Our findings demonstrate that hypolimnetic upwelling between the two basins frequently occurs during winter and could potentially be an important, but as yet overlooked mechanism for hypolimnetic-epilimnetic exchange and deepwater renewal in Lake Geneva, and probably in other multi-depth basin lakes under similar wind conditions.