Wintertime Coastal Upwelling in Lake Geneva: An Efficient Transport Process for Deepwater Renewal in a Large, Deep Lake

Abstract

Combining field measurements, 3‐D numerical modeling, and Lagrangian particle tracking, we investigated wind‐driven, Ekman‐type coastal upwelling during the weakly stratified winter period 2017/2018 in Lake Geneva, Western Europe's largest lake (max. depth 309 m). Strong alongshore wind stress, persistent for more than 7 days, led to tilting and surfacing of the thermocline (initial depth 75–100 m). Observed nearshore temperatures dropped by 1°C and remained low for 10 days, with the lowest temperatures corresponding to those of hypolimnetic waters originating from 200 m depth. Nearshore current measurements at 30 m depth revealed dominant alongshore currents in the entire water column (maximum current speed 25 cm s−1) with episodic upslope transport of cold hypolimnetic waters in the lowest 10 m mainly during the first 3 days. The observed upwelling dynamics were well reproduced by a 3‐D hydrodynamic model (RMSE 0.2°C), whose results indicated that upwelled waters spread over approximately 10% of the lake's main basin surface area. Model‐based Lagrangian particle tracking confirmed that upwelled waters originated from far below the thermocline, that is, >150 m depth, and descended back to around 150–200 m depth over a wide area after wind stress ceased. Observational and particle tracking results suggest that wintertime coastal upwelling, which can occur several times during winter, is an overlooked transport process that is less sensitive to the effects of global warming than convective cooling. It can provide an effective but complex 3‐D pathway for deepwater renewal in Lake Geneva, and other large, deep lakes with a sufficiently long wind fetch.

Rapid changes in river plume dynamics caused by advected wind‐driven coastal upwelling as observed in Lake Geneva

“…It has also been successfully applied to lakes (Dorostkar et al 2017). The capability of the model to realistically reproduce the stratification, mean flow, internal seiche variability, and upwelling in Lake Geneva was assessed and confirmed by Cimatoribus et al (2018Cimatoribus et al ( , 2019 and Reiss et al (2020) after careful calibration of the model. The model integrates the 3D Reynoldsaveraged Navier-Stokes equations on a sphere under the Boussinesq and both the hydrostatic and nonhydrostatic approximations (Marshall et al 1997).…”

“…In the near shore zone of large lakes, wind‐driven coastal upwelling is an important transport process that occurs due to offshore Ekman transport. It brings deep cold and dense water masses towards the upper layer of the nearshore zone by displacing the thermocline upwards, thus replacing the warmer water in that layer (Plattner et al 2006; Oesch et al 2008; Reiss et al 2020). During “full” upwelling, the thermocline reaches the lake surface (Csanady 1977).…”

“…During "full" upwelling, the thermocline reaches the lake surface (Csanady 1977). Coastal upwelling generates strong alongshore currents in the nearshore zone (Reiss et al 2020). A key feature of coastal upwelling is that, after strong wind events cease, the region of coastal upwelling progresses counterclockwise around the lake basin as a thermal front at a speed comparable to that of an internal Kelvin wave (Mortimer 1974(Mortimer , 1988Csanady 1977;Simons and Schertzer 1987).…”

Primary and Net Ecosystem Production in a Large Lake Diagnosed From High‐Resolution Oxygen Measurements

“…Due to its large size, the currents are affected by Coriolis force and the most energetic modes of internal motions are Poincaré waves in open waters (Lemmin, 2020; Lemmin et al., 2005) and Kelvin waves in nearshore regions (Bouffard & Lemmin, 2013), with periods of ∼16 h and ∼4–8 days, respectively. The nearshore areas are also affected by coastal upwellings, which can persist for several days (Reiss et al., 2020).…”

Deep-mixing and deep-cooling events in Lake Garda: Simulation and mechanisms

“…Assessing the occurrence of DMEs based on temperature stratification alone may lead to wrong conclusions in some lakes, because other processes (than surface cooling) affect DMEs (Michalski and Lemmin, 1995). Such processes include density currents due to river inflows and/or differential cooling between littoral and pelagic areas (Meybeck et al, 1991;Peeters et al, 2003;Laborde et al, 2010;Ambrosetti et al, 2010), the contribution of spring snowmelt and duration of ice cover (Flaim et al, 2019;Sadro et al, 2019), storm events inducing thermocline tilting and seiches (Imboden et al, 1987), as well as wind-driven secondary flows inducing up and downwellings at the lake boundaries (Piccolroaz et al, 2019;Reiss et al, 2020). Given all these contributing processes, it is not surprising that there are also lakes where an increase of DMEs has been observed in the last decades.…”