Chimney‐Like Intense Pelagic Upwelling in the Center of Basin‐Scale Cyclonic Gyres in Large Lake Geneva

Abstract

Basin‐scale quasi‐geostrophic gyres are common features of large lakes subject to Coriolis force. Cyclonic gyres are often characterized by dome‐shaped thermoclines that form due to pelagic upwelling that takes place in their center. At present, the dynamics of pelagic upwelling within the surface mixed layer (SML) of large lakes are poorly documented. A unique combination of high‐resolution 3D numerical modeling, satellite imagery and field observations allowed confirming, for the first time in a lake, the existence of intense pelagic upwelling in the center of cyclonic gyres under strong shallow (summer) and weak deep (winter) stratified conditions/thermocline. Field observations in Lake Geneva revealed that surprisingly intense upwelling from the thermocline to the SML and even to the lake surface occurred as chimney‐like structures of cold water within the SML, as confirmed by Advanced Very High‐Resolution Radiometer data. Results of a calibrated 3D numerical model suggest that the classical Ekman pumping mechanism cannot explain such pelagic upwelling. Analysis of the contribution of various terms in the vertically averaged momentum equation showed that the nonlinear (advective) term dominates, resulting in heterogeneous divergent flows within cyclonic gyres. The combination of nonlinear heterogeneous divergent flow and 3D ageostrophic strain caused by gyre distortion is responsible for the chimney‐like upwelling in the SML. The potential impact of such pelagic upwelling on long‐term observations at a measurement station in the center of Lake Geneva suggests that caution should be exercised when relying on limited (in space and/or time) profile measurements for monitoring and quantifying processes in large lakes.

The role of submesoscale filaments in restratification of the surface mixed layer and dissolved oxygen variability in large Lake Geneva: field evidence complemented by Lagrangian particle-tracking

“…In addition, the effect of upwelling on BI induced by cyclonic circulation is evident in the near-surface layer of CC in the southern part of the lake (Figure 8A). Due to divergent flows in the near-surface layers and convergent flows in the deeper layers, water from deeper layers can be brought up into the mixed layer in the center of the CC (Hamze-Ziabari et al, 2023). The turbidity of upwelled water from greater depths is expected to be lower than that of near-surface waters.…”

“…The exchange of heat, mass and oxygen between the lake and the atmosphere can be greatly facilitated by high vertical velocities [O(100 m d -1 )] produced by submesoscale dynamics (Mahadevan, 2016;Levy et al, 2018). In the pelagic zone of lakes, upwelling and downwelling associated with basin-scale or mesoscale circulations are considered to be the dominant physical mechanisms for the vertical transport of tracers (Hamze-Ziabari et al, 2023). However, it appears that vertical transport by submesoscale processes has previously been overlooked as an alternative.…”

“…Field evidence in Lake Geneva suggested that submesoscale filaments/fronts can potentially impact on the mixing of the water column, when the thermocline is in close proximity to the surface layer (Hamze-Ziabari et al, 2022a). Furthermore, a procedure was developed by Hamze-Ziabari et al (2022c); Hamze-Ziabari et al (2023) to detect mesoscale and submesoscale gyres/ eddies and their associated pelagic upwelling/downwelling in Lake Geneva. The procedure can be applied to predict when and where regularly-occurring gyre patterns and associated submesoscale filaments are expected to form and thus allows capturing them in field measurements.…”