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 which takes place in their center. At present, dynamics of pelagic upwelling in the Surface Mixed Layer (SML) of oceans and 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.

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

“…All data used in the paper are available from https://zenodo. org/record/7941268#.ZGOJtaVBxPY (Hamze-Ziabari et al, 2023).…”

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

“…Originally developed for oceanography, the code has also been successfully applied to lakes (Dorostkar and Boegman, 2013;Djoumna et al, 2014;Dorostkar et al, 2017). Prior to the present study, the model was calibrated for Lake Geneva and accurately reproduced seasonal thermal stratification, mean flow field and internal seiches (Cimatoribus et al, 2019), nearshore currents, coastal upwelling, inter-basin exchange and river plume dynamics (Cimatoribus et al, 2018;Cimatoribus et al, 2019;Reiss et al, 2020;Soulignac et al, 2021;Reiss et al, 2022), basin-and mesoscale rotational flows, as well as submesoscale currents (Foroughan et al, 2022;Hamze-Ziabari et al, 2022a;Hamze-Ziabari et al, 2022b;Hamze-Ziabari et al, 2022c). Here, using the model setup validated by Cimatoribus et al (2018), the model was run in hydrostatic mode with an implicit free surface.…”

“…Submesoscale filaments, which are typically observed as elongated patches at different temperatures/densities within mesoscale rotational motions (e.g., gyres and eddies; Hamze-Ziabari et al, 2022a), are the focus of this study. Submesoscale filaments are formed as a result of the rapid sharpening of a density gradient by horizontal deformation flows, a process known as filamentary intensification (McWilliams et al, 2009) or filamentogenesis (Gula et al, 2014).…”

“…Recent studies using high-resolution satellite imagery, numerical models and field observations revealed that the SML dynamics in lakes can also be characterized by submesoscale filaments and fronts (Foroughan et al, 2022;Hamze-Ziabari et al, 2022b). Hamze-Ziabari et al (2022a) reported that submesoscale filaments are formed in Lake Geneva, the largest freshwater lake in Western Europe, during the strongly stratified summer season, and that they are likely ubiquitous features of quasi-geostrophic gyres in large lakes. Foroughan et al (2022) also employed a combination of field observations, numerical modeling, and satellite images to document a persistent submesoscale frontal slick in Lake Geneva.…”

“…Foroughan et al (2022) also employed a combination of field observations, numerical modeling, and satellite images to document a persistent submesoscale frontal slick in Lake Geneva. 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.…”