Basin-scale gyres and mesoscale eddies in large lakes: a novel procedure for their detection and characterization, assessed in Lake Geneva

Abstract

Abstract. In large lakes subject to the Coriolis force, basin-scale gyres and mesoscale eddies, i.e. rotating coherent water masses, play a key role in spreading biochemical materials and energy throughout the lake. In order to assess the spatial and temporal extent of gyres and eddies, their dynamics and vertical structure, as well as to validate their prediction in numerical simulation results, detailed transect field observations are needed. However, at present it is difficult to forecast when and where such transect field observations should be taken. To overcome this problem, a novel procedure combining 3D numerical simulations, statistical analyses, and remote sensing data was developed that permits determination of the spatial and temporal patterns of basin-scale gyres during different seasons. The proposed gyre identification procedure consists of four steps: (i) data pre-processing, (ii) extracting dominant patterns using empirical orthogonal function (EOF) analysis of Okubo–Weiss parameter fields, (iii) defining the 3D structure of the gyre, and (iv) finding the correlation between the dominant gyre pattern and environmental forcing. The efficiency and robustness of the proposed procedure was validated in Lake Geneva. For the first time in a lake, detailed field evidence of the existence of basin-scale gyres and (sub)mesoscale eddies was provided by data collected along transects whose locations were predetermined by the proposed procedure. The close correspondence between field observations and detailed numerical results further confirmed the validity of the model for capturing large-scale current circulations as well as (sub)mesoscale eddies. The results also indicated that the horizontal gyre motion is mainly determined by wind stress, whereas the vertical current structure, which is influenced by the gyre flow field, primarily depends on thermocline depth and strength. The procedure can be applied to other large lakes and can be extended to the interaction of biological–chemical–physical processes.

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

“…This study focuses on Bise wind events that caused the formation of a cyclonic gyre and pelagic upwelling in the central region of the Grand Lac basin of Lake Geneva. This gyre is part of a three‐gyre pattern that was previously identified as the typical response to strong Bise winds blowing over lake (Hamze‐Ziabari, Lemmin, et al., 2022; Figure 1c). Pelagic upwelling is important for the ecological development of the lake, since it transports nutrients from the deeper layers into the phototrophic zone.…”

“…(2022), Hamze‐Ziabri, Fouroughan, et al., 2022), Hamze‐Ziabari, Lemmin, et al. (2022), Hamze‐Ziabri, Razmi, et al. (2022), and Reiss et al.…”

“…These strong uniform wind events can generate complex gyral flow pattern systems (Figures 1c and 1e). The surface currents in the lake's center are either cyclonic (counterclockwise rotating) after a Bise wind or anticyclonic (clockwise rotating) after a Vent event (Hamze‐Ziabari, Lemmin, et al., 2022; Lemmin, 2016). In order to monitor the long‐term water quality development of the lake, the Commission Internationale pour la Protection des Eaux du Léman (CIPEL) has measured physical and biological parameters regularly since 1967 at station SHL2 (Figure 1a).…”

“…It was previously shown that gyre patterns in Lake Geneva can be reliably predicted by numerical modeling based on forecasted COSMO meteorological data (Hamze‐Ziabari, Lemmin, et al., 2022). Guided by these numerical model forecasts, one or more transects for field measurements were selected within the main cyclonic gyre in the central part of the Grand Lac basin.…”

Deepwater Renewal in a Large, Deep Lake (Lake Geneva): Identifying and Quantifying Winter Cooling Processes Using Heat Budget Decomposition

“…(2019) showed that wind‐induced coastal downwelling can also transport coastal water downslope to ∼200‐m depth, thus contributing to the renewal of intermediate hypolimnion layers. Ever present basin‐scale gyres and mesoscale eddies (Hamze‐Ziabari, Lemmin, et al., 2022) spread these advected water masses in the intermediate hypolimnion layers throughout the basin. However, these processes do not reach the deepest layer (∼300‐m depth) of the Grand Lac .…”