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Unraveling submesoscale processes associated with meso- and basin-scale gyres in Lake Geneva

Seyed Mahmood Hamze Ziabari

In large lakes, basin-scale gyres and submesoscale eddies, i.e., rotating coherent water masses, play a key role in spreading biochemical materials and energy throughout the basin, thereby significantly impacting water quality. Due to their transient and spatially heterogeneous nature, detailed field measurements of these flow features are challenging and thus scarce. However, such measurements are crucial to investigate the spatiotemporal extent and dynamics of basin-scale gyres and submesoscale eddies, assess ecological implications, and validate numerical simulation results.Combining extensive field observations, three-dimensional (3D) numerical simulations, remote sensing data and statistical analyses, this thesis explores the dynamics of basin-scales gyres and submesoscale eddies in Lake Geneva, unraveling the implications for water quality heterogeneity and existing lake monitoring strategies. First, a novel framework is developed that permits study the spatiotemporal characteristics of gyres/eddies during different seasons by allowing efficient design of field sampling strategies with unprecedented precision in time and space. Building on this novel framework, the existence of submesoscale filaments caused by large-scale gyres is demonstrated for the first time in a lake. Then, the dynamics of pelagic upwelling within a cyclonic gyre in Lake Geneva are investigated. Finally, the spatial heterogeneity of water quality parameters caused by (sub)mesoscale flow features is revealed.Submesoscale filaments, well-documented in the ocean but poorly understood in lakes, are spatially-elongated bands often observed within large-scale gyres/eddies. Submesoscale currents have horizontal scales of 0.1â10 km, lifetimes of hours to days, and are characterized by Rossby and Richardson numbers both of O(1). Measurements in Lake Geneva demonstrate that submesoscale, cold-water filaments are formed at the edges and in the center of cyclonic large-scale gyres above the thermocline during summertime, confirming 3D numerical modeling results and remote sensing data. Field observations in Lake Geneva revealed that surprisingly intense submesoscale upwelling from the thermocline to the Surface Mixed Layer (SML) and even the lake surface occurred as chimney-like structures of cold water within the SML, as confirmed by Advanced Very High-Resolution Radiometer data. The potential impact of such pelagic upwelling on a long-term 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 mesoscale circulations in determining the spatial heterogeneity of water quality parameters remains poorly understood in large lakes. This thesis provides unparalleled evidence of dissolved oxygen (DO) variability caused by cyclonic gyres (CGs), anticyclonic gyres (AGs), submesoscale eddies, and filaments in both weak (fall) and strong (summer) thermal stratification conditions. In the presence of CGs, AGs, and submesoscale flows, significant lateral variations in DO, mixed layer depth, and metalimnion depth were observed. Such spatial variability of water quality parameters induced by (sub)mesoscale circulations challenges classical one-dimensional (1D) long-term monitoring programs for quantifying physical and biological processes in large lakes.

EPFL2023

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

David Andrew Barry, Mehrshad Foroughan, Seyed Mahmood Hamze Ziabari, Ulrich Lemmin, Frédéric Charles Soulignac

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.

2022

Detecting submesoscale cold filaments in a basin-scale gyre in large, deep Lake Geneva (Switzerland/France)

David Andrew Barry, Seyed Mahmood Hamze Ziabari, Ulrich Lemmin, Amir Mehdi Razmi

Submesoscale filaments are a well-documented feature of oceanic flow fields. Based on the analysis of high-resolution 3D numerical simulations, field observations and remote sensing imagery, this study, carried out in Lake Geneva, Western Europe's largest lake, provides for the first time evidence that submesoscale filaments also exist in a lake. Field observations confirm that submesoscale, cold-water filaments are formed at the edges and in the center of a counterclockwise rotating large-scale gyre during summertime above the thermocline, as indicated by 3D numerical simulation results and remote sensing. Cold submesoscale filaments were well-developed with sharp lateral temperature gradients (~1-2°C) during summertime. Locally, these filaments can significantly increase the depth of the upper mixed layer when a relatively strong thermocline exists in the near-surface layers and cause upward and downward vertical velocities comparable to those reported for oceanic filaments.

2022