Remontée d'eau

Résumé

Une remontée d'eau, en anglais , est un phénomène océanographique qui se produit lorsque de forts vents marins (généralement des vents saisonniers) poussent l'eau de surface des océans, laissant ainsi un vide où peuvent remonter les eaux de fond et avec elles une quantité importante de nutriments. Les phénomènes de remontée d'eau se localisent par leurs résultats : une mer froide et riche en phytoplancton. Concrètement, pour les pêcheurs, la remontée d'eau se traduit par une augmentation importante du nombre de poissons. Le phénomène inverse est la plongée d'eau. Océanographie physique Il existe au moins cinq types de remontées d'eau dont le plus connu est le type côtier (voir images):

Si des vents soufflent de la côte vers la mer, ils entraînent avec eux l'eau qui se trouve en surface. S'ils soufflent le long de la côte et que celle-ci est à leur droite dans l'hémisphère sud (ou à leur gauche dans l'hémisphère nord), la force de Coriolis va faire tourner les courant

Source officielle

https://fr.wikipedia.org/wiki/Remont%C3%A9e_d'eau

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Dynamics of wind-induced coastal upwelling and interbasin exchange in Lake Geneva during winter: Implications for deepwater renewal

Rafael Sebastian Reiss

Water quality in lakes is closely linked to hydrodynamics and often dominated by stratification, limiting the exchange between the epi- and hypolimnion. Thus, the vertical redistribution of biogeochemical tracers such as dissolved oxygen and nutrients by convective overturning during winter is an important process in annual lake cycles. In deep, monomictic lakes convective cooling often does not reach the deepest layers. Studies have shown that climate change will further reduce the efficiency of this process, motivating a good understanding of alternative deepwater renewal mechanisms.Combining field observations, three-dimensional (3D) hydrodynamic modeling, and particle tracking, this thesis explores wind-induced transport processes contributing to vertical exchange in Lake Geneva during winter. A first focus is on Ekman-type coastal upwelling at the northern shore. Then, wind-induced interbasin exchange and hypolimnetic upwelling between the Petit Lac (depth 75 m) and Grand Lac (depth 309 m) basins of Lake Geneva are investigated. Finally, the effect of stratification in the Petit Lac on the latter phenomenon is revealed by model-based momentum budget analysis.Coastal upwelling at the northern shore of Lake Geneva was investigated in the winter 2017/2018. Following a strong alongshore wind, nearshore temperatures decreased and remained low for 10 days, with the lowest temperatures corresponding to values found at 200-m depth. Current measurements at 30-m depth showed dominant alongshore currents with episodic upslope transport of cold hypolimnetic water in the lowest 10 m. Model results suggest that upwelled waters spread over ~10% of the Grand Lac's surface area. Furthermore, particle tracking confirmed that upwelled waters originated from far below the thermocline and descended back to these layers after the wind ceased.Exchange and upwelling between the lake's two basins was investigated in the winter 2018/2019. Following a strong along-axis wind, a two-layer flow established, with the downwind surface drift into the Grand Lac being balanced by opposing bottom currents into the Petit Lac and the current reversal occurring around the thermocline depth. Bottom temperatures at the confluence gradually decreased to values found in the deep Grand Lac hypolimnion. Furthermore, particle tracking revealed a current loop, whereby hypolimnetic water from below 150-m depth first upwelled into the Petit Lac, intruding ~10 km, i.e., half its length, and then descended back into the Grand Lac hypolimnion. Model results indicated a temporary doubling of the Petit Lac hypolimnetic volume during the upwelling.The observed interbasin upwelling occurred only during early winter, with the bottom inflow of deep hypolimnetic water driven by baroclinic pressure gradients at the confluence, produced by upwind upwelling at the western end of the Grand Lac. Once the Petit Lac was fully mixed, the wind-induced net volume exchange between the basins was reduced by > 50% and deep hypolimnetic interbasin upwelling was suppressed altogether.This thesis shows that wind-induced coastal upwelling and hypolimnetic interbasin exchange are complex 3D processes, regularly occurring during winter and providing important mechanisms for deepwater renewal in Lake Geneva. They cannot be represented by one-dimensional models frequently employed to predict climate change induced shifts and merit further attention when assessing deepwater renewal in large, deep lakes.

EPFL2021

Chargement

Wind‐Induced Hypolimnetic Upwelling Between the Multi‐Depth Basins of Lake Geneva During Winter: An Overlooked Deepwater Renewal Mechanism?

David Andrew Barry, Ulrich Lemmin, Rafael Sebastian Reiss

Combining field observations, 3D hydrodynamic modeling, and particle tracking, we investigated wind-induced interbasin exchange between the Petit Lac (PL) (depth 75 m) and Grand Lac (GL) (depth 309 m) basins of Lake Geneva in early winter. Following a strong 2.5-day wind event, a two-layer flow field developed, where the downwind surface drift into the GL was balanced by counterflowing hypolimnetic currents into the PL. Velocities in both layers exceeded 20 cm/s, with the highest values (27 cm/s) found near the bottom. For 3.5 days, hypolimnetic temperatures at the confluence decreased to values found in the deep GL hypolimnion at 180-m depth. Approximately 1.5 days after the wind event ceased, currents reversed and upwelled waters returned into the deep GL hypolimnion. The Coriolis force strongly modified the interbasin exchange dynamics, which were well represented by the model. Particle tracking revealed a “current loop,” that is, water from below 150-m depth first upwelled into the PL, intruded approximately 10 km (half its length), and then descended back into the GL hypolimnion. Model results showed that the PL hypolimnetic volume doubled during the upwelling. Low model-based gradient Richardson numbers and temperature inversions in CTD profiles indicated turbulent mixing between the upwelled GL and surrounding PL waters. Our findings demonstrate that hypolimnetic upwelling between the two basins frequently occurs during winter and could potentially be an important, but as yet overlooked mechanism for hypolimnetic-epilimnetic exchange and deepwater renewal in Lake Geneva, and probably in other multi-depth basin lakes under similar wind conditions.

2022

Chargement

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Convective winter cooling is an important process in the annual cycle of a lake ecosystem, because it brings oxygen to the deeper layers. However, in deep lakes, convective processes are often not strong enough to extend the mixing down to the deepest layers. This may become even more problematic in the future considering climate change and potentially milder winters. In large deep lakes, two other processes originating in the coastal zone can significantly contribute to winter deep water renewal: cold water density currents on the lateral slopes caused by differential cooling between the shallow literal zone and the deep off-shore region, and upwelling of deep water masses over the lateral slopes which results from wind forcing and may be influenced by the Coriolis force. Previous field experiments in Lake Geneva, the largest freshwater lake in Western Europe (max. depth 310 m), have shown that cold-water density currents are a common phenomenon during wintertime, frequently reaching the thermocline at O(100m) depth. As part of a project investigating winter mixing dynamics in Lake Geneva, we examine wind-driven, Coriolis-influenced upwelling in order to better understand its role in the deep-water exchange during the weakly stratified period (December-March). The study is based on field observations taken on the northern shore of the lake, 20 km west of Lausanne in the winters of 2017-19. Near-bottom water temperatures are inferred from a fiber-optic based Distributed Temperature Sensing (DTS) system, laid down on the lateral slope starting at the shore, with a spatial and temporal resolution of O(1m) and O(0.1°C), respectively. Temperature and current velocities in the water column are recorded by several ADCP and thermistor chain moorings. The data set is completed by regular CTD campaigns following wind events favoring upwelling. These field measurements were combined with a validated 3D hydrodynamic model (MITgcm) in order to analyze and quantify the exchange between the littoral zones and deeper parts of the lake. It was observed that following medium to strong northeast-bound wind events (i.e. winds parallel to the main axis of the central lake) lasting for at least one day, near-shore temperatures at all instruments suddenly dropped to the same temperatures as those in the deep open waters and remained low for several days. This indicates that, triggered by persistent wind-stress and under the influence of the Coriolis force, hypolimnetic water reached the surface in the near shore zone, where it is subject to heat and gas exchange with the atmosphere, before returning to the deep layers. Numerical modeling of these events showed comparable results and allowed to trace the origin of this cold water to the deep layers. Our field observations and the numerical results suggest that in addition to the previously observed cold-water density currents, wind-driven upwelling is a common phenomenon in Lake Geneva during the weakly stratified winter period and potentially a significant mechanism for deep-water renewal.

2019

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2019

Dynamics of wind-induced coastal upwelling and interbasin exchange in Lake Geneva during winter: Implications for deepwater renewal

Rafael Sebastian Reiss

EPFL2021