Wintertime deepwater renewal, which is important for heat–oxygen–nutrient exchange in lakes, is traditionally considered to be mainly driven by 1D vertical convective cooling. However, differential cooling between shallow and deep waters can produce density currents that flow into deep layers. In order to determine the role that these two cooling processes play in deepwater renewal, field measurements and 3D numerical modeling were combined to investigate heat content dynamics in Lake Geneva's large basin, the Grand Lac (maximum depth 309 m), during an exceptionally cold air spell in early 2012 where complete overturning had been reported. In a novel approach, the heat budget of the lake was decomposed, which allowed the identification and quantification of the heat budget components. The heat budget decomposition revealed that vertical convective cooling only penetrated to 200 m and that lateral advection was not only caused by density currents being discharged from the shallow littoral zone of the Grand Lac, but also from the Lake's shallow Petit Lac basin (maximum depth 75 m); the latter was found to be the main driver of heat content decrease in the deep layers of the Grand Lac below ∼200-m depth. These findings provide unique insight into heat exchange processes that cannot be obtained from field data or numerical simulations alone. Heat budget decomposition proved to be a powerful, universally applicable tool for quantifying the contribution of alternative deepwater renewal processes. This is important, since deepwater renewal by convective cooling is weakening due to persistent global warming.

中文翻译：

---

大型深湖（日内瓦湖）的深水更新：利用热收支分解识别和量化冬季冷却过程

冬季深水更新对于湖泊中的热-氧-养分交换非常重要，传统上被认为主要由一维垂直对流冷却驱动。然而，浅水区和深水区之间的冷却差异会产生流入深层的密度流。为了确定这两个冷却过程在深水更新中所起的作用，结合了现场测量和 3D 数值模型，研究了日内瓦湖大盆地Grand Lac（最大深度 309 m）在异常冷空气期间的热含量动态。据报道，2012 年初发生了彻底倾覆的事件。在一种新颖的方法中，湖泊的热量收支被分解，从而可以识别和量化热量收支的组成部分。热收支分解表明，垂直对流冷却仅渗透到 200 m，横向平流不仅是由大湖浅滨区排放的密度流引起的，而且还来自湖浅部小拉克盆地（最大深度 75米）；研究发现，后者是导致大湖深 200 米以下深层热含量下降的主要驱动力。这些发现为热交换过程提供了独特的见解，而这些见解无法仅从现场数据或数值模拟获得。事实证明，热预算分解是一种强大的、普遍适用的工具，用于量化替代深水更新过程的贡献。这很重要，因为由于持续的全球变暖，对流冷却带来的深水更新正在减弱。