Through the artificial manipulation of hydrological residence time, hydropower operations on alpine lakes should not only alter their hydro-biogeochemical characteristics but also modulate the degree of sensitivity of the lake to atmospheric forcing. These hypotheses were addressed by means of in situ observations that were combined with a three-dimensional lake model of a high-altitude pump-storage reservoir, i.e., Lake Corne, during the ice-free period. The fitted model (Pumping scenario) was used to simulate the hydrodynamics and oxygen concentrations of the lake under natural conditions (without pump-storage). Thereafter, the lake response to direct or catchment-mediated meteorological conditions (e.g., changes in inflows during storms) was investigated with a sensitivity analysis of the forcing parameters under both the pumping and natural hydrological scenarios. The pumping operation resulted in significant changes in water temperature, heat content and water mass stability during summer, as compared to a natural scenario. The lake hydrodynamics during the ice-free season were highly responsive to changes in the summer weather conditions, and more sensitive to realistic changes in cloudiness and water transparency than changes in air temperatures for both the pumped and natural scenarios. The spatial and temporal evolution of dissolved oxygen was comparatively less responsive to summer weather conditions, in both scenarios. The pump-storage operation from Lake Corne had a comparatively smaller effect on the lake functioning than the natural meteorological variability in summer. However, the pump-storage operation decreased the sensitivity of the lake hydrodynamics to changes in water transparency and limited water mass stability during summer.