The global wind energy industry is growing rapidly for its potential in combating global warming. For large-scale wind farms, wind-turbine wake interference remains a major challenge as it reduces power efficiency and increases structural fatigue. In this study, we discover a new wake physics mechanism which can be applied as a novel wake control method. By spraying fine water droplets into the wake, a negative buoyancy zone is created through the evaporation-induced cooling. The resulting negative buoyancy generates downstream entrainment of upper and lateral high-velocity airflow, accelerating wake recovery and increasing power generation in the turbine downstream. We perform large-eddy simulations of this process under various atmospheric conditions. In a three-15MW turbine array, this method yields net power gains of 4.9%-7.8% from the full-wake scenario after deducting the spraying energy cost. The power gain is found to elevate with lower ambient humidity, larger spraying flow rate and higher atmosphere stability. We further apply this method to a wind farm in Denmark consisting of eight-2MW turbines, where the net power gain reaches 24.5%. The large-eddy simulation with DPM framework used is validated by wind tunnel experiments, in which we also observe the accelerated wake recovery by cooling. These findings demonstrate that water-spray cooling can be a promising approach to control wind turbine wake and improve power generation in the wind farm worldwide.