<p>The ocean is in constant motion, containing eddies across a wide spectrum of scales. Outside the equatorial region, nonlinear interactions of ocean mesoscale eddies, ranging in scales from tens to hundreds of kilometers, drive a universal upscale kinetic energy (KE) transfer. This transfer is fundamental to sustaining large-scale flows and shaping the ocean kinetic energy spectrum, consequently maintaining an equilibrium state of the ocean. Yet, how this upscale KE transfer will respond to greenhouse warming remains poorly understood. By applying a spatial coarse-graining approach to a state-of-the-art high-resolution climate simulation, we reveal an overall weakening of the upscale KE transfer outside the equatorial region, predominantly in the deep ocean. This weakened upscale KE transfer is primarily caused by a diminished conversion from available potential energy to KE at mesoscales, due to enhanced stratification. Our findings suggest significant shifts in the ocean energy cycle in response to anthropogenic climate changes.</p>