Warm ocean provides essential energy for tropical cyclones (TCs). It is well recognized that the upper-ocean vertical temperature gradients substantially influence TC-self-induced sea surface cooling, thereby moderating TC intensification. Recent high-resolution satellite imagery reveals ubiquitous horizontal submesoscale sea surface temperature gradients (SSTGs) in TC-active regions, but whether and how they might affect TCs remains unknown. Here, we show that weak pre-storm SSTGs substantially suppress TC-induced cooling, whereas strong SSTGs enhance cooling under down-front winds. Down-front winds blow along the front with warm water on the right-hand side, and the associated current in the upper ocean pushing the cold, heavy water across the front to the warm side through Ekman transport under the influence of Earth's rotation. Aggregated globally, weak SSTGs favor TC intensification and lead to approximately 40% higher TC intensity than that over strong SSTGs. Further, approximately 70% of rapidly-intensifying intense TCs occur over weak SSTGs and approximately 60% of intense landfalling TCs experience below-average SSTGs in the days preceding landfall. Consequently, a decreasing trend of approximately 10% per decade in SSTGs, across all TC-active regions observed since 1993, should be conducive to a trend of increased TC intensity. Thus, weak pre-storm SSTG can be an early-warning for intense TCs with destructive impacts, and resolving submesoscale SSTGs is essential in operational prediction systems and in climate models used for TC projection.