The persistent increase in heatwaves has caused substantial economic and ecological damage. However, the contribution of decadal oceanic variability to the recent surge in heatwaves remains unclear. Here, using observations and simulations, we demonstrate that oceanic modulation drives decadal heatwave swings and trends. We quantify that the decadal component of heatwave cumulative intensity (HWCI) accounts for 57% of the observed increase in HWCI across the Northern Hemisphere from 2013–2021, with 44% attributed to increases in the smoothed component (HWCI_S) and 13% to enhancements in the anomaly component (HWCI_A). Notably, decadal oceanic variability contributed to 63% of the HWCI increase in the Northern Hemisphere during 2013–2021 and to 26% over 1985–2021. Regionally, oceanic modulation amplified HWCI by 58% in Europe, and contributed more than 20% in North Africa, southern North America, eastern China, and northern Central Asia during 2013–2021. The positive-to-negative phase transitions of the Atlantic Multidecadal Oscillation (AMO) and Interdecadal Pacific Oscillation (IPO) were identified as key drivers of this recent intensification. Model simulations incorporating AMO and IPO forcings closely align with observed HWCI decadal oscillations since 1940, further supporting these findings. Our results highlight that oceanic modulation can significantly amplify or dampen human-induced long-term heatwave trends, suggesting a potential slowdown in heatwave intensification in the coming decades as oceanic variability transitions to a new phase.

decadal,extreme events