Atmospheric water vapor contribution to interannual variability of Northern Hemisphere summer heatwaves.

Previous studies suggested that atmospheric water vapor, as a greenhouse gas, can amplify surface warming by enhancing downwelling longwave radiation (DLR), thereby potentially intensifying extreme heatwaves. However, the atmospheric water vapor anomalies associated with heatwaves and their radiative effects remain poorly quantified. Here, using radiative kernels to decompose DLR anomalies, we demonstrate that atmospheric water vapor exerts a substantial influence on summertime interannual-scale heatwave-related DLR across the Northern Hemisphere, despite DLR being largely dominated by near-surface air warming. This influence exhibits pronounced regional contrasts: mid‑ to high‑latitude heatwaves are typically accompanied by moist air, which enhances DLR, whereas heatwaves in India and western North America often coincide with drier conditions that reduce DLR and thereby partially offset the positive DLR anomalies caused by air warming. The dryness over India is linked to an interannual weakening of the Indian summer monsoon, while that over western North America is associated with soil‑moisture deficits. Our findings provide a quantitative assessment of the contribution of atmospheric moisture to extreme heatwaves.