Dual physical pathways shape subseasonal-to-seasonal predictability of North American summer heat extremes

Improving the subseasonal-to-seasonal (S2S) prediction of heatwaves is a critical scientific challenge due to their severe societal impacts and the limited predictability at these lead times. Here we show two dominant and physically distinct sources of S2S predictability for the weekly hot days across North America. The leading predictable mode, with its strongest loading over the southern U.S., reflects an ocean-forced mode associated with a Pacific Decadal Oscillation-like sea surface temperature pattern. In contrast, the second most predictable mode, with centers of action over northwestern and southeastern North America, is dominated by atmospheric internal variability associated with the summer Arctic Oscillation and summer North Atlantic Oscillation. Skillful week-3 forecasts of hot days are further characterized by quasi-stationary wave patterns coupled with regional soil moisture deficit, revealing dynamical and land-surface ‘windows of opportunity’ for extended S2S predictability. These findings identify the physical mechanisms governing North American heat extreme predictability and provide a scientific basis for improving extended-range early warnings of heat extremes.