Atlantic SST as a key source of subseasonal predictability in the April 2020 Siberian heatwave

The April 2020 Siberian heatwave was among the most extreme early-spring warming events on record, with widespread environmental and societal impacts, yet it was missed by most subseasonal-to-seasonal (S2S) forecast systems. Understanding the sources of predictability and their limitations remains a key challenge in extended-range forecasting of land heat extremes. Here we show that Atlantic sea surface temperature (SST) anomalies constitute a key remote contributor, modulating a downstream Rossby wave response and upper-level ridging over Siberia, while local land–atmosphere feedbacks, including snow–albedo and radiative processes, amplified the near-surface warming. Regional replay experiments demonstrate that Atlantic SST conditions can provide effective forcing for the development of large-scale circulation anomalies consistent with the observed event, whereas Siberian land conditions further enhance their persistence. Forecast failures are associated with systematic SST biases in the Western North Atlantic, which weaken downstream wave propagation along the mid-latitude waveguide. As background conditions such as Atlantic warming, Eurasian ridging, and declining snow cover continue to evolve, these results highlight that reducing SST biases is essential for improving subseasonal prediction skill for high-impact Eurasian heat extremes.