Extended Data Fig. 1: Atmospheric dynamics during June 2021 leading to the anomalous geopotential heights associated with the PNW heatwave.

See Text S1 for further discussion. (a–f): 500hPa Geopotential height (filled contours), 300hPa meridional wind speed (red and blue contours), and outgoing longwave radiation (OLR; green and dark brown contours) anomalies averaged over 9-day periods centred on the annotated date. For clarity, the meridional wind field is only shown poleward of 20°N and the OLR field is only shown within 90°E–100°W (roughly the Pacific Ocean). For example, (a) shows the 9-day mean surrounding 06/05, when geopotential heights were high in the PNW accompanying a heatwave, with centres of low and high geopotential height extending westward over the Pacific forming a tripole. By 06/10 (b)) the tripole had expanded longitudinally, placing negative geopotential height over the PNW, and begun to constitute part of a wavenumber-4 pattern in meridional wind and geopotential height encircling the midlatitudes. Over 06/10–06/20 (c–e)) this wavenumber-4 pattern moved slightly northward and shifted phase longitudinally, eventually placing high geopotential height over the PNW. Throughout the last two weeks of June (d–f)) the wavenumber-4 pattern persisted and amplified, causing extreme temperatures and dry soils in central Europe, Siberia, and the PNW, and was reinforced by a Rossby wavetrain emanating from the subtropical western Pacific.