Fig. 3: Synoptic configuration and forcing mechanisms.

a Number of days (shading) when a Saharan intrusion was detected in each grid cell during the June HW. Black dots represent areas where the occurrence of Saharan intrusions was unprecedented (since at least 1948). Lines depict the composite of 1000–500 hPa geopotential height thickness (dam) for the days when our algorithm detects heatwave conditions. The dashed contour at 580 dam indicates the minimum threshold for Saharan dust intrusion. Panel adapted from Sousa et al.¹². b Same as (a), but for the July HW. c Temporal evolution of the 1000–850 hPa temperature anomaly (°C, with respect to 1981–2010; grey line; right y-axis) and the contributors to the temperature tendency (°C day⁻¹; coloured lines; left y-axis) over WEU, from lag −8 to lag +8 days of the June HW onset there (24 June). Grey shading represents days with HW conditions in WEU. d Same as (c) but for the July HW (onset on 23 July). e Temporal evolution of the fractional area (%) dominated by each forcing during the June HW. Grey shading as for previous panels. f Same as (e) but for the July HW. A 3-day smoothing is applied to the series presented in (c–f).